A TRANSISTOR CAR RADIO WITH PUSH-BUTTON TUNING Bound... · A TRANSISTOR CAR RADIO WITH PUSH-BUTTON...

1962/63, No. 1

A TRANSISTOR CAR RADIO WITH PUSH-BUTTON TUNING

by D. PASMA *) and G. SPAKMAN *).

621.396.62:621.382.3:629.113

Two important, characteristics desirable in car radios are small dimensions and euse ofoperauon: As regards the first point, the transistor made a consider able advance possible. Thisarticle describes one of the most recently developed sets (type N5X04T), which is entirely equippedwith tronsistors. The receiver is tuned to the principal stations by push-buuons ; the mecha-nism employed and the measures taken to ensure accurate tuning are discussed at some length.

Differences between car radios and other receivers

Certain characteristics are required of car radioswhich are not so rigorously imposed, if at all, onportable sets or domestic receivers. We shall beginwith a brief review of these characteristics.The first point to be considered is the mechanical

construction. Although a car radio should be smallenough for it to be mounted in or under the dash-board of a normal car, it should be sufficiently sturdyto withstand the shocks and vibrations to which itis constantly subjected in a moving vehicle.

To screen the set against electrical interferencefrom the engine, it must be completely enclosed ina metal housing, and this in its turn calls for specialmeasures to ensure adequate heat removal. Further-

*) Radio, Television and Record-playing Apparatus Division,Philips, Eindhoven.

more the construction should make it possible tocarry out repairs quickly and easily. With the latterpoint in mind, the mounting of the set in the carshould be a simple operation.

Electrically, too, special demands are made oncar radios. The power has to be supplied by the carbattery, which may have a voltage of 6, 12 orperhaps 24 V, and which in some cases may beearthed at the positive pole and in other cases atthe negative pole. The set should be adaptable toeach of these situations. To limit the extra drainon the car battery, the current consumption of thereceiver should be as low as possible.

As regards circuitry, the demands made on acar radio do not differ much from those imposed ona good domestic receiver. A high sensitivity isrequired from both types of set. In the fairly small

27

28 PHILIPS TECHNICAL REVIEW VOLUME 24

aerials used on cars, the signal strength is in manycases not much more than 10 !LV,and reasonablereception at such a low aerial voltage should stillbe possible. For example, at a modulation depthof 30%, a power of at least 0.5 W should beavailable at the output. This fairly high power isnecessary in a car in view of the fairly noisy con-ditions usually present. For this reason, too, themaximum output should be fairly high; at the pres-ent time it is usual to be able to supply 5 or 6 Wto the loudspeaker without appreciable distortion.

Since considerable variations in signal strengthoccur when the car is in motion, a very effectiveautomatic gain control is necessary. In this respect,too, the demands are no lower than those made ona high-quality domestic receiver.Some of the above-mentioned requirements also

apply to other transportable receivers. Small size,for example, is always important. Since in mostcases, however, a smaller output is sufficient, thepower taken from the source and hence the heatgenerated are considerably lower. This makes iteasier to solve the problems of heat dissipationarising from the reduced dimensions.

In some respects the demands made on the receiver part ofmobile VHF equipment resemble those applicable to a carradio. Such equipment does not, however, like the car radio,have a continuously variable tuning frequency, but operateson various fixed frequency bands. This considerably simplifiescertain problems of circuitry. On the other hand, a mobileVHF unit, as professional equipment, has to meet even morerigorous demands in mechanical respects than a car radio.

Development of the car radio

The building of car radios was started as long agoas in the thirties. These sets were designed forreception in the long and medium wavebands. Thefirst receivers were "straight sets", i.e, they hadno frequency changer, but a switch was very soonmade to superheterodyne receivers.

The valves and other components then availablewere rather bulky, and the early sets, with built-inloudspeaker, had a volume of 8.5 litres. Sets of thissize necessarily had to be fitted outside the reachof the driver; usually they were fitted to the bulk-head (fire wall) between engine space and car in-terior. The driver operated the set by Bowdencables from a control box 1). With the developmentof smaller valves and components, the size of subse-quent series of car radios was gradually reduced, atrend which was helped by not building-in the loud-

1) J. W. Alexander, A car radio, Philips tech. Rev. 3,112-118, 1938.

speaker. Nevertheless, the sets were still too big tobe mounted near the driver, and the use of Bowdencables stillremained necessary. Because of the draw-backs attached to these cables (stiff running,backlash), efforts continued to be made to reducethe dimensions sufficiently to enable the set to bemounted in the dashboard. These efforts reachedfruition in 1945, with the aid of a new range ofsmall valves 2). .

The new set (type NX570V) consisted of twoparts. One part, which could be mounted in thedashboard, contained the receiver proper; the powerpack and the loudspeaker were contained in a sep-arate box which could be fitted to a suitable pointon the bulkhead. The two parts were connected by amulticore screened cable.

The volumes of the two parts were 1.6 Iand 4.7 Irespectively, giving a total of 6.3 litres. In thisrespect, then, not much progress had yet been made.An important advance, however, was that the man-ual controls were now on the receiver itself, thusdispensing with the need for Bowden cables.

A measure that contributed a great deal to thefurther reduction of volume was the change fromtuning by means of ganged variable capacitors topermeability tuning, using inductors in which theposition of the ferrite core can be varied. Thisconsiderably reduces the size of the tuned circuits.A further advantage of permeability-tuned induc-tors is that they are much less sensitive than capa-citors, with their large vanes, to mechanical vibra-tions, and thus cause less microphony. Yet anotheradvantage of these inductors is that, given a welldesigned mechanical system, they are much morereadily adaptable to push-button tuning. We shallreturn to this point presently. Finally, it is worthmentioning that using a capacitive aerial and atuning circuit having a constant capacitance and avariable inductance, the voltage multiplication Q isvirtually independent of the tuning frequency.(This means that the sensitivity of the receiver ismuch less dependent on the tuning frequency.)

