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MEASUREMENTS AND AN INTRODUCTION TO FAULT FINDING

GOLBORNE 2015

This years’ talk is on Measurements and an Introduction to Fault Finding. I’ll start by going over my

background in electronics then revisit the 2012 talk to go over various aspects of measurements and

how to interpret them. This is probably one area where beginners have difficulty.

I’ll also describe the effect on the unit being tested of the measurements as in some cases the act of

connecting a meter can either cause the unit to fail or to start working and go on to describe various

components and their failure mechanisms.

In the second part I’ll go over the basics of fault finding. Now everybody will have their own views on

the best way to fault find, usually built up over many years of experience and in many cases in a

commercial service environment. Others will have little experience and will need significant

guidance. Witness the Vintage radio forums where beginners are often guided through the fault

finding and repair process remotely. In some cases it can take many posts to make a measurement

and interpret the result whereas a more experienced person would probably have taken a few

minutes to make and interpret the same measurement. It’s all part of the learning experience but

there’s nothing better than bringing a radio or TV back to life using your own efforts. It’s probably a

greater thrill for a beginner than for someone who’s been doing it for years. How many of us can

remember their first successful diagnosis and repair?

And finally I’ll talk about replacing components and finding alternative parts.

Background

Before I start I’ll briefly describe my experience in both fault finding and general electronics.

I’ve been interested in electronics for most of my life and in the 1960s started doing repairs to radios

and TVs for friends and relatives to supplement my pocket money. I had decided that I would rather

design equipment rather than repair it so after A levels I did an electronic engineering degree at

university then moved out into the big wide world earning a living in the military, commercial and

most recently consumer electronics fields.

During the design, and more importantly, the testing phase, my previous experience of diagnosing

and repairing faults came in very useful. It is much more difficult to diagnose a fault on a brand new

prototype piece of equipment than on a mature piece of equipment.

Failure Analysis

In my last job one of my tasks was failure analysis. This is different to repairing equipment where the

aim is to find the fault, repair it and get it back to the customer as soon as possible. In failure analysis

the aim is not necessarily to repair the item, although quite often the item is repaired, but to find

the fault and investigate the cause. If the same fault keeps re-occurring then further investigation is

undertaken and measures can be put in place to correct the manufacturing or test process in the

factory. In many cases a single unit being investigated will be damaged irreparably during the course

of the investigation. Of course the investigation can also show faulty components being fitted. I’ll

mention a few cases I’ve been involved in to show how failure analysis and normal fault finding

differ.

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We had a case where a rectifier in the power supply went short circuit shutting the unit down. The

first case took some time to investigate but it soon became a “stock” fault with many failures being

recorded. I could diagnose the fault within seconds of powering the unit up. It actually took longer to

open up the unit to confirm the fault than to do the diagnosis. We had information on the build

dates of these units and the numbers of failures so I plotted the build date and number of failures

and it soon became apparent that there were two peaks relating to certain build months. Further

investigation showed the majority of faulty rectifiers had two date codes. We sent faulty rectifiers

back to the manufacturer and to an external test house for investigation and eventually the

manufacturer was audited by our parent company and their manufacturing process for those

rectifiers was considered as not up to the required standard. The manufacturer was removed from

the list of suppliers following that audit. We changed to a different rectifier manufacturer and the

failure rate for units built after that date due to failure of the rectifier dropped to zero.

The Safety Warning

Before we go any further here’s the usual warning about the high voltages used in radios, especially

valve radios and TVs. Most of the equipment described in this talk is powered from the mains,

typically 230V. This voltage can be fatal if used improperly. Also the HT side of valve radios can also

give you a nasty shock. Always take care when working on a mains powered unit, switching off and

disconnecting from the mains before replacing components. Inevitably the unit must be powered up

to make measurements and under these circumstances it will often have been taken out of its case.

It is recommended that you keep one hand in your pocket when making measurements and make

the connections to the test points before switching on.

The use of an isolating transformer when working on AC/DC sets, this includes TVs, is recommended

as in some cases only a single pole switch was fitted by the manufacturer which can result in the

chassis becoming live when switched off. Note that only one item should be connected to an

isolation transformer at a time. See last years talk on Power Supplies for more details.

Ensure the transformer is rated for the units connected to it. 200VA is probably adequate for most

radios and 1950/60s B & W TVs but up to 1000VA may be needed for first generation colour TVs

which can consume up to 400W. CRT colour TVs also have a switch on surge due to the degaussing

coils operating to demagnetise the CRT so the transformer must be rated to cope with that.

Some of the earlier 1930s/40s TVs used mains derived EHT which will be fatal if touched. For a

beginner these are probably best avoided until more experience is gained.

Take care with the mains and treat it with respect for, unlike James Bond, you only live once.

Hopefully this hasn’t put you off so we’ll begin with Measurements.

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Test Equipment and Measurements

This section is the environmentally friendly part of the talk as some of it is being recycled from the

2012 talk on test equipment and describes the effect of component tolerances and instruments on

the measurements. I feel it’s worth repeating as these two factors can have a major influence on the

measurements and highlights the need to interpret the results of any tests.

Before I go over the measurements what test equipment do you need? There are many items of test

equipment that can be useful for fault finding, such as signal generators and oscilloscopes but the

basic minimum is a multimeter.

Multimeters come in two basic flavours, analogue and digital. Each has its own advantages and

disadvantages and it’s down to your own preferences. Analogue meters, such as the AVO 8, are

useful where voltage are varying such as when aligning a radio whereas digital meters can give a

more precise reading. Most analogue multimeters have DC and AC voltage ranges, usually up to

1000V, DC current ranges up to 10A and resistance ranges. Some analogue multimeters have AC

current ranges. Range selection is usually manual.

The cheaper digital multimeters are usually manual range selection with similar voltage, current and

resistance ranges but often have a simple transistor tester. Higher spec digital multimeters are often

auto ranging and can have additional features such as frequency and capacitance measurements.

Whatever meter you have, take time to get used to it and how to operate it.

Component Tolerances and Measurements

All electronic components have a tolerance in their values. The most common examples are resistors

and capacitors where you’ll see the values quoted as say 10kΩ ± 5% or 100µF ± 20%.

When components such as resistors are manufactured no two parts will come out with the exactly

the same value even though they have been made from exactly the same material and have been

through the same processes. There will be a slight variation in the values. In the early days

manufacturers used to measure the resistor values, and place them in various bins according to the

measured value. Those that were closest to the required value, within 5%, were then marked as 5%

parts (gold band) those that were within 10% were marked as 10% parts (silver band) and the rest

that were within 20% of the required value had no band. The 5% and 10% parts were sold at a

premium. That is why if you measured a new 20% resistor it would be between 10% and 20% of the

marked value.

As manufacturing processes improved components, such as resistors, could be made to much closer

tolerances and in many cases the actual values could be trimmed to within 1% or better of the

required values. These days 1% and 2% resistors are readily available and are relatively cheap.

However the voltage rating of the most commonly used resistor types is generally unsuited to valve

circuits.

For most manufacturers of valve consumer electronics 20% resistors were perfectly adequate and,

more importantly, were cheaper than the closer tolerance parts, which were only used in cases

where the value was critical. An example of this is the biasing of the typical class B transistor output

stage of a transistor radio where 5% resistors were used, all the other resistors being 10% or 20%.

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The effect of the tolerances can be shown by examining a simple resistive divider across a fixed

voltage from a 12V regulator.

The voltage at the junction Vj is

12 * R1

(R1+R2)

12v regulator and potential divider.

If we set both resistors to be the same value the voltage at the junction will be 6V won’t it. Well it is

until we take into account the resistor tolerances.

If we assume both resistors are 10kΩ then the worst case values for each resistor are.

Tolerance 20% 10% 5% 2% 1%

Max 12kΩ 11kΩ 10.5kΩ 10.2kΩ 10.1kΩ

Min 8kΩ 9kΩ 9.5kΩ 9.8kΩ 9.9kΩ

How will this affect the voltage at the junction?

Taking the worst case conditions for the resistor values, with one being the maximum value and the

other being the minimum value, the junction voltages are.

Tolerance 20% 10% 5% 2% 1%

Max 7.20V 6.60V 6.30V 6.12V 6.06V

Min 4.80V 5.40V 5.70V 5.88V 5.94V

The nominal voltage is 6V in all cases but this shows that if the divider is built with 20% resistors the

voltage at the junction can be anywhere between 4.8V and 7.2V and still be within specification. The

tighter the resistor tolerance the closer to the required 6V the voltage will be.

Now let’s add in another factor, the tolerance of the voltage regulator powering this circuit. Typically

these have a tolerance of ±4%. For a 7812, 12V regulator, this will result in the output voltage being

between 11.5V and 12.5V.

Adding the regulator tolerance into the circuit the resulting voltages at the junction are thus.

Tolerance 20% 10% 5% 2% 1%

Max 7.49V 6.86V 6.55V 6.36V 6.30V

Min 4.61V 5.18V 5.47V 5.64V 5.70V

So we have moved from a nominal voltage of 6V to a voltage, which can be up to 1.5V from the

nominal value and still be in specification.

R1

R2

Vj

12V regulator

+12V

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The Effect of Measuring the Voltage.

The voltage at the junction Vj is now

12 * R1a

(R1a+R2)

Where R1a is the parallel combination of

R1 and the voltmeter resistance.

The Effect of the Voltmeter on the Potential Divider.

When we come to measure the voltage at the junction the most common method is to use a

multimeter. What effect, if any, will this have on the reading?

The actual voltage measured will depend on the values of the resistors used and the meter being

used to measure it. Let’s take 3 different meters and examine the effect of each one on the circuit.

