BEWARE OF ADSwmsinc.org/N7EBG/heathkitpdf/N4NAB SB-200 Rebuild.pdfDec 10, 2008 · This page is a...

This page is a rebuild job on a Heathkit SB-200 amplifier, done by John, N4NAB.

December 10, 2008

Notes for the record:

Below is the ad for the SB-200. Somewhat miss-leading as the amp in fact was not working. I asked Dave to assure

me it would at least key and get some amount of power out before we finalized a price. He called back and said it

would not key. He thought the keying relay was stuck. As you can see below he was asking $325 plus shipping.

I offered $175 including shipping and that's the final price.

The amp arrived on December 10, 2008, well packed, tubes separate. It was not in as good of shape as I had hoped

for but none the less okay for my project, even if its just parts.

I am following down the restoration path of Andrey E. Stoev, AE1Z, who has his project documented at

http://blog.kotarak.net/2008/03/sb-200-part-1.html , with some very helpful information and several good

modifications.

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BEWARE OF ADS……………………………..

Listing Number: 700369 Date Ad was Placed: 11/11/08 Name: Dave Callsign: XXXX Category: AMPHF

Heading: For Sale : Heath SB-200 Amplifier

Ad Message:

N4NAB SB-200 http://www.boatanchors.org/N4NAB.htm

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Amp has clean face plate and clean chassis ( NO). Cabinet is in great shape as well ( NO). Powered supply upgraded

(NO). Unit was fully operational last time in use (???) . Tubes are in good shape, no discoloration (No) full output 80 -

15 , slightly less on 10m. Great amp for 6 meter conversion or as a beginners 1st amp. Comes with manual. Asking$325 plus actual shipping costs.

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Initial Visual Inspection December 11, 2008

Following are pictures of major sections of the amp prior to any cleaning and power up. Tubes were shipped

separately and have not been installed. Will power up with VARIAC without tubes to check HV Section.


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This is the front panel with the cabinet removed. Its okay but I don't like the dated and faded Heathkit green. Knobsare okay but also green.


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The original High Power Supply board is dated. The paper capacitors don't appear to be damaged but I will find outshortly upon power up. This will be replaced with a Harbach drop in up-grade.


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The meter appears okay but I plan some by-pass capacitors and use half of the filament voltage (6.3v, which alsopowers the meter bulb) to power LED indicators for power on and a stand-by switch. I will pick up the 3v here on

the meter.


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The transformer appears to be okay and the tube sockets are tight. I will replace capacitors on the sockets. The unit issetup for 120vac but will be changed to 240vac. The left rear of the chassis is bent from a drop!


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The fan motor will be replaced and fan blades re-balanced using the Harbach drop in unit. The originals do not moveenough air and are very noisy.


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The keying relay looks burnt and corroded. It will be replaced with a Harbach upgrade. Also the RF Input is an RCAtype connector mounted right to the left of the relay and will be replaced with an SO-239.


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The bottom of the chassis is clean and open. The board on the lower right is the HV Power supply which will bereplaced and the open area will be used for the Harbach Soft Start and Soft Key board mounting.

APPLYING POWER- The Initial Test December 12, 2008


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The power was brought up on the VARIAC in steps of 20vac starting about 30vac. Each step was allowed about 10minutes operation before going to the next voltage level. At the final operating voltage (my house line voltage) of

122vac ± 2vac ,the Plate (no tubes) was 2350 volts. I let it run for about an hour at the 122vac setting to cook in the

old caps and insure no failures with time. The manual specifies a range of 2400 volts as acceptable. Other

observations were no smoke, no shorts and the meter light worked. Also, the fan was quite noisy and out of balance.


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Using the “Initial Check” section of the Building Instructions to verify several items, a check of the resistance

between both anode caps and chassis was 174 kO.

180 kO was specified in the manual. Next the tubes go in and other items will be checked.

I powered up with the VARIAC very slowly but “no glow” on the tubes. I shut down quickly to avoid any damage to

the tubes.