In the earlier sets the power pack accounted for asubstantial part of the total volume. To obtainthe anode voltage for the valves from the car battery,use was made of a vibrator. This converted thebattery voltage into an alternating voltage, whichwas stepped up and rectified to produce the requiredDC supply of about 220 V 3). Rectification was

2) G. Alma and F. Prakke, A new series of small radiovalves, Philips tech. Rev. r., 289-295, 1946.

3) J. Kuperus, On the construction of vibrators for radiosets, Philips tech Rev. 6, 342-346,1941, and also the articlein reference 1).

1962/63, No. 1 TRANSISTOR CAR RADIO 29

effected either' by using a separate contact on thevibrator or a rectifying valve. Since the unit alsocontained a transformer, together with smoothingcapacitors and choke, it always took up a fair amountof space. The ohvious line of development was to-wards a receiver in which the battery voltage couldbe used directly for feeding the valves. It waspossible to make valves that could operate on ananode voltage of about 6 V, with the exception,however, of the output valve; no valve could delivera power of 2 to 5 W on such a low voltage as this.For this reason the construction of such a receiveronly became possible after the advent of the tran-sistor. One of the first transistors capable of deliv-ering a sufficiently high power (type OC 16) wasused in the output stage of a car radio, the otherstages of which were equipped with valves for low-voltage operation. The ouput stage containedone OC 16 transistor, or two of them in push-pull.In these sets, called "hybrid receivers", or VT(valve-transistor) receivers, which were first soldin 1957, a vibrator was thus no longer needed.Since this made it possible at the same time to

dispense with the transformer and smoothing com-ponents (choke and capacitors) a considerable re-duetion in volume was achieved. The radio-fre-quency and intermediate-frequency circuits werecontained in a housing of 1.3 Ivolume, which couldbe mounted in the dashboard, whilst the audio-frequency section was mounted, without the loud-speaker, in a separate housing of only 0.9 1. Thevolume was thus 2.2 1. Owing to various sizes ofloudspeaker being used, the total volume varied3 and 4.2 1. This considerable size reduction waspossible despite the fact that the set could deliver6 W, while only 2 W could be obtained from theearlier car radios.Although substantial space saving was achieved

by eliminating the vibrator, transformer, rectifierand smoothing components, the gain was to someextent offset by the need to introduce extrasuppressor filters into the supply lines. The likeli-hood of ignition interference was considerablygreater now that the valves were directly connectedto the battery.An intractable problem in the development of VT

sets was the design of an effective automatic gaincontrol. The main difficulty was that at the lowanode voltage of 6 V, even a very low negative gridbias reduced the anode current of the valves tozero. As a result, severe distortion occurred whenthe sets came into the proximity of strong trans-mitters. Although this trouble could be avoided bycareful design, the normal spread in the data of the

valves and other components made it impossibleto guarantee the absence of this undesirahle effectin all sets.

The difficulty descrihed is much less serious atan anode voltage of 12 V. In countries where thegreat majority of cars are equipped with 12 Vbatteries, these VT sets are therefore still beingmade.

The mechanical and electrical construction of thefirst VT sets was identical with that of other receiv-ers; "the components, for example, were intercon-nected in the conventional way by copper wire.1;0.such very compact units there was a considerablerisk of short-circuits, and in fact these occurredrepeatedly after installation in the vehicle. Thisdifficulty was overcome by the use of printed wiringin car radio receivers, which not only substantiallyreduced the risk of wiring faults, but also allowed afurther reduction of dimensions.

When transistors were sufficiently far advancedto enable them to he used for radio-frequency andintermediate-frequency amplification, it becamepossible to build receivers which operated entirelyon transistors. All-transistor car radios are nowalso being made, which are both smaller and con-sume less current. The entire set (excluding theloudspeaker) can now be contained in a housing of1.7 litres, the dimensions of which allow it to bemounted in the dashboard. A receiver of this kind(type No. N5X04T) will be described in this article.

The circuit

The set is designed for reception in the long-waveand medium-wave bands. It is equipped with tentransistors and three germanium diodes, the variousfunctions of which are illustrated in figs 1 and 2.These figures show the basic circuits, omitting non-essential parts. We shall consider some particularsof these circuits.

Fig. 1 represents the radio-frequency, inter-mediate-frequency and detector sections. I is theradio-frequency stage, which uses a transistor (TrI)ofthe type OC170. K; and K2 are the two RF tuningcircuits, using permeability-tuned inductors. Thewayin which the RF tuning circuits and theoscillatorcircuit are switched over from the long-wave to themedium-wave band is not shown in this diagram.The aerial is connected to a capacitative tapping ofKl. The aerial lead incorporates a choke, LI' forsuppressing ignition interference from the engine.To enable the set to be connected to aerials of dif-ferent sizes, a variable capacitor Cl is included inthe screened aerial Iead (see also fig. Sb).

The RF stage I is followed by the self-oscillating

30 PHILlPS TECHNICAL REVIEW VOLUME 24

Fig. 1. Simplified diagram of the radio-frequency, intermediate-frequency and detector circuits of the N5X04T car radio.Among the parts omitted are the circuit elements requiredfor switching from themedium-wave to the long-wave band. Thesymbols +, - and ± indicate that the relevant points areconnected to the positive or negative supply cable or to avoltage divider between these cables. The various parts of thediagram that may be regarded more or less as separate unitsare surrounded by dot-dash rectangles. I RF stage. II mixer

mixer 11, which has a transistor (Tr2) of the typeOC 44. K3 is the oscillator circuit, which is tuned,like Kl and 1(2' by varying the position of a ferrox-cube core in the inductors. A tuned transformer,consisting of the tuned circuits 1(4 and K5, forms thefirst intermediate-frequency band filter.The mixer is followed by two intermediate-fre-

quency stages III andI V, containing transistorsTr3 and Tr4 of the typeOC 45. The second andthird IF band filters areformed respectively bycircuits K6,1(7 and K8,Ko.The inductors of thetwo' primary circuits K6and Ka are provided witha few extra turns (shownat the bottom ofthe coils)which are connected viacapacitors to the inputend of the relevant tran-sistors. This largely elimi-nates the feedback in thetransistors. (This techni-que is known as neutral-ization.)