Meter 1 - Typical analogue meter 20kΩ/V on the 10V range presents a resistance of 200kΩ to the

circuit being tested.

Meter 2 - Typical cheaper digital multimeter. Presents a resistance of 1MΩ to the circuit being tested

irrespective of the range setting.

Meter 3 - Typical digital multimeter. Presents a resistance of 10MΩ to the circuit being tested

irrespective of the range setting.

The voltage is measured across the lower resistor, R1, and the meter will reduce the effective value

of this resistor. The effective value will depend on the value of the resistor. The following table

shows the effect for values of 1kΩ, 10kΩ, 100kΩ and 1MΩ.

1kΩ 10kΩ 100kΩ 1MΩ

20kΩ/V meter (10v range) - 200kΩ 0.995kΩ 9.524kΩ 66.667kΩ 166.667kΩ

DMM - 1MΩ 0.999kΩ 9.901kΩ 90.909kΩ 500.000kΩ

DMM - 10MΩ 1.000kΩ 9.990kΩ 99.010kΩ 909.091kΩ

It can be seen that the higher the values of resistor in the divider the greater the effect of the meter

resistance.

If we assume the junction voltage is 6V, the voltages actually measured by the different meters are.

1kΩ 10kΩ 100kΩ 1MΩ

20kΩ/v meter (10V range) - 200kΩ 5.985V 5.854V 4.800V 1.714V

DMM - 1MΩ 5.997V 5.970V 5.714V 4.000V

DMM - 10MΩ 6.000V 5.997V 5.970V 5.714V

As would be expected the meter with the highest resistance, the 10MΩ DMM has the measured

value closest to the actual value as it presents the lowest load to the circuit.

R1

R2

Voltmeter

Vj

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The tables do not take into account the tolerance and measurement accuracy of the meter itself.

Analogue meters are typically ± 2% (AVO 8) and digital meters are typically 0.3% ±1digit (Fluke 79).

This will affect the actual reading taken.

Many service manuals for older equipment will have voltages specified for various test points. When

measured with a modern DMM the measured value will often differ from the specified value

because of the different loading of a DMM compared to the meter used to measure the voltages

originally, usually an AVO. It’s also important to note that the service manual will often specify which

range the meter was set to. A 20kΩ/V meter will present a resistance of 200kΩ to the circuit on the

10V range but on the 100V range the resistance will be 2MΩ which can result in apparently different

voltages being measured.

Whenever you measure a voltage, current or check a waveform you need to ask the question is it

what I was expecting?

If you measure the voltage on the collector or anode of a small signal amplifier what level of voltage

would you expect? To coin a phrase is it in the right ballpark. If you have service data with expected

voltages and it says for example the collector voltage is 9V and you measure 8.7V would you worry?

In that case no but if it’s 0V or close to the supply voltage then yes something is wrong and needs

further investigation.

It’s always worth measuring the supply voltages first as if these are wrong then all the other voltages

will be wrong. In many cases incorrect supply voltages will be the cause or a significant contributor

to the fault.

There are similar effects when using a scope in a circuit. In this case if it is being used to make

measurements on a tuned circuit the effect of the capacitance of the scope must be taken into

account. The standard input impedance of a typical scope is usually quoted as 1MΩ in parallel with a

capacitance of 30pF. Using a 10:1 scope probe alters this to typically 10MΩ and 3pf. Even this small

capacitance can affect the tuned circuit. It’s easy to demonstrate this by tuning an AM radio to a

station near the high frequency end of the Medium Wave and connecting a scope probe to the non-

earthy end of the local oscillator coil. The radio will be detuned because of the effect of the

capacitance of the scope probe.

This demonstrates that when measuring any signal the effect of the measuring instrument must be

taken into account. This is especially important in high impedance circuits or RF circuits where the

capacitance of the measurement probes can affect the circuit operation.

Current Measurements

For current measurements there are two alternatives. The most obvious one is to break the circuit to

be measured and connect the meter in the break. Using this method it is advisable to start on a

higher current range of the meter and work down. This will prevent trying to pass 100mA through

the 50µA range of the meter, then trying to repair the meter! Connect the meter then switch on and

note the current. The range can then be changed to suit the current. It may be necessary to switch

off the unit before changing range but be aware that the peak current at switch on may overload the

meter.

The alternative method is to measure the voltage across a resistor inserted into the circuit. This does

assume that you know the value of the resistor. It does have the advantage of not overloading the

meter but involves a calculation. The resistor value should be low enough so as not to affect the

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operation of the circuit but high enough to give a sensible reading on the meter. It is also possible to

use an existing resistor in the circuit but you may have to measure its value before using it to

measure the current.

The current (I), in amps, is

I = V

R

Where V is the measured voltage and R is the value of the resistor in ohms.

Resistance Measurements

Resistance measurements should always be made with the power to the unit turned off as not all

meters will withstand full HT being applied to the input on the resistance range.

There are several things to be aware of when measuring resistance. It can be done in circuit but it is

better to either remove one end of the resistor or remove it completely from the circuit before

measuring. If measurements are made in circuit the effect of all the other parallel paths should be

taken into consideration. Measurements on lower value resistors are generally easier using this

method but be aware that in solid state circuits a parallel base emitter junction can give different

readings depending on the polarity of the connection. Often you will find the resistance will start

low and rise due to the capacitors across the supply charging. In that case wait a few seconds until

the reading stabilises. The resistance should be no higher than the value of the part being measured,

any higher and it is possible that the resistor is high in value or open circuit.

As an example let’s look at the potential divider board used earlier. We know the values of the

resistors used but what value do we see if we measure them in situ. Take one of the 1kΩ resistors,

R1, what are the parallel paths?

Demo board circuit. 12v regulator and potential dividers.

There are three sets of resistors, the 10kΩ, 100kΩ and 1MΩ branches all in parallel. Together these

are equivalent to an 18kΩ resistor. This is in series with the other 1kΩ resistor making a resistance of

19kΩ which is in parallel with R1. This is equivalent to a resistance of 950Ω.

78M12 +12V

1k

1k

10k

10k

100k

100k

1M

1M R8 R1

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If we measure across the 1MΩ, R8, the measured value is 500kΩ. This demonstrates that higher

values of resistance are more affected by parallel paths.

Resistance is one measurement where a digital meter can be an advantage as it will give you an

exact reading.

Diode Measurements

Another advantage of a digital meter is that many include a diode measurement which indicates the

voltage across the circuit being measured. This can be used to advantage to check if there is a

parallel diode path by reversing the leads. If the voltage is the same then it is probable that any

parallel diode is not having an effect or the parallel resistance is low enough to prevent the diode

conducting.

With the diode measurement a voltage of approximately 0.1V to 0.3V indicates a germanium diode,

a voltage of approximately 0.6V to 0.7V indicates a silicon diode and a voltage of approximately

0.45V indicates a schottky diode. If you have a diode option measurement on your meter it is worth

checking a few known diodes and transistors of all types to get a feel for the typical readings for

each device.

Another use for the diode measurement is to

check LEDs. In the forward direction the “colour”

of the LED can usually be determined by the

forward voltage.

The LED will also light up dimly while being

measured to confirm the colour.

Note that many DMMs use a maximum of 3V for the diode measurement which may not be enough

to measure a Blue or White LED.

Making the Measurements

Next, the practical side of making measurements. Most voltage measurements are made with

respect to the ground of the unit under test. This will normally be the chassis which will be the 0V

connection and to which the negative terminal of a meter is usually connected. However there are

some radios & TVs where the chassis is not the most negative voltage. Also note that in early

transistor radios which have a positive earth i.e. the ground is connected to the positive terminal of

the battery, with these it is best to connect the meter positive terminal to ground.

The meter range should be set to suit the voltage being measured e.g. if the voltage is specified as

60v the range should be set to 100v. If in doubt set the range to suit the HT supply and change to a

lower range. This will prevent any damage to the meter, especially an analogue meter where the

pointer could be damaged if the pointer hits the end stop,

An auto ranging multimeter will automatically change range to suit the voltage being measured.

Make a firm connection to ground. This allows you to make measurements single handed, keeping

your other hand out of the way of any high voltages. A crocodile clip or a probe as shown in the

picture is the best way to make the connection.

LED type Typical Forward Voltage

Infra Red 1V

Red 1.6V

Yellow 1.8V

Green 2V

Blue 3V

White 3V

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Chassis connection using Test Probe and Crocodile Clip (BAC31 chassis)

Be aware of the voltage rating of the multimeter leads. The meter may be capable of measuring

1000V but if the probe leads are not rated for 1000V it may not be safe to hold the probes when

making measurements. Meters made by major manufacturers such as Fluke, AVO have probe leads

rated at 1000V or greater whereas the leads and probes supplied with the cheaper meters may not

have the same voltage rating.

Note that due to the way the HT battery is connected

in battery valve sets the HT negative terminal is not

normally a suitable ground point.

The unit obviously has to be powered up to make

voltage measurements and usually the probe can be

moved easily and quickly between the test points.

However in some cases where making a connection to

the test point is difficult the unit can be powered

down and the connection made using a wire soldered

to the test point or the test probe can be clipped on.

The unit is then powered up and the measurement

made. This is especially relevant if the voltages at

power up need to be measured, as it is not always

easy to hold a test probe on a particular point and turn

the unit on.

When making measurements take care not to short

out adjacent pins on valve bases or other components

as this can have catastrophic effects. Full HT on the grid of a valve does not do it any good! If in

doubt make the test connection first, check it’s OK, then switch on.

One further point is that many analogue meters are calibrated for use in a horizontal position. If the

meter is used vertically the accuracy may suffer.

