SAE 2200 Rebuild

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The rebuild of my SAE 2200 is progressing nicely, thanks to input from @llwhtt and @hamrules. I've got the driver board done and tested and it's working fine. Next I'm working on the power supply board.

I was getting ready to solder in the six new rectifier diodes p/n: SAE11-0098 and I double checked a service bulletin (SAE2200-3) which deals with incorrect circuit board markings for some early units including mine. This bulletin refers to the "5 amp bridge rectifier diodes". Are these diodes rated 5 amp? Other sources (Wardsweb) indicate that these are 3 amp, 200 volt diodes. I have plenty of these, but no 5A on hand. Do I need to go with 5A?

 

[hamrules]

Once again I don’t have a copy of that bulletin either.

 

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The bulletin was referring to incorrect cathode markings on the board. The diodes I took out did not have any markings except the cathode ring. Do you recall what diodes you used on your 2200?

 

[hamrules]

That I saw. It has the diodes marked as 1N5400 but should be 1N5402. I am not positive, but I believe I used 1N5407's. For some reason I do not have it marked down but I do have an old package with some left which makes me believe they were the ones

 

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Those are all 3A rectifiers. The main difference is reverse voltage. It is 50v for the 1n5400, 200v for the 1N5402, and 800v for the 1N5407. I have 1N5408 on hand which are 1000v.

Craig agreed that 3A is fine, so any of these, except 1N5400 would be a good choice.

I'm almost done with the PS board and the last board is the output. So far it is going well. I will post some photos soon.

 

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The 2200 restoration is progressing nicely. I've got the Power Supply board done, including new main filter caps. I have tested it with the work done to both the Driver and PS boards with the original condition output board and the amp continues to function well -- biases correctly and DC offset is about 22mV on CHA and 26mV on CHB which is within the published 50mV spec. I did replace the differential pairs, but for now left the stock "select" resistors (750k) in place. When I get everything else done, I will put in 2M trimpots to see if I improve the DC offset.

 

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I have a question about the cement 0.2 ohm 5W resistors (R69,70,71,72) on the output board. Are these non-inductive resistors?
I want to replace all four - two are original and two were replaced as a result of an output transistor failure in 1981. They all measure fine, but I would like to have them all the same.
The only replacements I was able to locate were on eBay. These are new in package NTE 0.2ohm 5W 5% - I don't know if they are inductive or non-inductive - see photo. I try not to buy electronic components on the Bay, and also avoid NTE, but figured a resistor would be safe. I was not able to locate anything similar at the Mouser, Digikey, or Newark. The only thing making me hesitate to install them, is if the originals were NI and the new ones are Inductive, that could be an issue. @hamrules @llwhtt or other SAE gurus I would love to get your input on this. Thanks.

 

[hamrules]

I honestly don’t know what the difference would be to your ear if anything you’d be able to hear. On the ones I have replaced I think there was only one amp that had NI type in it or at least marked with NI anyway.

 

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Have you replaced these resistors in any of your SAE amps? If you did where did you get them?

 

[hamrules]

Yes I have replaced them. Never purposely bought NI type though. I’ve usually gotten them from Mouser or Digikey.

 

[ivandezande]

I have a question about the cement 0.2 ohm 5W resistors (R69,70,71,72) on the output board. Are these non-inductive resistors?
I want to replace all four - two are original and two were replaced as a result of an output transistor failure in 1981. They all measure fine, but I would like to have them all the same.
The only replacements I was able to locate were on eBay. These are new in package NTE 0.2ohm 5W 5% - I don't know if they are inductive or non-inductive - see photo. I try not to buy electronic components on the Bay, and also avoid NTE, but figured a resistor would be safe. I was not able to locate anything similar at the Mouser, Digikey, or Newark. The only thing making me hesitate to install them, is if the originals were NI and the new ones are Inductive, that could be an issue. @hamrules @llwhtt or other SAE gurus I would love to get your input on this. Thanks.

NTE? Cmon man.
Use https://www.digikey.com/en/products/detail/te-connectivity-passive-product/SQPW5R22J/2365728
Induction is irrelevant in this application.

 

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Thank you for the link.
I tried many times, but could not get the search engines on DigiKey or Mouser to find these, probably because I was looking for 0.2 ohm, not 0.22 ohm. It seems like a small difference (1/100th of an ohm), but it is 10% on a resistor with 5% tolerance. Perhaps this level of precision is not needed in this part of the circuit.