Gro-,-,-,_, '-ffj

ii.rjLFI

·i?tlii

D2i+ i

i_,-=,_,_~_j

7.891

stage. III and IV IF stages. V detector stage. VI, VII andVIII parts of circuit involved in automatic gain control.Cl is a variable capacitor incorporated in the aerial lead (seealso fig. 5b). The maximum capacitance variation is 60 pF,which makes the set suitable for connection to aerials whosecapacitance (including that of the screened aerial cable) isbetween 45 and 105 pF. An "Automignon" record player canbe connected to the terminals marked G. In that case the con-nection in the receiver between two points has to he broken.

The detector stage V is a normal diode detectorusing a germanium diode Dl of the type OA 79.The potentiometer RI is the volume control, andpotentiometer R2 with capacitor C2 form the tonecontrol.

The parts of the circuit denoted by VI, VIIand VIII form the automatic gain control.

f'iïi-'-'-'-':-'-iI .. I

iPI

I

72 !I

ff~LSii

. !L._._._._._._._ ...J7892

L._._._._._._._.J

Fig. 2. Simplified diagram of the audio-frequency section. IX emitter-follower stage.X and XI audio-frequency amplifier stages. XII output stage. LS loudspeaker.P negative-feedback network. SI speech/music switch. If the set is operated froma 6 V battery, optimum matching between the transistors Tr9, Trio and theloudspeaker is obtained by using only part of the primary of transformer T2• For thispurpose switches S2 and Sa are used (shown here in the position for 12 V). If the set isconnected to a 12 V battery, switch S4 is used to introduce a resistor into the commonemitter lead of Tr9 and Trio; this resistor is short-circuited in the case of a 6 V supply.


The automatic gain control works as follows. The signalvoltage on the primary of the last intermediate-frequencytransformer, Ks, is rectified by the diode D2 (type OA 79)and the rectified signal is fed via a filter to the base of the firstIF transistor Tra. Consequently, if the signal strength increasesthe emitter-collector current and the gain of Tra decrease.The change in the voltage across the emitter resistance Raof Tra now controls, through the intermediary of transistorTr6, the gain of the radio-frequency transistor TrI" To allowthe reception ofvery strong signals (e.g. 1V or higher) withoutoverloading the IF amplifier, a diode Dl (also of type OA 79)is connected between the emitter of TrI and the top of theresistor R4 in the collector lead of TrI. During the receptionof small signals the emitter of TrI is at a negative potentialwith respect to the top of R4. As a result Dl is cut off. Largesignals, however, make this diode conduct, chiefly becauseof the drop in the voltage across R4. Between the base and theemitter of TrI there now appears a positive voltage of ahout I V,cutting off this transistor completely and allowing the verystrong aerial signal to reach the mixer transistor Tr2 only viainternal capacitances, thus undergoing a strong attenuation.

Fig. 2 shows a simplified circuit diagram of theaudio-frequency section. The first audio-frequencystage, IX, consists of a transistor Tr6 of the typeOC 75, in the common-collector (emitter-follower)configuration. This stage provides no voltage am-plification; its presence in the circuit is to enable acrystal pick-up (of an "Automignon" record-player)to be connected to the terminals G (see fig. I).As a crystal pick-up has a high internal resistanceand thus calls for a large load resistance, it wasnecessary to give the first audio-frequency stage alarge input resistance. One means to this end is touse a transistor in the common-collector connection,the input resistance of which is much higher thanthat of transistors in other arrangements. A conse-quence of this high input resistance is that thedetector stage V also has a high input resistance.This made it possible to connect the detector inparallel with the last IF circuit Kg, and no tap onthe coil of Kg was necessary for this connection.

Stages X and XI are audio-frequency amplifiers.The coupling between IX and X is effected by thetwo capacitors C3 and C4 in series. The capacitorwith the lower capacitance, C4, can be by-passed by the switch SI. When SI is open the lownotes are not so strongly reproduced, which is anadvantage for the reception of speech (speech/music switch). Transistors Tr7 and Trs are typesOC 71 and OC 79 respectively. The latter transistoris coupled via a transformer Tl to the push-pulloutput stage XII. This contains two transistors oftype OC 26 (Tr9 and TrIO). The loudspeaker LSis connected across the output transformer T2• Ifrequired, two loudspeakers can be connected inparallel. To ensure proper matching in that case,

a tap is provided on the secondary of T2• Negativefeedback is provided between stage XI and stageXII via the network P in the figure. The outputstage can deliver a power of 6 W, at which thedistortion is 10%.The diagram infig. 3 again simplified, represents

the power supply circuit. The battery is connectedto the terminals a and b. The chokes L2 and L3'together with capacitors C5 and C6, form the inter-ference-suppressor filters. The on/off switch S5is combined in the conventional way with thevolume control RI (see fig. I).To adapt the set for operation on a 6 V or 12 V

battery, according to requirements, various con-nections have to be changed. The switches used forthe purpose are S2' Sa, S4' S7 and Ss in figs 2 and 3;

r'-'-'-'-'-'-'-'-'-'---'-'-'-'-'-'-'-'-:C~~---------?--~---,

SL

+e

Fig. 3. Simplified circuit diagram of the power pack. S6 on/offswitch. The switches marked S7 and Ss serve for adapting thereceiver to the car battery voltage (6 V or 12 V). Thebattery is connected between points a and b. The supplyvoltage for the output stage XII is obtained from the pointmarked e, use thus always being made of the entire voltageavailable (6 or 12 V). All other stages, however,are fed frompoint d. At both battery voltages the supply voltage in theset is kept at 6 V by opening or closing switch S7 as the casemay be. The dial illumination bulb SL is connected directlyto the 6-Vbattery (Ss closed). If the battery voltage is 12 V,a resistance is connected in series with the lamp by openingSs· The double-throw switch S6 enables the set to be operatedirrespective of whether the positive or negative battery ter-minal is connected to earth: turning the switch to one or theother position ensures that points c, d and e always have theindicated polarity.