Typical circuit of battery valve set showing

the HT –ve taken to 0V via R13 to provide

the grid bias for the output valve V4.

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EHT Probes.

The maximum voltage for most multimeters is typically 1000v which is satisfactory for the majority

of applications. However there are occasions where higher voltages need to be measured. One

instance is the EHT on a CRT TV. For these measurements an EHT probe is used. This is simply a

series of high value resistors, with a suitable voltage rating, forming a potential divider to reduce the

high voltage to a level suitable for a multimeter.

EHT probe circuit

The resistors are selected to give the correct reading of the EHT voltage when connected to a meter

with an input impedance of 10MΩ and set to the 10V range.

It’s not always necessary to measure the EHT current for a CRT but if you need to the meter should

be well insulated as shown in the picture below.

When using an EHT probe or current meter always ensure the connections are made and are secure

before switching on. Ensure that the ground connection of the EHT probe is securely made. If this

becomes disconnected, the meter will become unsafe as it will be raised to the EHT potential.

Once you are satisfied the connections are secure and safe, power up the equipment. After making

the measurement, switch off and allow the high voltage to dissipate before removing the probe or

current meter.

Do not touch the meter while the equipment is switched on.

EHT probe connected to meter EHT current meter

Series of high value resistors

HV

Connection

Ground

Connection

Multi

meter

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Whenever you make a measurement or conduct a test the question you have to ask is “Are the

results I am getting the results I was expecting?” If making voltage or current measurements are the

results close to those in the service manual. If they are not, what could cause them to be different?

Components

We’ll move onto the passive components that actually make up a TV or radio. Some are more

reliable than others and some are always suspect. I’ll be briefly describing the components used in

the vintage radios & TVs rather than components used in more modern radios & TVs although the

modern parts can be subject to similar faults.

Resistors

Most early resistors are either carbon composition or wirewound. The carbon composition types

comprise a rod of material containing carbon. The amount of carbon depends on the value required.

Some resistors, mainly the higher power types, are of the “dog bone” type where the leads are

soldered to the ends of the rod. The lower power resistors, up to about 2W, encase the rod in a

ceramic tube. The value and tolerance is designated using either the body, tip, spot method or by

rings on the ceramic body.

These carbon composition resistors are prone to changing vale as they age. Higher values tend to

rise in value and lower values tend to fall in value. In a lot of cases a small change in value doesn’t

make a lot of difference to the circuit operation but if the resistor forms part of a biasing circuit the

change can cause problems.

Modern resistors are coded with standard colour code rings indicating the value and tolerance.

Some resistors, particularly those used in TVs from the 1950s and early 1960s, have an extra salmon

pink band. These are high stability resistors used where the value was critical. You won’t find many

of these in a typical valve radio because of the cost but you may find one or two in the timebase

circuits of TVs. See the appendix for more details of the colour code.

Resistors

Above – Typical dropper resistor from an AC/DC set

Left – Typical styles of resistors used in vintage equipment

The bottom resistor is actually a thermistor but shows the

construction of a “dogbone” resistor

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The dogbone types are normally coded as “body, tip, spot” to indicate the value. Here the body is

the first digit of the value, the tip is the second digit of the value and the spot is the multiplier. If the

resistor has a 20% tolerance there will be no further markings but if it is 10% or better the other

“tip” will be coloured with the appropriate colour.

Capacitors

Capacitors come in many flavours; the most common types found in vintage equipment are

Electrolytic, waxed paper, silver mica and ceramic. There are also the more modern types using a

plastic film.

The silver mica, Mullard “mustard”, ceramic and trimmer capacitors are generally reliable whereas

the electrolytics, Hunts and waxed paper capacitors are almost always suspect.

The basic structure of a capacitor is of two metal

plates separated by a dielectric. The capacitance

depends on the area of the plates, the separation of

the plates and the dielectric material. The greater

the area (A), or the smaller the separation (d), the

greater the capacitance. The required value and

application will often determine the type of

capacitor to be used.

The dielectric used will also determine the stability

and other characteristics of the capacitor. These

characteristics determine the use to which the

capacitor is used.

Silver Mica Capacitors

Silver Mica capacitors have the electrodes deposited on mica sheets and are usually very stable and

have a low temperature coefficient but are only available in lower values, typically less than 1000pF.

They are also capable of being made to very close tolerances typically 5% or less, which is one of the

requirements for tuned circuits. They are also reliable with a low failure rate. However they are

Basic capacitor construction

Capacitors

Left – Plessey electrolytics

and Hunts.

Centre – TCC electrolytic

and waxed paper. Silver

Mica and Mullard

“mustard”.

Right – Beehive and

compression trimmers,

tubular and disc ceramic.

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more expensive than other types of capacitor and their use in domestic radios ad TVs is confined to

circuit areas such as RF tuned circuits that benefit from their stable characteristics.

Ceramic Capacitors

Ceramic capacitors are often used for decoupling where a precise value is not required as they often

have wide tolerances and, depending on the dielectric used, some will have a capacitance that varies

with both the applied voltage and ambient temperature. They are made by either by interleaving the

two electrodes with a ceramic dielectric or depositing the electrodes on the inside and outside of a

ceramic tube. They are also fairly reliable.

Wax Paper Capacitors

Waxed paper capacitors, used extensively in radios and TVs from the 40s to the 60s, on the other

hand, are one of the replace on sight components. These use aluminium foil as the electrodes with a

dielectric of waxed paper. The foil and paper are wound into a roll which is then stuffed into a paper

tube and the ends sealed with wax to form the familiar capacitor. The dielectric has the unfortunate

characteristic of slowly absorbing moisture decreasing the leakage resistance between the

electrodes over time.

Film Capacitors

The waxed paper capacitor was used in radios and TVs up to the 1960s when alternative types

became available. These still used aluminium for the electrodes but this was deposited onto a plastic

film as the dielectric and wound into a roll as with the earlier types. These plastic films are more

stable than the paper previously used and are significantly less prone to absorb moisture. Examples

of this type of capacitor are the Mullard mustard type and the yellow types sold by BVWS and

others.

Electrolytic Capacitors

Electrolytic capacitors are made using aluminium foil with a thin layer of aluminium oxide as the

dielectric and an electrolyte between the two foils. These can have very high values of capacitance

but because of the nature of the dielectric they are polarised. Electrolytic capacitors are normally

used in power supplies and for coupling purposes in transistor circuits where the lower impedances

necessitate higher capacitance.

Basic Aluminium electrolytic capacitor construction (courtesy of Wikipedia)

http://en.wikipedia.org/wiki/Electrolytic_capacitor

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The diagram shows the aluminium foils have been etched to increase the surface area. Earlier

capacitors did not etch the foils and were therefore larger for the same value. The thickness of the

dielectric also determines the voltage rating.

They do have a wearout mechanism as the electrolyte will evaporate over time, made worst at

higher temperatures. Electrolytic capacitors also have a specified life based on temperature and

ripple current. Also the dielectric can deteriorate over time. Sometimes this can be “repaired” by

reforming. I’ll explain this later.

Inductors and Transformers

These can come in all sizes from small coils of a few turns intended for VHF front ends to large

power transformers capable of delivering 10s or even 100s of watts. They all have the same basic

construction, wire wound round a former with or without a core. In the case of power and audio

transformers and power supply chokes the former has an iron core. Coils and transformers intended

for RF circuits often have an adjustable ferrite core. Most wire used for winding coils is insulated but

some VHF coils, where the turns are widely spaced, are often wound with uninsulated wire.

Many coils used for AF circuits are wound with thin wire which can suffer from corrosion causing the

wire to break. Coils with thicker wire can also suffer from corrosion but as there is more copper they

are less prone to open circuits. If you are unlucky enough to have a coil or transformer with an open

circuit winding, it is often worth undoing the winding as the break may be close to the end of the

winding.

Fortunately with transformers it is often possible to have it rewound and there are several forum

members who offer rewinding services.

Variable Components

The components described previously all have fixed values. There are also variable versions of these

components used where adjustment is needed. The most common types are variable capacitors,

used for tuning and variable resistors used for volume and tone controls.

Variable Resistors Variable Capacitors Preset Inductor and

Permeability Tuner

15

These components have their own failure mechanisms. Variable resistors and capacitor rely on a

spring contact between the fixed and moving parts for their operation. This can be a source of

problems as corrosion and lack of use can increase the resistance between the fixed and moving

parts and cause noisy operation. In addition the resistive track can wear out due to constant useage

over the years. When used as a volume control with a power switch the first part of the track gets

most wear.

Switches, typically used for wave change, can also be affected by corrosion and contamination of the

switch contacts especially if they have not been used for some time.

Variable inductors can suffer from corrosion on the wire similar to fixed inductors. The preset

inductors can suffer from the core sticking or breaking due to use of inappropriate tools for

adjustment.

Active Components

As well as the passive components, radios and TVs contain many active components to amplify and

process the signals. The main types of active components are valves and transistors. In spite of

popular belief, valves are much more reliable than people think. They will often withstand significant

overloads and continue working once the overload has been removed. Transistors are also reliable

although they are more prone to failure if overloaded, however there are certain types of transistor

that are notorious for a specific failure mechanism. These are the OC17x and AF11x TO7 cased

transistors used extensively in the 1960s. These have a tendency to grow tin whiskers from the

inside of the case which will eventually reach the transistor die shorting it to the case which is

normally grounded. There are various methods to deal with this and I’ll go into more detail later.

Lockfit transistors introduced in the late 1960s by Mullard are also showing signs of faults becoming

noisy or just failing.