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The resistance between Lug 3 on V1 and V2 to ground is specified in a range between 5000 and 15,000O.Measurement indicated the value was 389,000O, a problem for sure. I checked the circuits on the heater and cathode

since the plate voltage had checked out at 2350v. The heater and grid circuit checked okay but upon visual inspection

of the tube socket V1, I found the 10uH RF choke was unsoldered from Lug 4 on V1 and over to Lug 1 on V2.

I cleaned up the lugs and soldered the choke in place.


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Again, I powered up with the VARIAC very slowly and the tubes “glowed” and everything checked out okay.


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All the old components thus far have held up. My next task is to determine if the unit will key up. The keying relayappears to be in bad shape. I have been working on this portion of the amp most all day and will tackle the keying

issues tomorrow.

Problems Finally Appear………….. December 22, 2008


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I have been letting the unit run on the VARIAC for about five hours. The house breaker tripped and of course the

amp was the problem. It was not clear what failed. After a careful inspection, I suspected the HV board or the

transformer. I reset the two internal 8 amp breakers and powered up and it tripped the internal breakers again.

I then carefully removed the two S1 secondary wires from the board and attached the VOM set at 600VAC and

powered up the unit on the VARIAC. The voltage came up nicely and I stopped at 400V out of the transformer. In

addition the heater S2 taps worked fine as well as S3 taps on the fan. The transformer is okay.

I then physically removed the HV board and could see the diode string was physically damaged as some of the diodes

were in pieces. The 125uF caps tested at 92uF and two bleeder resistors were open.


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I was going to replace the HV unit anyway and this just sped up the work that is necessary. I removed and tested theHV caps and they were in poor physical and electric condition as well.

The HV board has traces flaking off and was burned in a couple of places. I suspect arcing here was the cause of the

failure.


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I am waiting until after the holidays to order the Harbach mods and have been working on re-wiring the meter andtapping the meter connectors to get 3vac to power an ON/OFF LED. I replaced the connector strip with a 5 lug

versus 3 lug, and built a small rectifier circuit (1N4003 diode, 100uF cap and two .01uf to prevent RF from getting

into the meter circuit. The meter is 6.3VAC and I took half for the LED and completed the circuit with a 47ohm

current limiter.


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Also I have decided not to use the old cabinet enclosure but use Plexiglas. The old cabinet was bent and I could notsuccessfully straighten it. I also plan to build a new square front panel from scratch as well. It will not look like an

SB-200 rebuild!


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I built a new front panel. I did not like the old round Heathkit panel and in addition it was scratched. Cutting thesquare hole for the On-Off Switch was a challenge. I used a Dremmel with a small cutting disc and it worked well.

The Plexiglas Enclosure


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I also repainted the transformer and cleaned the chassis; it looks much better.

In addition I have gone over all the solder joints on the tube sockets and unsoldered and re-soldered suspects.

January 8, 2009


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I have gotten back to work after the holidays. I am waiting on the arrival of the Harbach order to start the

re-assembly of the HV section, installing the new fan and new keying relay. In the meantime I have finished the front

panel work and the Plexiglas cabinet.

January 13,2009

The Harbach order arrived and work has started on the HV Board. The dowel in the picture was used to hold the

bleeder resistors up off the board about ¼“while I soldered them in place. The new diodes are 1N5408's. The six caps

are 180uF @ 250v replacing the eight 125uF paper caps.


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January 15 ,2009

The new fan was installed using new wire and the blades were balanced. The new keying relay was also installed and

the RCA RF Input jack was replaced with a new Teflon SO-239. New coax leads replaced the older coax.

The New Fan


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The Old Relay, RCA Jack and New SO-239

(Notice the contacts are rusted closed!


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January 18, 2009

The new HV board was installed and the meter and new On/Off LED wired into the meter circuit. Almost ready for

power!


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Before applying power, I was a little nervous! I re-checked all the new wiring twice and went over all thecomponents to insure I had not disconnected anything in handling the amp while doing all the other work.