789)

further particulars are given in the captions. Thedouble-throw switch S6 in fig. 3 enables the set tobe used in cars in which the positive pole of thebattery is connected to earth as well as in thosewhere the negative pole is earthed.

In the actual receiver it is a particularly simplematter to switch from one supply voltage to another.The leads to the switches S2' S3' S4' S7 and Ssterminate at contact points which are all disposedon a small panel. A multipole plug effects thenecessary connections. The contacts are so arrangedthat, in order to change over from a 6 V to a 12 Vsupply, or vice versa, the plug simply has to bereversed. The polarity switch S6 also takes theform of a reversible plug. Both plugs can be seenin fig. 6.

r----------------------------------------------------------------- - - -----

32 PHILIPS TECHNICAL REVIEW VOLUME 24

ConstrnctionThe problems involved in the constrnction of a

car radio are not only concerned with the efforts toreduce dimensions, mentioned at the ontset. Smalldimensions are also important in portable receivers,but in their case the constrnction is simplified in asmuch as the manual controls and the tuning dialcan be disposed if necessary on different sides ofthe set. Since a car radio has to be monnted in thedashboard, the controls and dial all have to be onthe same side, and indeed on one of the smallestsides in view of the relatively small space availableon the dashboard. An added difficulty is. that itshould be possible to operate the set wearing fairlythick gloves, and therefore the controls should notbe too small. The receiver type N5X04T measures180 X 174X 54 mm. The space thus available for atuning dial and the controls was 54X 180 mm.This side also had to accommodate the means ofsecuring the set in the dashboard, to which we shallreturn presently.

The receiver can be tuned by means of fivepush-buttons, which select three stations in thcmedium-wave band and two in the long-wave band.A control knob is provided for tuning to otherstations in the normal way. The facility for tuningto a station by pressing a push-button is of consid-erable importance in a car radio, since it meansthat the driver's attention is not so distracted aswhen tuning by ear with a knob. If he knows thesequence of the five stations to which the push-buttons are preset, he can operate the set by touchalone.

The construction of the set was substantiallyinfluenced by the choice of push-button tuning.We have already mentioned that permeability-tuned inductors are preferable for this purpose tovariable capacitors. The rectilinear movement of the'inductor cores in the tuning process, and the smallmass of the moving parts, make it easier in this.case to design a simple and accurately functioningtuning system than if conventional variable capac-itors were to be used.In a receiver without push-button tuning, the RF

tuning section is not tied to a particular part of thehousing. The mechanical coupling between the tuningcontrol, the variable inductors and the dial cursor canalmost invariably be satisfactorily effected by a sys-tem of cords or other coupling elements, particularlysincethere need be no fixed relation between the tunedfrequency and the position of the tuning control:the station is after all tuned in byear. In the caseof push-button tuning, however, such a mechanismis unsuitable owing to the considerable precisionrequired to adjust the tuning device by means ofeach push-button. The motion of the push-buttonsmust be transmitted as directly as possible to theferrite cores. The inductors should therefore bebrought forward as much as practicable, roughly inthe middle of the set, so that they form a singleassembly together with the push-buttons. To achievethe desired compactness the remainder of thecircuit should be grouped around this assembly soas to waste as little space as possible. Fig. 4. illus-trates how this is done in the receiver under dis-cussion. F is the front plate of the set (seen from

Fig. 4. Sketch showing the lay-out of the principal parts of the N5X04·T car radio. F frontplate. U dial. Al to As printed-wiring panels. B cooling plate. H chassis plate. L inductors.Tro one of the two output transistors. RI volume control. R2 tone control.

1962/63, No. 1 TRANSISTOR CAR RADIO

7900

a

7901

b

Fig. 5. The receiver with the housing removed.a) Right way up. Panel A4 is raised to allow access to the components beneath it.b) Upside down. In the foreground can be seen the variable capacitor Cl in the aeriallead (see fig. I), enabling the set to he matched to aerials of different capacitance. Theletters denoting various components correspond to those used in figs 1 to 4.

33


the rear), on the outside of which is the dial U.Immediately behind are the inductors L of theradio-frequency and oscillator circuits. The fiveplates marked Al to A5 are printed-wiringpanels 4). On these panels are mounted mostof the smaller components, such as resistors andcapacitors. Panel Al carries most of the corn-ponents of the radio-frequency section, panels A2

and A3 carry the intermediate-frequency anddetector components, and A4 and A5 carry the audio-frequency circuits, 'with the exception of some heavycomponents like the transformers Tl and T2 (seefig. 2). The latter are mounted directly on the metalchassis.

Although much less heat is generated in transis-tor sets than in receivers fitted with valves, in thiscase it was nevertheless necessary to pay attentionto cooling. The most heat is generated in the twooutput transistors, Tr 9 and TrIO; these are thereforefixed to the aluminium plate, B, which forms the A,----' __ ....jj-.........c:back cover of the set. This plate is provided withcooling ribs and the transistors are mounted on theoutside, so that they are in fact outside the actualset. (In fig. 4 only one of the output transistors isvisible; see also fig. 5.)