It should be remembered that many of the components used in vintage radios and TV were chosen

on cost and that they would have performed perfectly well in the equipment when it was new but

the manufacturers did not expect it to last 60 or more years, probably closer to 10 years or, more

importantly, until the guarantee had expired. As vintage enthusiasts we are now experiencing the

deficiencies in these components. Fortunately it is not difficult to find modern alternatives to replace

the faulty components.

16

Fault Finding

Fault finding on any equipment can take many forms. In military equipment there is usually a rigid

pre-defined procedure where the fault is diagnosed down to “box” level and the box replaced to

keep the equipment going. The faulty box is then returned to the second service level where it is

diagnosed down to individual board level. The board is replaced, the unit re-tested and then

returned to service. Faulty boards are then sent to the third service level where they are diagnosed

down to component level, repaired, retested and returned to service. There is usually complex

automatic test equipment (ATE) to aid the diagnosis and minimise the time taken to return

equipment to working order.

In the consumer world we are not so lucky. There may be equipment available which can be plugged

in, such as code readers for car diagnostics and fault codes on TVs and VCRs, but these do rely on a

certain level of functionality in the unit and the ability of the software writer to predict what

happens when a fault occurs. They can certainly point to the area that is probably at fault but if that

fails to cure the problem them it’s down to good old fashioned fault finding. Although there are

many items of test equipment available to aid the fault diagnosis, a multimeter is probably the best

item to have, certainly for the initial stages of the diagnosis.

Everyone involved with fault finding and repairs will almost certainly develop their own way of doing

things based on their abilities and experience. It’s not an overnight process. Many service engineers

would attend Radio & TV servicing courses at college and gain recognised qualifications but these

days these courses have largely been discontinued. These courses could be compared to a driving

test where you are trained following a set curriculum but the real learning takes place once you have

passed the test.

For beginners it can be difficult to work out the best way to start finding a fault. This part of the talk

is intended to give beginners and others, who may wish to develop alternative methods, an

introduction to fault finding. There is no single correct way to fault find as you may go round in

circles trying to find a fault only to find it was something trivial that would have been found quickly

with an alternative approach. The main thing is to learn from each fault finding experience. The

more you learn the quicker you can become at finding the fault and repairing it. Some of the

techniques I’ll describe are those I have used over the years I’ve been involved in electronics both

privately and professionally.

Often there is more than one fault so following a logical procedure such as getting the audio stages

of a radio working first and using these to monitor the earlier stages will help to reduce the time to

fix the set.

There is no easy way to learn fault finding skills. Read up on the subject, follow the various threads

on the forums, learn to follow the circuit diagram, learning what each component does and what

effect a fault in that component has on the circuit operation. I’ve put a list of suggested reading in

the appendix although this is not exhaustive.

To illustrate the basics let’s assume you have a radio or TV with a fault. You may know what the fault

is or have a vague description like “it doesn’t work”. Most of the vintage radios and TVs bought

these days would be expected to have at least one fault but we must assume that it did work once. If

possible you should try to establish what was happening prior to the fault occurring but this is not

always possible so we’ll assume we have no information about the radio or TV (we may be lucky and

find it may not even have a fault).

17

I’ll concentrate on fault finding on radios as this is probably easier for beginners to deal with than

jumping in at the deep end with a TV. The principles behind fault finding are the same for both

radios and TVs.

How Do We Start?

The first thing to do is examine the radio. What we are looking for here is

1. To identify the make and model.

2. Note the condition of the radio.

3. Is it mains, battery or mains battery?

4. Is it valve, transistor?

5. Is it complete/ does it look as if anything is missing?

6. Are there any obvious signs of damage, broken components, burnt PCB etc.

7. Is there any evidence of any work having been done on it?

If the make and model can be identified then service data can usually be obtained. This is much

easier these days with Pauls service data DVD, the internet etc. When I was doing this as a hobby

back in the 60s, getting service data often meant making a trip to the local library and hunting

through the service books to find the service data then copying it out by hand.

Let’s take a look at four radios and identify the circuit areas. Here we are trying to identify the type

of radio and the various areas of the radio such as the audio preamp and output stages, the IF stages

and the frequency changer. Knowledge of the typical valve or transistor types used in these stages is

useful here.

Firstly what type of radio are they. This is where good observation is required, checking all the

available clues.

Radio 1 - GEC BC402

There’s no mains transformer but there’s a dropper resistor so it’s almost certainly an AC/DC set.

Only one valve was present when I found it but it’s almost certain that the valves will be U series

with 100mA heaters. The variable capacitor has four sections, two high capacitance and two low

capacitance, there are the remains of a ferrite rod aerial with two coils therefore it has VHF FM, MW

and LW. There are 6 valve holders one of which is in a separate “module”, the VHF front end.

Radio 2 - KB DR10

There’s a mains transformer so this one is an AC only set. The tuning capacitor has two high capacity

sections so it’s an AM only set. There are four positions on the wave change switch so it’s probably

got a Gram input with LW, MW & SW.

Radio 3 - Bush BAC31

No mains transformer or dropper resistor but two battery plugs are fitted and the valves have B7G

bases and the tuning capacitor has two high capacity sections so this is a battery valve set with LW

and MW.

Radio 4 - Ever Ready Sky Leader

This is a transistor set. The tuning capacitor and push button switches indicate it’s an AM only set

with LW and MW.

18

Radio 1 – GEC BC402 chassis Radio 2 – KB DR10 chassis

Radio 3 – Bush BAC31 chassis Radio 4 – Ever Ready Sky Leader

19

Of course it helped that I knew which models radios 2, 3 and 4 were but it took a little detective

work to identify radio 1 as a GEC BC402.

Next we’ll identify the various stages of the radio.

A typical superhet radio will have a frequency changer, IF amplifier, demodulator, audio pre-amp

and audio output. If it’s mains powered there will be a power supply. Let’s take a look at three types

of AM radio. A mains powered valve set, a battery valve set and a transistor set and see what valves

or transistors are typically used.

Function Typical Valves/Transistors Used

Valve Set - Mains Valve Set - Battery Transistor Set - Battery

Frequency changer E/UCH81, 6BE6 DK91, DK96 OC44, AF117

IF amplifier E/UF89, EBF89, 6BA6 DF91, DF96 2 x OC45, 2x AF117

Demodulator EBF89, 6AT6 DAF91, DAF96 OA70

Audio pre-amp 6AT6, E/UCL82 DAF91, DAF96 OC71, OC81D

Audio output E/UCL82, E/UL84,6BW6 DL94, DL96 OC72, OC81

Power supply EZ80, UY85, 6X4 - -

Note that in a valve set, one valve can have more than one function as a single valve can contain

several separate elements, a sort of thermionic integrated circuit!

A valve radio with VHF will have an additional valve or valves for the VHF tuner, typically a U/ECC85

although many earlier VHF sets had EF80s or an ECC81 in the tuner. There will also probably be a

U/EABC80 as the AM and FM demodulators and the audio pre-amp.

With transistor sets one device will normally perform one function but with valves there can be

more than one “device” in the valve. This is where interpretation of the valve number is a great help.

The Pro-electron code, as used by most European valve and semiconductor manufacturers, gives a

good indication of the type of valve. It comprises between two and four letters followed by two or

three numbers. Let’s take the ECH81 as an example.

E C H 8 1

The first letter indicates the heater or filament voltage or current. In this case E is a 6.3V

heater.

The remaining letters indicate the types of valve. In this case C is a Triode and H is a

Heptode.

The first number indicates the valve base. In this case 8 is a 9 pin B9A base.

The remaining numbers are serial numbers although in the case of RF valves an even

number generally indicates a straight characteristic and an odd number indicates a variable

µ.

In the above cases

The EBF89 is an RF pentode with two diodes and a 6.3V heater on a B9A base.

The ECL82 is a triode and output pentode with a 6.3V heater on a B9A base.

The DAF 91 is an RF pentode with a single diode on a B7G base and a 1.4V filament.

20

The OC series of transistors have their numbers derived from the valve codes where the O indicates

a 0V heater and the C indicates a “triode”! Diodes are indicated by numbers beginning with OA.

The later Pro-electron code shows the semiconductor material and its intended function. E.g.

A F 1 1 7

A indicates it’s a germanium transistor

F indicates it’s intended for RF applications.

More information on the coding systems used can be found in the link in the appendix.

The stages can also be identified by the proximity of various components. The IF amplifier(s) will

normally be located between the IF transformers and the frequency changer will be close to the

tuning capacitor and the audio amplifier will be close to the output transformer or the loud speaker.

Dropper U119 N119 DH109 W119 X119 B109

Resistor (UY85) (UL84) (UABC80) (10F18) (UCH81) (UCC85)

Output Transformer 2nd

IF Transformer 1st

IF Transformer VHF Front End BC402

Tuning Capacitor 6K8 1st

IF Transformer 6K7 6Q7 2nd

IF Transformer 6V6 6X5 Mains Transformer

DR10

21

Identifying the various stages from the transformers and other components can also help as it is not

unknown for the valves in a valve radio to have been removed and re-inserted in the wrong sockets

or even the wrong valves inserted in the sockets. These may be reasons why the radio doesn’t work

of course.

During the identification stage, look for any suspect components. The usual ones are waxed paper

capacitors, Hunts capacitors, the red and black Plessey electrolytic capacitors and any capacitors that

look as if they are leaking. Also look for overheated resistors, broken components and bad wiring.

Make a note of these for replacement later.

Now we’ve identified the various sections of the radio we can make some measurements.

Mains Input Measurements

For a mains powered radio, either valve or transistor, measure the resistance across the mains lead.