Everything appears to be a go. It was late, I was tired, and I used a rare case of good judgment to wait until tomorrow

to install the front panel, put on the new knobs and bring up the power.


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January 19, 2009

Today is the day I have been anticipating for more than a month now. The panel is on and looks pretty good with thereplacement knobs.

Missing is any lettering which I have not found a good solution. I have investigated screen printing but found it takes

a special ink.

I cautiously connected the amp to the VARIAC and began to increase the power. The LED popped on and the meter

came to life instantly. At 15-20VAC, the fan spun up. I put the meter on Plate Voltage and watched it come up to

2400 volts as I reached 122 VAC on the VARIAC. Everything looks good!


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The next task is to install the soft start and soft key modules and put some drive into the amp.

January 20, 2009

The Harbach SoftKey and SoftStart Modules were assembled and installed with no problems, just tedious as the main

wire bundle had to be separated. The method for mounting the SoftStart was changed from the suggested Harbach


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method. That method was to use RTP/ Silicone and glue the board upside down to the chassis. I choose to mount the

module on an L shaped piece of Plexiglas which I held in place with the mounting bolt on the transformer.

The SoftKey is a very small board and was mounted on a 3/8” insulated standoff

with #6 screws. While I was working in this area I chose to replace two old electrolytic capacitors, C1 and C3. C1 is


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a 20uF, 150v cap in the grid circuit and C3 is a 2uf, 150v cap in the grid portion of the meter shunting circuit.

The unit was powered up, again on the VARIAC, and HV check out at 2400v, an improvement over the original caps.

The SoftKey was tested by shorting the antenna relay (RCA) to close the relays. All worked well, no problems.

I connected my Kenwood TS-430S (versus my ProIII) for testing. Keying the unit with my straight key and gradually

increasing the drive resulted in close to 600 watts out on a dummy load.


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More Problems……………surprise!

A problem with the meter circuit………..only the HV volts position would read on the meter. No grid or plate

current, relative power or SWR. I suspected the 5 position rotary switch after tracing out all the circuits and lifting

several resistors for testing. I cleaned the switch with DeOxit DL5 and it looked much better, however, I still had a

problem. After exhausting my ideas, I called in the big guns, W8UT , Al Parker. After several hours of tracing circuits


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and checking components, he too agreed it was the switch. As a BoatAnchor collector and repair expert, he

“happened” to have a Centralab 9 pole, 5 position rotary switch, which was adapted as a replacement for the 2 pole,

5 position Heathkit switch.

After some very tedious work in removing and reconnecting the switch………it didn't work correctly either. The grid

and plate currents were pegging the meter and the SWR and relative power positions show no indications. The HV, as

before, worked fine. We quit for the day, both tired from about 4 hours with our heads in the amp. Al called me the

next day and informed me that the two yellow wires going into the switch were backwards!

I had made a color sketch of the switch connectors and Al had taken digital photos before we started the replacement

process. The wires were crossed by the original kit builder and the meter never worked correctly. It now works greatand with new switch should perform well for many QSO's. After more testing I think I may be completed with the

electronic portion of the amp. I have to install all the Plexiglas covers and determine how I can label the panel. I have

tried several Avery Label products but so far nothing has worked satisfactory or had satisfactory appearance. Several

views of the new cabinet:


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I have had several CW and SSB QSO's with good results. I am very pleased with the outcome and the lessonslearned. My only job now is to solve the labeling problem.

I searched the web for solutions for labels since screening printing costs were out of the question for one panel. I

tried a number of Avery Clear Labels that would not adhere. I finally found a water slide decal inkjet paper ( like

labels on the old plastic models) at http://www.siriusrocketry.com/Saturn51.htm in packs of three sheets, an ample


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supply. Using Microsoft Word to design the labels it came out looking pretty good! I used Krylon Clear Coat to seal

the panel after applying the labels and they seem to blend in fairly well.

Done !! Learned a lot and got a nice amp to boot !!

The final product of several months' labor is getting about 650 watts out with 60-70 watts drive.

A great little amp …………………


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Radio to home page

this page last edited 2/24/09


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