Opposite panel ..11'3 are the volume control RIand the tone control R2 (see also fig. I), which areoperated with the aid of two concentric shafts, andthus take up little space on the front plate. Theon/off switch (55' see fig. 3) is as usual coupled to RI'Mounted on the other side of the front plate is themechanism (not visible in fig. 4) for tuning to sta-tions that cannot be selected with the push-buttons,and also the speech/music switch (51 in fig. 2).The operating shafts for the tuning control and 51are also concentric. The part of the set to the leftof the compartment marked H in fig. 4 containsthe power supply components (fig. 3).

The connection points for the printed-wiringpanels are mostly arranged around the periphery,enabling each panel to be easily disconnected andreplaced if necessary. Panel A4 is connected bymeans of flexible wires, and is held in place betweenlugs. It can thus be pulled up, as shown in fig. Sa,without breaking any of the connections, thusallowing easy access to the components beneath it.Various other measures have been taken to en-

sure ready access to the majority of components.The chassis is so designed that after disconnectingonly one lead and removing two screws, the rearsection can be turned through 90° with respect to

4) R. van Beek and W. \V. Boelens, Printed wiring in radiosets, Philips tech. Rev. 20, 113-121, 1958/59.

the front section. It can be fixed in that positionwith two screws.

Fig. 6 shows a receiver opened up in this way,which is not only useful for making repairs but isalso used in assembly.

Fig. 6. For assembly or repair work, the rear portion of thereceiver can be turned through 90° with respect to the frontpart. Cv and Cp are reversible plugs for adapting the set tobatteries of different voltage (6 V or 12 V) and different po-larity. Ai' A2' A3 are printed-wiring panels.

The tuning mechanism

We shall now consider in more detail the mecha-nism of tuning by means of the five push-buttons.Each push-button ensures that the ferrite cores inthe inductors of both RF circuits and in the os-cillator circuit are moved into pre-set positions.The mechanism employed is shown in a very sim-plified form in fig. 7. The cradle N pivots aroundthe line AA' and is coupled to the bar 11, Themovement of this bar is transmitted by rods Tto the ferrite cores in the inductors L. Only oneof the five push-buttons, D, is represented in thedrawing. Mounted on the push rod E of this buttonis a semi-circular segment S. When the button isdepressed, the cradle N takes up the position cor-responding to the position of the segment, andthis in turn brings the bar 11 into the positioncorresponding to the tuning of the receiver to a


Fig. 7. Simplified sketch of tuning mechanism. D push-button. E push rod. S segment.N cradle which turns about the line AA'. W1worm. W2 wormwheel. K tuning knob.J1, J2 and Ja moving bars. T tie-rods. L inductors. I indicator block.

particular station. Since the segments S of thefive push-buttons all have different positions, theoperation of each button tunes the set to one offive different stations. When none of the buttonsare depressed, the cradle can be moved by meansof the control knob K, which is coupled to the shaftof the cradle N by a gear system consisting of aworm W1, a wormwheel W2 and a friction clutch(not shown).Each segment S in its normal position is locked

to its push rod E so that, whenever a push-buttonis depressed, the cradle N takes up the same posi-tion. The position of S, however, can be alteredvery simply. A mechanism, not drawn in fig. 7,loosens segment S when the push-button is pulledoutwards. By now tuning with K to a certainstation and then pushing D in again, Stakesup the position corresponding to that of N.When the push-button is next depressed, S is againlocked to E. In this way, then, a station oncetuned in with the tuning control K can repeatedlybe obtained by depressing one of the push-buttons.The push-buttons also operate the waveband

switch. The sliding contacts of this switch are mountedon a strip of insulating material which is connectedto the sliding bar J2 shown in fig. 7. The latteris provided with notches, one of which (Z) can beseen in the figure. Whenever a button is depressed,the chamfered end of the relevant rod Epushesagainst the bevelled edge of one of these notches,causing J2 to take up one of the two extreme posi-

,\II

If

tions, and thus setting the waveband switch to oneof the two wavebands. The notches in J2 are soarranged that the long-wave band is switched inwith two ofthe push-buttons and the medium-waveband with the three others.Another sliding bar Ja is moved at the same time

as J2; this serves to disconnect the friction clutchbetween the wormwheel and the cradle shaft whenone of the push-buttons is depressed. (We shallreturn to this point later.)

The use of a sliding switch instead of the conven-tional rotary type made it possible to place the con-tacts immediately under the inductors, thus givingshort connections and a very convenient layout.

Directly behind each push-button there is a tiltingblock 1 marked with an arrow. When a push-buttonis depressed, the block is tipped into a position inwhich the arrow becomes visible in front of anopening in a screen (not shown in the drawing).When the button is released, the block remainslocked in that position. When another push-buttonis depressed the block is unlocked and springs backinto its original position, an arrow then appearingabove the other push-button. In this way it canalways be seen which button was last depressed,allowing rapid identification of the station selected.When the receiver is tuned either by push-button

or tuning control, the station indicator must movealong the dial. Its movement must be perpendicularto that of the sliding bar J1•The most obvious methodof coupling the tuning mechanism to the cursor


is to use a cord passing over pulleys. A corddrive system, however, could not be used herebecause the friction involved would detract fromthe high precision requiredofthe tuning mechanism.The simple linkage mechanism adopted, and shownin fig. 8, caused much less friction.

The link HI pivots about a fixedpoint 1 and carriesat end 3 a spindle which rests in a slot in the bar Jl(see fig. 7). The end 2 is hinged to a strip H2' whichcan slide in a slot in a fixed plate Q (The spring 4keeps H2 pressed against one side of this slot.)The other end of H2 carries the cursor Ha, which iseasily detachable. U denotes the position of thedial. It can easily he shown that the pivoting ofHI (caused by the spindle 3 following the movementofJl) results in a practically linear movement of Ha;with the dimensions of the components used in thisset, the maximum deviation from a straight line isonly 0.3 mm. The cursor thus travels straight alongthe dial with sufficient accuracy.