This should be infinite with the on/off switch in the off position. With the switch in the on position

there should be a reading. If the set has a mains transformer the reading should be typically 100 to

200ohms. For an AC/DC set the reading should be approximately 2400ohms, the resistance of the

dropper resistor and the heater chain (100mA heaters).

If the reading is still infinite there are a number of possible reasons. These include

1. Open circuit or blown fuse.

2. Faulty on/off switch.

3. Faulty mains transformer.

4. Faulty dropper resistor (AC/DC set).

5. Open circuit heater or missing valve

(AC/DC set).

Tuning Capacitor

OC44 – Frequency

Changer

Local Oscillator Coil

1st

IF Transformer

OC45 1st IF Amplifier

2nd

IF Transformer

Sky Leader

Ferrite Rod Aerial

OC45 2nd

IF Amplifier

3rd

IF Transformer

Audio Driver Transformer

2x OC78 Audio Output

Audio Output Transformer

OC78D Audio Driver on

underside of chassis

Local oscillator coil

1st

IF transformer

DK91 – Frequency Changer

DF91 - IF Amplifier

2nd

IF Transformer

The tuning capacitor is on the

underside of the chassis.

DAF91 – Demod & AF Preamp DL94 Audio Output BAC31

22

If the resistance drops to a very low value this could be due to

1. Short circuit turns on mains transformer.

2. Short circuit suppression capacitor.

If there is a reading of a few hundred ohms with the on/off switch in the off position the on/off

switch probably has welded contacts which could indicate a fault with the mains transformer.

In the case of an infinite or very high resistance reading check

1. The mains plug for correct wiring

2. The fuse in the mains plug. This should be 3A maximum for a typical radio but often a 13A

fuse will have been fitted.

3. The internal mains fuse in the radio, if one is fitted. It could be open circuit or even be

missing.

4. The switch contacts on the switch. This may be a single pole or double pole. Measure the

resistance across the contacts in the on and off positions. There should be a very low, <1Ω,

in the on position. Often the on resistance will be a few hundred ohms and will vary. This is

normally due to contaminated contacts. A method of dealing with this is detailed in the

appendix.

5. Measure the resistance of the mains transformer primary. Note that many radios had a

mains voltage adjustment plug which sometimes had a fuse fitted. It’s also possible that this

fuse is missing or open circuit.

6. In an AC/DC set check the resistance of the dropper and the continuity of the valve heaters

as described later.

For a short circuit check

1. The suppression capacitor. Often it is simplest to cut it out of circuit before applying power.

2. The mains transformer. If you have the service manual these often have the resistances of

the transformers and inductors. A reading which is significantly different to any published

value would require further investigation.

A suppression capacitor across the mains may at first appear OK but if it’s a wax paper type it can fail

once mains power is applied. I had a TV once where the fuse blew after about 45 minutes due to a

faulty suppression capacitor. Once cut out the set stopped blowing fuses.

HT Supply Measurements

Check the resistance across the HT supply (this is applicable to both valve and transistor circuits).

There should be an initial low reading, probably a few hundred ohms for a transistor set and a few

thousand ohms for a valve set. The reading should increase over a few seconds as capacitors across

the HT rails charge up. If it’s low and stays low there is probably a short circuit or low resistance

component across the supply. Capacitors are the usual culprits.

Reforming Electrolytics

One thing that is often done on valve sets is to reform the electrolytic capacitors in the power

supply. Over the years these can dry out, the dielectric can deteriorate, loose capacitance and

become less effective. They can be replaced or reformed.

23

Reforming is just applying a voltage near to their maximum working voltage, and limiting the current

supplied to the capacitor and monitoring either the current or the voltage across the capacitor. This

can be done without removing the capacitor from the set. Initially the current will be high but as the

dielectric reforms the current will fall.

If the capacitors are being reformed in circuit there are other checks that can be done while the

reforming voltage is applied.

It’s not necessary to remove the valves during this procedure as the heaters will not be powered up

however it is advisable to remove them if they are battery valves as it is possible to damage the

delicate filaments should a test probe slip.

Check the voltage rating of any capacitors across the HT supply as applying too great a voltage can

have explosive results.

For a radio using indirectly heated valves apply a voltage of typically 250V to 300V or for a battery

valve set apply a voltage of typically 90V via a resistor, observing the correct polarity, across the

capacitor. Monitor the voltage across the resistor and calculate the current or monitor the voltage

across the capacitor. You are looking for a low voltage across the resistor or a voltage across the

capacitor close to the supplied voltage.

The current will initially be “high”, several mA, or the voltage across the cap will be low, but as the

capacitor reforms the current will drop and the voltage will rise.

Simple Capacitor Reformer

Reforming can take several hours so be patient. While you are waiting you can measure the voltages

on the screen grids of the valves. In the IF and frequency changer stages these are usually decoupled

with a wax paper capacitor so if the voltage is low, allowing for any series resistor, the capacitor

must be leaky. Similarly measure the voltage on the control grid of the output valve. This should be

0V, any voltage there is a sure sign that the coupling capacitor, also known as ‘That Cap’, (C4 in the

diagram on the next page) is leaky. Any capacitors that show signs of leakage should be replaced.

Specifically ‘That Cap’ must be replaced before the set is switched on to prevent damage to the

audio output valve and the output transformer. Many people, myself included, replace all wax paper

10k 5W

+ve

To capacitor to

be reformed

-ve

250v 4x 1N4007 22µF 4 x

Secondary Bridge rectifier 450V 33k 2W

320V

240V

160V

80V

Mains transformer

24

caps as a matter of course. It is often advisable to do this especially if you are doing it for someone

else as it can prevent any call backs when the capacitors not replaced start leaking.

It is also worth measuring the voltages on the anode

pins of the valves as this will indicate continuity of the

output transformer and IF transformer primary

windings

Reforming is not always successful in which case the

only option is to replace the capacitors.

Valves

Many beginners assume the valves will need replacing,

after all they are in sockets and Grandads radio was

always having its valves replaced. Just because they are

easy to replace it doesn’t mean they will be faulty.

Valves are generally more reliable than people think.

It’s more often than not the components around them

that are faulty especially after 40 or 50 years.

A simple check on the valves is to check the heater or

filament continuity. Using a multimeter on the ohms range connect it across the heater pins usually

pins 4 & 5 of a B9A base, pins 2 & 7 of an IO base, pins 3 & 4 of an indirectly heated B7G based valve

and pins 1 & 7 of a directly heated B7G based valve. Note that some valve have tapped heaters or

filaments, notably the ECC81/2/3 and their equivalents 12AT7, 12AU7 and 12AX7 which have a

centre tap on pin 9 and the DL92/4/6 which have a centre tap on pin 5. If in doubt consult a suitable

data book or valve data website. Valve pin numbering is always done looking at the underside of the

valve in a clockwise direction from the gap in the case of B9A and B7G bases or the pip of the spigot

of an IO or MO type base.

Another thing to watch for is the valve base itself. Some types have contacts which are notorious for

breaking and making poor or intermittent contact with the valve pin. If a faulty contact is found

either the whole base can be replaced or if the contacts are removable the faulty one can be

replaced by an unused contact from another valve base. Battery valve radios usually have a few

unused contacts on the valve bases.

In an AC set the valves are usually in parallel across a winding on the mains transformer Thus if one

heater or filament has failed the others will not be affected. In an AC/DC set the heaters are in series

and one open circuit heater will prevent all the valves operating. In a valve battery set the filaments

are usually in parallel but in a valve mains battery set the filaments are in series as it is easier to

derive the filament supply from the main HT supply if the current required is 25mA or 50mA rather

than the 125mA or 250mA required if they were in parallel.

For battery valve or transistor radios check the on/off switch operates correctly. Valve sets usually

will have a double pole switch, switching both the HT and LT but in transistor sets a single pole

switch is more usual.

It can be useful to measure the resistance across the battery connector in a transistor radio. With

the set switched on the resistance should be a few kohms. A low resistance would indicate possible

short circuit output transistors or a short circuit decoupling capacitor.

“That Cap”. The coupling capacitor

between the anode of the pre-amp

valve and the grid of the output valve.

25

Powering Up

Having identified the stages, ensured the valves are in the correct positions, reformed the power

supply electrolytic capacitors and made sure there are no shorts on the HT rails, it’s time to power it

up. The following applies to mains sets.

There are two schools of thought on this one

is connect it straight to the mains, the second

is connect via a lamp limiter or a variac. It all

depends how confident and brave you are!

The lamp limiter is simply an incandescent

light bulb, typically 60W or 100W, connected

in series with the mains supply to the unit.

The bypass switch allows the lamp to be

shorted out to allow full mains to be applied.

The lamp limiter will limit the fault current to

a safe value. It will also limit the voltage to the

set so the performance may not be the same

as when it’s on full mains. However at this

stage we are just trying to ensure there no

damage will be done by powering it up.

If you power it up via a Variac, start at a low voltage and slowly increase the voltage carefully

observing the set.

I normally connect a meter across the HT supply to monitor the HT voltage as the set warms up but

note that with a lamp limiter the HT will be lower than specified.

When power is applied, good observation is needed. If a lamp limiter is used the lamp will briefly

light up brightly then fall back to a dull glow. If it lights up brightly there is almost certainly a low

resistance across the mains supply.

1. Are the heaters lighting up?

2. Is there any smoke?

3. Is the HT supply about right. Note that in sets with a solid state rectifier the HT will rise

immediately to a higher value until the valves warm up. The HT in sets with a valve rectifier

will rise slowly as the rectifier warms up.

4. Are there any noises from the speaker? There may be a low level hum present showing the

speaker is functional.