Tuning precision

As mentioned in the foregoing, a very high degreeof precision is required of the tuning mechanism.Some figures will illustrate this. In the inductorsused in the N5X04T set the total variation of theself-inductance is achieved by displacing the ferritecore 15 mm. This is sufficient to cover a frequency

o

Fig. 8. Sketch of dial cursor mechanism. HI link with fixedpivoting point 1; the link carries at 3 a spindle which slides in aslot in Jl (see fig. 7). H2 strip connected with HI at point 2.Q chassis plate. HI cursor. U dial. 4 spring.

illustrated in fig. 9. The push rod, which was shownin fig. 7 for simplicity as a single rod, consists inreality of two parts capable of moving relative toone another, and denoted by El and E2 in fig. 9.The part El' which carries the push-button D, can

Fig. 9. Cross-section of push-button. El and E2 two parts of the push rod; El slides insidethe sleeve 1 of E2• 2 and 3 leaf springs. S segment. 4 steel ball. 5 screw. D push-button.

_£,_._.

range of 1100 kcts in the medium waveband. Ifwe now specify that, when one of the push-buttonsis depressed, the set should be tuned to the requiredstation with a deviation of no more than 0.5 kc/s,then the average precision with which the coresmust be displaced is (0.5/1100) X 15 mm ~ 7 (J.m.This maximum permissible deviation is the sum ofthe deviations that may be due to misalignment ofthe segments S (see fig. 7) and the play in, and de-formation of, the drive system. The componentsof this system must therefore be light and veryrigid, and springs must be used to ensure that anyplay in the connections has no effect on the adjust-ment of the cores. Once the segment S has beenfixed in position, any movement in relation to thepush rod E must be prevented by very rigid clam-ping. The construction used for this purpose is

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slide in a bushing 1 which is attached to E2• Thesegment S is pivoted on the screw 5. Fig. 9represents the situation in which S can movefreely. When El is now moved to the right in rela~tion to E2' a leaf spring 2 presses a hardened steelball 4 to the right and causes a leaf spring 3 toclamp the edge of the segment S. Very considerableleverage is exerted by 3 as a result of its specialprofile. The force with which S is clamped is roughly40 kg, which is sufficient to prevent any slip in thesegment, even in frequent use.

To prevent displacement of the segment S inthe pivoting point, the hole through which the screw5 protrudes is not round but slightly V-shaped(see fig. la). This ensures t~at there is no play atthis position when the segment S is pressed againstthe cradle N (see fig. 7).

1962/63, No. 1

= 26".

TRANSISTOR CAR RADIO 37

Fig. 10. Segment used inpushbutton mechanism (de-noted by S in figs. 7 and9). The centralopening isslightly V-shaped to reduceplay.

To minimize the chance of cradle rebound, amechanism is used by means of which, whenever apush-button is depressed, the friction clutch betweenthe wormwheel and the cradle shaft is put out ofoperation. For ease of illustration, this mechanismwas omitted in fig. 7, but is drawn separately infig· 11, together with the friction clutch in cross-section. The wormwheel W2 can be turned onthe shaft A' of the cradle N; fixed to the sameshaft is a diaphragm M of beryllium copper, whichis pressed into a tapered recess in the wormwheelby a retainer 1. The pressure is applied by the spring4 via a lever 3 and a screw 2. When one of the push-buttons is depressed, the sliding bar J3 moves tothe right (see fig. 7), as a result of which the lever 3also moves to the right against the action of the

The precision with which the cradle returns tothe same position when one of the push-buttons isdepressed depends, among other things, on thetorque required to move this component with thebar J1, the tuning cores and the cursor mech-anism. This torque is determined to a large extentby the friction clutch by means of which the cradleshaft A' is connected to the wormwheel W2 (seefig. 7). Apart from increasing the torque necessaryto turn the cradle when the wormwheel is stationary,the friction clutch also causes the cradle to reboundslightly whenever a push-button is depressed, largelybecause the depression of the button produces slighttorsion in the cradle. There may even be some re-bound in the friction clutch itself. The consequenceis an additional error in the reproducibility of thepush-button tuning.

Operation of the tuning control K mayalso cause.some rebound in the friction clutch, primarily dueto vibrations to which the set is subjected after thetuning.The degree of precision

required in the alignmentof the cradle appears fromthe following figures. Theangle through which thecradle can turn is roughly40°. In the medium-waveband this corresponds toa frequency coverage of1100 kcJs. If we specify0.2 kcJs as the maximumpermissible detuning dueto rebound, then themaximum angle overwhich the cradle may re-bound is (0.2JllOO) X 40°

N

Fig. 11. Cross-section of the mechanism which disconnectsthe friction coupling between the tuning control and the cradlewhen one of the push-buttons is depressed. N cradle with shaftA'. W2 wormwheel, M diaphragm. 1 retainer. 2 screw. 3 lever.4 spring. Ja sliding bar.

spring 4. The pressure of the retainer 1 on the dia-phragm M is then removed, thereby disconnectingthe coupling between the wormwheel W2 and theshaft A'. When the push-button is released, thecoupling is again restored by the spring 4.Another factor influencing the precision of the

tuning mechanism is that, when a push-button isdepressed, the cradle is not only turned, but slightlybent. The bending produced by the force Fd exertedby a push-button is illustrated infig.12, exaggerated

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Fig. 12. Simplified sketch of the cradle N, bent under the forceFd exerted by one of the push-buttons. W1 worm. JJ72 worm-wheel. 1 steel ball. 2 leaf spring. a distance from centre ofwormwheel to centre of ball bearing. f displacement of worm-wheel due to bending of the cradle.


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Fig. 13. The two parts of the metal housing are provided withlugs which are held together by two stainless-steel pins, thusensuring good electrical screening.

for the sake of clarity. The effect disappears whenthe push-button is released. Since the wormwheelW2, which meshes with the stationary worm W1,

also moves upon the bending and rebound of thecradle, the resultant displacement of wormwheeland cradle may cause an impermissible tuning errorif suitable measures are not taken to prevent it.To give some idea of the amount of bending per-missible, it may be mentioned that a displacementf of the wormwheel over a distance of 5 l1.mcan causea detuning of 0.6 kcjs.The wormwheel displacement produced by a

given degree of bending is less the smaller is thedistance a between the centre of the right-hand ballbearing and the centre of the wormwheel. Amongthe measures therefore taken -to reduce this distanceto as little as 3 mm was to mount the right ball-bearing on the outside of the chassis and to providethe left-hand side of the cradle with a ball thrustbearing, in which the ball1 is forced outwards by aleaf spring 2. The cradle was also given such aprofile as to make it very rigid. These and similarmeasures made it possible to reduce the detuning inquestion to a permissible value.

The receiver housing

In designing the receiver housing, two importantfactors had to be taken into account. Firstly, thehousing had to prevent the penetration of electricalinterference from the engine. Secondly, provisionhad to be made for opening and closing the setquickly and easily. These requirements would be in-compatible if one were to build a metal housing,the various parts of which were screwed together,for effective screening in that case would call forthe use of numerous screws to ensure good electricalcontact at many points over the parts of the hous-ing. This, of course, would make it impossible to

open and close the set quickly. A satisfactory solu-tion was found in a housing consisting of two partseach fitted with lugs in which two pins of stainlesssteel can be inserted (see fig. 13). The lugs act ascontact springs, thus ensuring effective electricalscreenrug.The cooling block B (see fig. 4) is slid on to the

bottom part of the housing at the rear, and isclamped between the two parts when the housing isclosed. It is connected to the bottom part of thehousing by only one screw, which in most cases isthe same screw used for securing the set in the car.In such cases, the receiver housing is in fact "screw-less" after removal from the dashboard.

The receiver is mounted in the dashboard bymeans of two heavy threaded bushings, fitted con-centrically with the operating shafts. If the openingin the dashboard does not correspond to thedimensions of the set, a special ornamental plate canbe used. If necessary the set can be additionallysupported at the rear.

Fig. 14 shows a photograph of the completereceiver, and in the title photograph it can be seenmounted in the dashboard of a car. The set can alsohe mounted in the car in other ways; for exampleit can be mounted in a cover suspendcd under thedashboard.

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Fig. 14. The complete car radio receiver N5X04T.

As the loudspeaker is not incorporated in the set,it can be set up in any appropriate part of the car.If required, two louds peakers can be used, connectedin parallel to the tapping available for that purposeon the ontput transformer.

Summary, After briefly considering the requirements to bemet by a car radio, the authors review the development ofthese sets in the last twenty years. A descrip tion follows ofone of the latest car radio receivers, type N5X04T, whichis entirely equipped with semiconductor devices, viz. tentransistors and three germanium diodes. The circnitry andconstruction are discussed, with special emphasis on the tuningmechanism using push-buttons, which calls for very highprecision to ensure reproducible tuning.

1962/63, No. 1 39

ABSTRACTS OF RECENT SCIENTIFIC PUBLICATIONS. 'BY THE STAFF OFN.V. PHILIPS' GLOEILAMPENFABRIEKEN

Reprints of those papers not marked with an asterisk * can be obtained free of chargeupon application to the Philips Research Laboratories, Eindhoven, Netherlands, wherea limited number of reprints are available for distribution.

2954: B. Jansen: A rapid and accurate method formeasuring the thickness of diffused layers insilicon and germanium (Solid-state elec-tronies 2, 14.-17, 1961, No. 1).

Surface-layer thicknesses can be measured verypractically by making use of the brittleness ofsilicon and germanium, thus eliminating cumber-some, precise grinding and polishing procedures.The germanium or silicon slice is broken in a specialmanner producing a rather flat cleavage surface, inwhich any disturbing fracture line is more or lessperpendicular to the very sharp, long edges. Inthis surface the exposed poN junctions are markedas very thin lines by one of the well-known methods(electrolytical, chemical). The distances betweenthese lines or to the edge of the cleavage surface aremeasured under a high-power metallographic mi-croscope with an eyepiece micrometer. Magnificationfactors mostly used are 400 or 600 times. Very thinlayers of about 1 [Lmare still measurable. The methodis used for diffused layers as well as for alloyedcontacts or combinations as in the case of alloydiffusion.

2955: J. Ubbink: Moderne ruisarme versterkers,1.Masers (Ingenieur 73, 01-06,1961, No. 3).(Modern low-noise amplifiers, 1. Masers; inDutch.)

An introduetion to the maser (which givesMicrowave Amplification by Stimulated Emissionof Radiation), a new sort of low-noise amplifier.Three types of masers are discussed. The ammoniamaser is not suitable for use as an amplifier, but itis an excellent frequency standard. Solid-statemasers, which make use of the magnetic propertiesof e.g. ruby (A1203 containing a small percentage ofCr3+ ions), are better amplifiers. The ruby can beused in two ways: in a cavity resonator or in a wave-guide. The first method has several disadvantages;the second is much better, if a system is used inwhich the group velocity of the microwaves is muchsmaller than in free space (a "slow-wave" struc-ture). A practical execution of such a maser isdescribed.

2956: H. Mooijweer: Moderne ruisarme versterkers,Il. Parametrische versterkers (Ingenieur 73,

023-032, 1961, No. 7). (Modern low-noiseamplifiers, Il. Parametrie amplifiers; inDutch.)

Discussion of the principle of operation of para-metric amplifiers. The effective input noise tempera-ture is higher than that of a maser (see 2955), but aparametrie amplifier is simpler to make. Someactual amplifiers and some possible designs aredescribed. The article contains extensive referencesto the literature.