5. As the valves warm up there listen for any noise from the speaker. A hiss is a good sign again

showing that the speaker and probably the audio amplifier are both functional.

6. If there is a loud hum this is almost certainly due to bad electrolytic capacitors in the power

supply. If these have been reformed then this has not been successful and the capacitors will

have to be replaced.

If there is any smoke switch off but if possible try to see where it’s coming from. This can be difficult

as your natural instinct will be to switch off quickly. Often the smoke is just the dust burning off a

dropper resistor.

Lamp Limiter Circuit

L

Mains In

N

E

L

To Unit

Under

Test

N

E

Bypass

Switch

Incandescent Light Bulb

Note that the lamp must be an incandescent light bulb.

CFL or LED light bulbs are not suitable for this circuit.

26

If the set powers up with no issues it’s time to apply full mains. Again follow the checks above.

Once the set has powered up it’s time to identify the circuit area at fault.

If there is noise from the speaker turn up the volume control. There may be crackles meaning the

control needs cleaning. If the back ground noise increases as the volume is turned up it means there

is some functionality in the tuner stages.

If there is no noise from the speaker, check the speaker itself as it could be open circuit. Many sets

had an external speaker connector with a switch, usually a screw which is unscrewed to disconnect

the internal speaker. Check the internal speaker is selected and try connecting an external speaker.

In a set with an earpiece socket try plugging in an earpiece. If sound is heard check the earpiece

socket as it is not unknown for these to have high resistance contacts.

If the internal speaker is open circuit or is disconnected it is not advisable operate the set as damage

could occur to the output valve or the output transformer.

Many AC only sets have a Gram input for connecting a record deck. If this is fitted connect an audio

source e.g. the output of an Ipod, and check for any output from the speaker. If you are lucky the

audio stages will be OK. Take care when connecting devices such as Ipods as these have stereo

outputs and the Gram input will be mono. There could also be a DC voltage on either the Ipod

output or the Gram input. The following circuit should be used to minimise damage to both devices.

Typical 3.5mm stereo jack cable Suggested circuit

If the audio stages are not functioning check the voltages as shown on the following pages. The

circuits are for typical audio amplifiers comprising a pre amplifier stage and output stage. A pre

amplifier stage is more common in valve sets with the pre amp valve often housing the demodulator

and AGC diodes. Typical valves are 6AT6 and EBC90.The pre amp can also be combined with the

output amplifier such as an ECL82.

Transistor audio amplifiers normally use a class B push pull output stage with a driver with or

without a pre amplifier stage. A 6 transistor radio would not have a pre amp but a 7 transistor radio

would almost certainly have one. Early transistor radios used transformers to couple the driver to

the output transistors and to couple the output transistors to the speaker. Later radios dispensed

with the transformers using PNP and NPN transistors driven directly from the driver transistor.

Red

White

Screen

2 x 1kΩ 0.1µF

Gram

input

Ground

The values are not critical and any values close to the

suggested values can be used.

27

Test

Point Function

Typical

Voltage

1 Pre Amp Anode 150V

2 Output Valve

Screen Grid 245V

3 Output Valve

Cathode 7 - 12V

4 Output Valve

Anode 250V

1 Pre Amp Base 1.4V

2 Pre Amp Emitter 1.1V

3 Pre Amp Collector 4.5V

4 Driver Base 1.8V

5 Driver Emitter 1.5V

6 Output Bases 0.15V

7 Output Emitters 0.05V

1 Pre Amp Base 1.6V

2 Pre Amp Emitter 1.3V

3 Pre Amp Collector 4.5V

4 Driver Base 0.7V

5 Driver Emitter 0.4V

6 Driver Collector

& Output Bases 4.4V

7 Output Emitters 4.5V

Typical Audio Stages and Voltages

Top – Single ended Valve Amplfier

Middle – Push Pull Transformer Coupled Transistor Amplifier

Bottom – Transistor Complimentary Pair Amplifier

1

2

3

4

1

3

2

4

5

6

7

1

3

2

4

6

5

7

28

The voltages given are roughly what you would expect but check with the service data for the exact

voltages to be expected. If the voltages are significantly different further checks are required.

When the audio stages are working the investigation shifts to the tuner section. Here it can help to

know how a superhet works. In simple terms the incoming signal is converted to a single frequency

signal, the Intermediate Frequency or IF, by mixing it with the output of the local oscillator. This

signal is then amplified by the IF amplifier and demodulated by the detector to produce an audio

output.

A fault could occur in any one of these stages.

The first step is to measure voltages in each of the stages.

Check the voltages at the anodes or collectors of each of the valves or transistors as shown in the

following diagrams.

Typical Valve Frequency Changer (V1) and IF (V2) Stages with Suggested Test

Points. (V3 has the Demodulator (pin 5) and AGC (pin 6) Diodes plus the Audio

Pre-amp)

Test Point Function Typical Voltages (HT voltage = 250V)

1 Frequency Changer

Screen Grid 100V – 150V

2 Frequency Changer

Anode 250V

3 Local Oscillator Anode 150V

4 Local Oscillator Grid -ve

5 IF Amplifier Screen Grid 150V

6 IF Amplifier Cathode 1V – 3V (varies with signal level)

7 IF Amplifier Anode 250V

1

2

3

4

5

6

7

29

The voltage on the anode of the frequency changer and IF amplifier valves should be very close to

the HT supply voltage. If either of these voltages is low or missing check the continuity of the IF

transformer winding.

The voltage on the grid of the local oscillator (pin 9) should be negative if the local oscillator is

oscillating.

A low voltage on the screen grid of either valve could indicate a leaky decoupling capacitor,

especially if a waxed paper capacitor is used.

Typical Transistor Frequency Changer (TR1) and IF (TR2 & 3) Stages with Suggested Test Points.

Test Point Function Typical Voltage (Supply voltage = 9V)

1 Mixer emitter 1V

2 Mixer collector 9V

3 1st

IF emitter 1V (varies with signal level)

4 1st

IF collector 9V

5 2nd

IF emitter 1.2V

6 2nd

IF collector 9V

Note that this circuit uses PNP transistors and therefore the meter positive lead should be

connected to the ground connection. The OC44 and OC45 transistors were used in early radios. Later

radios used AF117 transistors which did not require the neutralising components R10, C9, R11 and

C10.

As with the valve circuit the voltage on the collectors should be close to the supply voltage. If they

are low check the continuity of the IF transformer.

In the above circuits if any voltage or current measurement is not what was expected or significantly

different to that specified in the service data further investigation is required to establish the cause.

The emitter voltage or the cathode voltage in conjunction with the emitter or cathode resistor will

1

2

3

4

5

6

30

give an indication of the collector or anode current in the particular stage. If it is out of specification

check the biasing components.

Also note that the AGC line can affect the voltages on the controlled IF amplifier. In a valve circuit

this is a high impedance circuit and as such any leakage due to the AGC line decoupling capacitors

(C6 in the valve circuit) can significantly affect the voltages around V2. The equivalent capacitor in

the transistor circuit is C5. Leakage in this component will affect the voltages around Tr2.

As an example let’s

consider the effect of

faulty components on

the amplifier circuit

shown right.

We’ll just consider the

components around TR1

and measure the

voltages and look at the

waveform at the output

for several fault

conditions.

There are eight possible resistor faults with the specified resistor either high or low in value.

However several of these have similar faults e.g. R1 high has very similar symptoms to R2 low.

The table shows the measured values for

the circuit compared to the normal

values and the effect on these of the

resistors R1 – 4 going high and low. The

exact voltage will depend on the extent

to which the value has changed.

The output signal will also be affected, in

some cases where the value is only

slightly different there will be no

discernible effect on the waveform.

To demonstrate the effect on the circuit and the signal waveform each of the resistors R1 to R3 are

disconnected from the circuit and R4 is increased in value and the effect on the transistor voltages

and the output waveform are shown on the following page.

The supply voltage was measured as 11.7V.

The input is a 1kHz sine wave set to give an output signal of 2V pk – pk. Note that the onscreen

display shows the Y channel set to 0.2V/div but the input is connected via a 10:1 probe.

With R1 O/C there is no bias on the base of TR1. Therefore there is no emitter current and hence no

collector current and the collector voltage is close to the supply voltage and there is no signal

output.

TR1

collector TR1 base

TR1

emitter

R1 High > Normal < Normal < Normal

Low < Normal > Normal > Normal

R2 High < Normal > Normal > Normal

Low > Normal < Normal < Normal

R3 High < Normal < Normal < Normal

Low > Normal > Normal > Normal

R4 High > Normal > Normal > Normal

Low < Normal < Normal < Normal

31

With R2 O/C the current into the base of TR1 causing the transistor to saturate. The transistor comes

out of saturation on the negative peaks of the input signal.

With R3 O/C there is no current in the collector and also the emitter therefore the collector voltage

drops to virtually the same as the emitter. As the only current through the emitter is the base

current the emitter voltage is low.

With R4 high the emitter and hence the collector current will be lower shifting the collector voltage

nearer the supply voltage. As the gain is essentially the ratio of the collector to emitter resistors this

is reduced as shown by the lower amplitude signal.

Fault C B E Collector Waveform

Normal 9.54V 1.08V 0.44V

R1 O/C 11.67V 0V 0V

R2 O/C 2.35V 2.67V 1.97V

R3 O/C 0.09V 0.6V 0.03V

R4 High 11.19V 1.13V 0.51V

32

Stock Faults

When a manufacturer puts a product such as a radio or TV into production, it will have been

subjected to a series of tests to ascertain its performance, reliablility etc under many different test

conditions but these tests will have been conducted on a limited number of samples.