2957: M. J. Sparnaay and J. van Ruler: The ad-sorption of oxygen gas on germanium andsurface conductivity (Physica 27, 153-162,1961, No. 2).

A new method is described for studying electricalsurface properties of semiconductors. The (verysimple) method consists of resistivity measure-ments, at oxygen pressures ranging from 10-9to 10-2 mm Hg, of thin germanium single crystals,the diameter of one sample varying from 1 mm to10-2 mm. The crystals can he given the desiredshape by "burning off" the germanium at 700°C inan oxygen gas pressure of 10-2 mm Hg. By thismethod the roles played by surface conductivityand bulk conductivity can easily be separated.Results are given for intrinsic germanium and arein qualitative agreement with results obtained byHandler et al.

2958: J. L. Meijering: On the thermodynamics ofthe Au-Pt system (Phys. Chem. Solids 18,267-268, 1961, No. 2/3). .

Comment on a publication by Weiss and Tauer,who claimed to be able to explain the asymmetry ofthe phase diagram of the system Au-Pt frommeasurements of the specific heat only. It is shownthat the phase diagram calculated by the methodof Weiss and Tauer in fact differs in two essentialpoints from that found experimentally.

2959: C. Haas: Vibratiespectra van kristallen (Ned.T. Natuurk. 27, 105-118, 1961, No. 4). (Vi-bration spectra of crystals; in Dutch.)

Data about vibrations in crystals obtained bymeasurement of specific heat and thermal conduc-tivity are difficult to interpret. This is because all the


vibrations in the crystal contribute to these quanti-ties. This article discusses two methods wherebymore selective information about the lattice vibra-tion can be obtained: infrared spectroscopy andRaman -spectroscopy.

2960: A. Schmitz: De tunneldiode (Ned. T. Na-tuurk. 27, 133-142, 1961, No. 4). (The tunneldiode; in Dutch.)

Esaki published his article on the tunnel diode inJan. 1958. The most important property of thisdiode is a part of the J-V characteristic with anegative slope. Various workers have studied theproperties and applications of this diode. Thisarticle gives a survey of the present state of know-ledge in this field.

2961: M. J. Sparnaay: Elektrische Doppelschichten(Originallectures IlIrd Int. Congr. Surf. Act.,Cologne, Sept. 1960, Vol. Il, pp. 232-253,Universitätsdruckerei Mainz GmbH). (Elec-trical double layers; in German.)

An electrical double layer is often formed at theboundary between two phases which contain asufficient number of free charge carriers. Thisphenomenon is very important in colloidal systems,but also in semiconductors, as regards both surfaceeffects and poN junctions. In this article a survey isgiven of the theory of these double layers for variouscases; the points of similarity and dissimilaritybetween the different cases are pointed out.

2962: J. Bergsma, J. A. Goedkoop and J. H. N. vanVucht: Neutron diffraction investigation ofsolid solutions AlT~Dn (Acta crystallogr. 14,223-228, 1961, No. 3).

AlT~ absorbs hydrogen readily. This articledescribes an investigation, carried out with the aidof neutron diffraction, of the manner of incorpora-tion of the hydrogen in the AlT~ lattice. In fact,deuterium was used instead of hydrogen, in connec-tion with the demands made by neutron-diffractiontechniques. The investigation was restricted to solidsolutions of composition AITh2Dn. with n = 0, 2,3 and 4. It was found that the hydrogen is taken upin tetrahedral interstices between thorium atoms,just as in thorium which contains no aluminium.If n = 4, all the available tetrahedral interstices arefilled. At lower values of n, i.e. when the filling isincomplete, no order could be found in the distribu-tion of the deuterium atoms over the available sites,even at a temperature of 82 OK. See also Philipstech. Rev. 23, 69, 1961/~2 (No. 3).

2963: P. Massini: Self-absorption correction forisotopes. emitting weak 'beta rays (Science133, 877-878, 1961, No. 3456).

Hit is desired to determine the radiation intensityper mg of a radioactive sample from measurementsof the radiation actually emitted, a correction mustbe applied for the absorption by the sample itself.The correction factor is often given as a function ofthe mass of the sample. There is disagreement in theliterature as to the form of this correction curve forf3-emitters. The author shows that this disagreementmay be due to differences in the geometry of thesample and the measuring set-up.

2964: H. C. Hamaker: Examples of designed ex-periments (Industr. Qual. Control17, No. 9,16-20, 1961).

Textbooks on the application of statistics to ex- -perimental investigations usually give exampleswhere the design of the experiment is taken as given,and the experimental results are successfully pro-cessed with the aid of standard statistical methodssuch as variance analysis. In practice, the situationis often different. This publication describes twocases from industrial practice where the helprendered by the statistician consisted in analysingthe problem and designing a suitable experiment orseries of experiments on the basis of this analysis.The results of these experiments spoke for them-selves: there was no need to subject them to anyform of statistical analysis.

This aspect of the task of a statistician is oftenneglected in textbooks of statistics. See also Philipstech. Rev. 22, 105, 1960/61.

2965: P. C. van der. Willigen: Booglasmethodesvoor staal en de rol die de waterstof daarbijspeelt (Chem. Weekbl. 57, 170-176, 1961,No. 14). (Methods of are welding of steel,and the role of hydrogen in such methods;in Dutch.)

A short survey of the historical development ofarc-welding methods for steel. Welding electrodeswith an "acid" coating are still used on an immensescale. A characteristic of the acid coating is that itproduces much hydrogen during welding. Hydrogenhas come to be regarded as an enemy of good weld-ing, for reasons which are stated in this article.Hydrogen-free welding methods (submerged-arcwelding, use of electrodes with basic coatings,welding with a bare wire in a protective atmosphereof argon or CO2) are therefore gaining more andmore ground. Hydrogen in steel is also one of thecauses of the cracking of enamel.

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