Once it’s in production, variations in the production process, changes in component suppliers will

cause a variation in the performance of the products. If the design is good and has been thoroughly

toleranced out to account for these variations then there should be no problems. However life is not

that simple and inevitably units will fail. If the manufacturer then analyses the failures it will soon

become apparent that certain components may be more prone to failure. As a result modifications

can be made in the production process and if necessary notices sent out to service departments to

change certain components.

Classic cases are the main capacitor in the Philips G11 chassis, the 47k sync separator screen resistor

in the Thorn 1500, R7 & R8 in the Quad 405.

If necessary a recall can be made although many manufacturers would just suggest modifications

when the set came in for service.

These days many errors or faults can be corrected by updating the software but remember

debugging software is often replacing one bug by another!! Manufacturers don’t like doing these

updates as to update the software using the OAD (Over Air Download) method can cost them a lot of

money. In many cases they will wait to “cure” a number of bugs before releasing an OAD update.

Many modern updates can be done via the internet either direct to the device or via a USB memory

stick.

Intermittent Faults

I’ve left the best or should it be the worst until last, the INTERMITTENT FAULT, the bane of many an

engineers life. These can be caused by any number of issues including mechanical and thermal

stresses.

By their very nature they can occur at random and often will not occur when you are ready to find

the fault which will almost certainly involve a significant amount of luck, a lot of perseverance and

even more frustration.

Mechanical causes include dry joints, hairline cracks in PCB tracks, bad connections on sockets.

Often moving the set or PCB can cause or cure the fault.

Some faults can occur when the set warms up due to the expansion of components. This can cause a

hairline crack to expand and cause an open circuit or a pin on a connector to expand and touch

another pin or component.

How do you find an intermittent fault?

1. Look carefully at the symptoms to try to identify the circuit area affected.

2. Monitor test points with a meter or scope. Get an idea of what the correct voltages should

be and when the fault occurs check the monitored test point.

33

3. Tapping the chassis may induce the fault. Again monitor test points and check what happens

when the chassis is tapped.

4. Examine the soldered joints carefully for dry or badly soldered joints. It’s not unknown for a

connection to have missed soldering during manufacture with connection being made

purely by mechanical means until time takes its toll and the connection becomes

intermittent.

5. Check any ground connections made via screws. Check the tightness of the screw noting that

there may be contamination or corrosion on the screw.

6. Examine PCBs for hairline cracks in the tracks. A meter can be used to check the track

continuity. Gently flexing the PCB can also help reveal a hairline crack.

7. Warm the board with a hair dryer or cool it with freezer spray but be careful as rapid cooling

can irreparably damage semiconductors and it doesn’t do a hot valve much good either!

When using this technique try to narrow down the area being cooled or heated to an

individual component.

Replacing Components

When radios and TVs were brought into the workshop for repair the main aim was to get them

repaired and back to the customer as quickly as possible. Most manufacturers had a parts list but it

was often more convenient to use alternative parts which were more readily available from the likes

of Radiospares (later called RS components). They would sell components, such as universal output

transformers, which could be used to substitute most manufacturers output transformers by

selecting the appropriate connections.

These days original replacement components are not available although you may be lucky and find

NOS (New Old Stock) parts but usually you’ll have to use modern components. Be aware though that

NOS parts, mainly resistors and capacitors and AF117 type transistors, may be subject to the same

faults that the parts in the equipment have.

Resistors

When replacing resistors in a valve circuit the voltage rating of the resistor must be taken into

account. There are two aspects to the voltage rating one based on the value and power rating of the

resistor and one based on the voltage rating of the resistor package.

E.g. Based on the power rating the maximum voltage that can be applied to a 1kΩ 0.25W resistor is

15.8V whereas the maximum voltage that can be applied to a 100kΩ 0.25W resistor is 158V.

Theoretically the maximum voltage that could be applied to a 1MΩ 0.25W resistor is 500V but if you

examine the datasheet for a typical modern 0.25W resistor you’ll probably see it is 250V or 350V.

Therefore a 1MΩ 0.25W resistor would be prone to failing if used near its maximum power rating.

For use in valve circuits the voltage rating of the resistor package should be greater than the HT

voltage. Fortunately suitable resistors are still made for use in power supplies. The resistors are

larger and more closely match the sizes of the resistors originally used.

As with the capacitors some early sets used resistors with non-preferred values. As these almost

certainly had a tolerance of 20% replacements can be the nearest preferred values.

34

Capacitors

The most common types of capacitor that will need replacing are the waxed paper type, Hunts and

electrolytic types. The waxed paper and Hunts types can be replaced with the film types. Many of

the values originally used will not be available as the original values were no always preferred

values. In most cases the actual value is not critical and in any case the components will have a

tolerance of typically 5% to 20%.

Original value Nearest preferred value

0.04µF 0.047µF

0.05µF 0.047µF

0.025µF 0.022µF

0.03µF 0.033µF

5uF 4.7µF

8µF 10µF

16µF 10µF or 22µF

The only situation where the actual value is critical is in tuned circuits. Fortunately tuned circuits

normally use Silver Mica capacitors which are normally very reliable. Should a Silver Mica capacitor

need replacement it may be necessary to make up the value from several preferred values in

parallel.

The voltage rating of the replacement capacitors should be equal to or greater than the rating of the

original.

Be careful when replacing the reservoir capacitor, the first capacitor after the rectifier, in a valve set

as there is a maximum capacitance specified for the rectifier valve. This should not be exceeded.

Inductors & Transformers

These are probably the most difficult components to replace as they are often specific to a single set

or sets from a manufacturer. The best source is a scrap set of the same or similar type.

With mains transformers it is possible to use alternative parts from a scrap set provided it has similar

outputs.

Output transformers are specified by their turns ratio to match the loudspeaker impedance to the

Anode Resistance, ra, of the output valve. However this is not as critical as it may seem and there is

a wide tolerance on the turns ratio so provided the turns ratio is reasonably close to the required

value it will work.

When replacing the mains or output transformer or any smoothing choke with an alternative type

check the fixings to make sure the replacement will physically fit.

It is almost always possible to have a transformer or choke rewound and there are several forum

members who offer this service.

IF transformers can usually be replaced with transformers of a similar type provided the IF frequency

is the same.

35

Inductors for the RF stages are probably the most difficult components to replace as they are specific

to each set. The alternatives here are parts from a similar scrap set or attempt a rewind.

Valves

In spite of not being manufactured for many years most types of valves are still available as either

NOS or tested ex equipment. Some valves are still being manufactured mainly for the audio and

guitar amplifier market. The quality of these valves can vary and prices for original valves favoured

by the audio market can be very high. (On Ebay earlier this year someone was selling a “rare”

Mullard ECC83 for £148!)

In some cases where a valve is not available or is very expensive, alternatives can be used but may

require a change of base and/or biasing components. Examples are using a UL84 in place of the

unreliable UL41. In some cases an adaptor can be used to convert one base to another.

It’s also worth noting that some valves are effectively available with alternative bases. E.g. the 6BW6

is basically a 6V6 with a B9A base although both valves are readily available.

Transistors

Germanium transistors used in early radios are no longer available but many are still available as

NOS. In many cases it is possible to use an alternative type provided the replacement is specified for

the intended function, e.g. an OC71, audio transistor, would not be a suitable replacement for an

OC45 RF transistor but an OC45 could be tried as a replacement for an OC71.

Some silicon transistors have a reputation for unreliability. These include the Mullard Lockfit

transistors. Fortunately the die used for these transistors is used in other transistors. E.g. a Lockfit

BC148 can be replaced by a TO18 BC108 or a TO92 BC548. It should be noted that the BC548 family

of NPN transistors includes the BC546 (65V Vceo), BC547 (45V Vceo), BC548 (30V Vceo) and BC549

(30V Vceo low noise). These types and their PNP equivalents are classed as general purpose

transistors and can be used to replace a large number of transistors. It’s always worth trying one of

these as an alternative but check the voltage rating is compatible with the supply voltage.

It’s at the extremes of the operating parameters, frequency, voltage, current etc. that the exact

transistor is required.

AF117 types

The TO-7 germanium transistors, such as the AF117 and OC170, suffer from the problem of tin

whiskers growing from the case to the transistor die, shorting out the die to the case. As the case is

connected to ground these whiskers stop the transistor functioning. The transistor does not have to

be connected and have power applied for the whiskers to grow as they can be found in unused NOS

parts.

For more information on this problem see

http://www.vintage-radio.info/whiskers

There are several ways to deal with the problem.

1) Cut the screen lead. By removing the connection to ground the transistor can function

correctly. This will not work if more than one of the transistor pins is shorted out. It can also

36

affect the stability of the circuit by removing the screening. However it is a quick and simple

method to check for the problem.

2) Remove the whisker. Remove the affected transistor from the circuit. You can check for a

short between the screen and other leads with a multimeter on its ohms range. Join the

Emitter, Base and Collector leads together. Charge up an electrolytic capacitor, a value of

47µF or 100µF is suggested, to 9V to 12V. Connect one lead of the capacitor to the transistor

screen lead and the other to the other joined pins. The capacitor will discharge through the

short and the resulting current pulse will blow the short. Check for shorts between the

screen and other leads with the multimeter. If the short has gone refit the transistor and

hopefully it will work. Note that the whiskers may return in the future as they do not stop

growing.

3) Replace the transistor with an alternative type.

The usual replacement transistor for the AF117 is

the AF127. This is a direct replacement in a TO-72

package which does not suffer from the whisker

problem. However the pinout is different so the

leads will have to be “joggled” to ensure the

correct connections. As the leads would have to

cross it is recommended to sleeve them.

4) Another alternative transistor is the BF450. This is a PNP silicon transistor. However the

biasing may need changing and it may not work in all stages of a radio.

It has been noted that some TO1 transistors such as the AC128 can suffer from the tin whisker

problem but it only becomes apparent when the case is connected to ground when fitted in a

grounded heatsink.

Other Components

Potentiometers and switches can cause crackling when operated. In many cases this can be cured or

at least an improvement made by applying switch cleaner. The usual way is to spray the cleaner into

the control and operate it several times with the power off of course. If no improvement is made the

control will have to be replaced with either a pot from a scrap set or a new pot which may be a

different size. Provide it is the correct value it should be OK. Note that volume controls are log rather

than linear.

Switches can also suffer from poor connection especially if they have been left in the same position

for years. With the set switched off, spray switch cleaner onto the contacts and operate the switch

several times. This will usually improve the switch but in extreme cases the switch will have to be

replaced.

Loudspeakers used in valve sets normally have an impedance of 3Ω. If a speaker is faulty almost any

speaker from a valve set can be used provided it will physically fit. Early transistor sets, with an

output transformer also used a 3Ω speaker but later sets without an output transformer used higher

impedance speakers. These should be replaced with a similar impedance speaker.

Mechanical parts can also suffer from aging. The classic case is parts made from Mazak commonly

known as “Monkey Metal”. Aging causes the metal to break up. It is not normally possible to repair

these parts and replacement with a more modern alternative is usually the only option.

Underside view of packages

37

Knobs on radios often get lost or damaged. Often it is possible to obtain replacements from scrap

sets from the same manufacturer. If this is not possible, either the damaged originals can be

repaired with epoxy or new ones made by creating a mould from an undamaged knob.

Dial glass is vulnerable to breaking especially if sent through the post in inadequate packing. The

marking on the glass can also become detached especially if cleaned too enthusiastically! Again glass

from a scrap set is one option but there are people on the forums who have scanned dial glass’s,

printed transfers and re-applied them to either the original glass or a replacement glass. There are

also forum members who have recreated dial glass for some sets. A search of the various forums

should find scans and the details of members with recreated dials.

Cabinets can also need attention which can range from a complete rebuild to a quick wipe down. I’m

not proposing to go into any detail on cabinet restoration but things to consider are woodworm,

damaged veneer, stains, broken Bakelite. Again the forums are a good source of information and

advice on how to proceed.

To Re-stuff or Not to Re-stuff That is the Question

When replacing wax paper capacitors the question that often arises is “Should I just replace the

capacitors or should I remove the contents of the original capacitors and fit the modern part inside

the case?” It is entirely up to the person undertaking the repair. I normally would not re-stuff

capacitors on the underside of a chassis but have re-stuffed some capacitors on the top side of the

chassis for appearance reasons. I have also re-stuffed the larger can electrolytic smoothing

capacitors with modern parts.

If capacitors have been re-stuffed it may be a good idea to leave a note inside the equipment to say

which capacitors have been re-stuffed to prevent perfectly good components being replaced by a

future owner who thinks the set hasn’t been restored!

Solder

The electronics industry has moved to lead free solder for environmental reasons and any new

electronic equipment placed on the market must use unleaded solder. However some sectors,

mainly medical and aeronautical, are still permitted to use leaded solder due to concerns about

reliability of the joints.

As far as I know leaded solder can be used to repair equipment originally built using leaded solder

which includes vintage equipment. It is however, not recommended to mix leaded and unleaded

solder as it can change the tin-lead ratio and cause unreliable joints. If you have to use both leaded

and unleaded solder for whatever reason, separate soldering irons for each type of solder are

recommended. Note that the iron temperature is higher for unleaded solder. If you’re using a Weller

soldering iron, number 7 bits are suitable for leaded and the hotter number 8 bits are suitable for

unleaded.

Leaded solder is still available from the likes of CPC, Farnell, RS components and others.

38

Summary

The basic requirement for test equipment is a multi-meter. Whether it is analogue or digital is not

important but whatever meter you get it is important that you learn how to use it. Read the

instruction manual if there is one, ask on forums etc. and get used to operating it. Try it out by

measuring known voltages e.g. a fresh battery, known resistors etc.

This applies to any other test equipment you get. As I said in the 2012 talk you do not have to get a

whole range of test equipment at once. I’ve spent over 40 years gathering my test gear collection,

some new, some secondhand, some working and other pieces needing repair. If you can get user or

service manuals with the equipment that is a bonus, however manuals can usually be obtained from

the internet or from other users who are willing to copy their manuals.

Whatever test equipment you use you need to be able to interpret the results of any test you

conduct. This is hard at first but with experience you’ll soon learn to determine whether the reading

is acceptable or needs further investigation.

Whenever you conduct a test you need to think, what result am I expecting, what level of voltage or

what type of signal would I expect to at the test point? Based on the results of an individual test you

should then be able to work out what further tests are required.

Learn how to read a circuit diagram and identify the various stages and try to work out what each

component is for. Manufacturers do not usually put in components unnecessarily as it costs them

money!

Learn to identify components such as IF transformers, audio transformers etc.

Learn to recognise common valve and transistors and what their typical function is e.g. ECH81 is a

frequency changer in a valve set and an OC44 performs the same function in a transistor set.

With practice it will become easier to recognise the various stages and their functions in a radio or a

TV.

Practice soldering and removing components on both tag strips and PCBs

Don’t start on complex or rare set. Get a cheap common set to practice on and work up the more

complex sets.

Don’t switch on a set “to see if it works”. More damage can be done to expensive parts such as the

output transformer if the coupling capacitor to the output valve (that Cap) is leaky.

39

APPENDIX

Fault Finding Articles

Practical Wireless and Practical Television, later Television, magazines have often featured articles

on fault finding both for beginners and for more experienced people.

Television in particular was dedicated to the TV trade and featured articles on servicing of specific

models, general servicing and developments in TV technology. These latter articles are well worth

searching out as they can give a lot of background information on how developments such as

Teletext, Nicam etc work. Many members of the forums, including myself, have written articles for

Television.

One prolific author was the much respected Les Lawry-Johns who not only wrote on servicing of

specific models but wrote a series in the late 70s for beginners under the name of S. Simon (S for

Simple!!).

The following is a list of useful articles and books for fault finding. It is not exhaustive but if you can

find them they will give an excellent introduction for beginners.

1) Repairing Radio Sets series 1. H. W. Hellyer & Gordon J. King. Practical Wireless April 1967

to September 1967*.

2) Repairing Radio Sets series 2. H. W. Hellyer & Gordon J. King. Practical Wireless April 1968

to September 1968.

3) Repairing Radio Sets series 3. H. W. Hellyer & Gordon J. King. Practical Wireless January

1969 to March 1969.

4) TV Servicing. Beginners Start Here. S. Simon. Television October 1977 to August 1980.

*This series was accompanied by a record of typical symptoms.

Books

Gordon J. King wrote many books on Radio and TV servicing. These are well worth hunting

out.

Mazda Book of PAL Receiver Servicing.

Websites

This is Paul Stennings website. There are detailed pages on the operation of radios plus a lot of

useful information on fault finding, repair and restoration.

http://www.vintage-radio.com/repair-restore-information/index.html

Service Data

Paul also has vintage service data available either as a DVD with all the service data or for individual

download.

http://www.service-data.com/

40

BVWS

Service data, in the form of the Trader sheets covering Radios and TVs, is available as part of the

membership pack when joining the British Vintage Wireless Society.

The BVWS also sells suitable replacement capacitors. These are available to members by post or at

the various meets they organise.

Vintage Forums

There are a number of forums for the vintage radio and TV enthusiast. These are probably the most

popular in the UK and are a source of useful information and advice on repairing and restoring

vintage equipment.

http://www.vintage-radio.com/

http://golbornevintageradio.co.uk/forum/

http://www.forum.radios-tv.co.uk/

Resistor colour code

Resistor and capacitor

colour coding

(Mullard/Philips data)

41

Valve Numbering Systems and Base Details.

Details of valve numbering systems can be found by following this link.

http://www.vintage-radio.com/repair-restore-information/valve_valve-numbering.html

Details of valve bases can be found at the following website.

http://www.radiomuseum.org/forumdata/upload/uk_valve_bases_pin_numbering.pdf

Dealing With an Open Circuit On/Off Switch.

I cannot guarantee that this technique will work every time but I have used it successfully a number

of times when I’ve found an open circuit on/off switch.

One problem with the mains on/off switch

on a radio or TV which hasn’t been used

for some time is that the contact surfaces

become contaminated leading to a high

contact resistance. Usually switch cleaner

would be used to clean the contacts but

this doesn’t always work and often access

to the contacts is difficult. This technique

uses a spark to break down this surface

contamination.

Connect the switch as shown in the circuit

shown left. The 10k resistor is to limit the

current. The inductor can be the primary

winding of a mains transformer.

Connect lead with an insulated crocodile clip across the switch and set the switch to the on position.

Turn on the 250V supply and after a few seconds carefully disconnect the insulated crocodile clip.

The circuit acts like a car ignition system and the high voltage from the back EMF from the coil

creates a spark across the faulty switch. If you are lucky this will break down the contamination on

the switch contacts. Switch off the 250V supply and disconnect the switch. Check the resistance

across the switch. If you are lucky this will have dropped from a few hundred ohms to a fraction of

an ohm. Operate the switch and check it is open circuit in the off position. Operate the switch a few

times and check it is operating correctly.

10k 7W

Mains transformer

Primary winding

Faulty switch in

On position

+ve

250V

DC

-ve

Insulated clip

connected

across the

switch

Take care with this circuit as high voltages are

involved