 

[hamrules]

Thank you for the link.
I tried many times, but could not get the search engines on DigiKey or Mouser to find these, probably because I was looking for 0.2 ohm, not 0.22 ohm. It seems like a small difference (1/100th of an ohm), but it is 10% on a resistor with 5% tolerance. Perhaps this level of precision is not needed in this part of the circuit.

They'll work fine.

 

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UPDATE: The 2200 rebuild is complete. I replaced almost everything in it, except for the transformer, relay, and coils on the output board.

I have owned this unit since it was about 6 months old (1976). It is an early (SN: 0809) unit that had several service bulletins applied to address mis-marked boards, and failure prone Fairchild output transistors.


The unit died and produced smoke in 1980 and was taken to an SAE factory authorized service center for in-warranty service. It received in-warranty service again in 1981 to address issues with the display. With no additional service since 1981, the unit was still functioning in 2023, but there was audible hum from the transformer, and barely audible hum from the speakers. The display stopped working again several years ago, and had been flaky for years before that.

Despite having the work done by an authorized SAE service center, the “smoke” repair only addressed the one channel that failed (ChA). Replaced components on this channel included the outputs, drivers, and a few diodes and resistors. SAE issued a service bulletin to replace the Fairchild output trasnsistors in both channels whenever a unit came in for service, but this was not done during either service. The un-applied service bulletins and the hum were the motivation to completely rebuild the amp.

Quick testing before starting work showed the following:

- The unit functioned on both channels
- The bias was able to be adjusted to 1.1vDC
- DC offset was 38mV ChA, and 21mV ChB, with inputs shorted, which was within spec of +/-50mV.
- Clipping was observed at 30v which is 112.5 watts
- Distortion at 1kHz at 50W measured 0.2%. Probably limited by my HP 334A analyzer.

Prior to servicing, schematics, and service manual with several service bulletins were obtained from stereomanuals.com, and other sources.

In the following posts, I document the work done with before and after photos.

 

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Display board

The display contains discrete transistors, rather than op-amps used in later units. I replaced the two 10uF 50v electrolytic capacitors on the display board which corrected the issues with the display, and it now functions properly on both channels.

 

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Driver Board

This board showed evidence of a failure during the 1980 event that took out the outputs. Several components for ChA had burned and were replaced, including the drivers, pre-drivers (limiters) and some diodes and a 120 ohm resistor. Other issues found included:

- One of the Channel A limiter transistors (Q13) was replaced incorrectly during the 1980 service. This transistor, originally would have been a 2N5210 NPN TO92 with a pin configuration of EBC. The transistor found on the board was a BC237 an NPN with a CBE pin configuration. However, this transistor was installed to match the INCORRECT board markings – it was installed CBE when it should have been installed EBC. It is unclear how the amp functioned with this transistor installed backwards.

- The repair to the divers, only replaced the ChA drivers not ChB.

- There was a service bulletin (2200-3) that identified incorrect board markings, and the need to replace resistors R55,56,57,58 which were 470 ohms when the unit was built (and marked this way on driver board) with 330 ohm resistors – this was not done when the unit was serviced.

Before I did anything, I replaced the differential pairs with discrete 2N5210 and 2N5087 (same as were found on the board) that I matched for hFE on an Atlas DCA55. I then re-checked DC offset, which dropped from 38/21mV to 26/18mV (ChA/B).

Then I replaced everything on the driver board. I also corrected the board markings where needed. In place of the “select” resistors (R21, R22) which were 750k I installed 2M trim pots. However the lowest I could get the DC offset with the trimmers was ~12-15mV, good, but still more room for improvement. I then put in 2.2M ohm fixed resistors resulting in ~8 -11mV DC offset readings, which is better. I will probably replace the fixed resistors with 3 or 5M trimmers.

 

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Power Supply Board

There was no evidence of prior service or any visible issues on this board. All components on this board were replaced except for the relay. The relay was inspected and looked very good, and was cleaned carefully with a strip of paper soaked in Deoxit 5. Board markings for diodes and one cap were corrected per service bulletin 2200-4.

 

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Output Board

The output board showed signs of major failure of one of the 1k resistors for ChA, with soot and burn marks on the nearby chassis panels, and significant burn marks on the board. The trace from this resistor to the bias pot burned away, and had been replaced in a prior repair.

The ChA output transistors had also been replaced (as part of the 1980 service). However, the outputs for ChB were the original Fairchild transistors. SAE Service Bulletin 2200-02 called for the replacement of the Fairchild output transistors for both channels whenever the units came in for service. Clearly this service was not performed on ChB.

All components on the output board were replaced except for the coils and transistor sockets. The heat sinks were disassembled, cleaned, and reassembled with new Dow 340 paste. Output transistors were replaced with On Semi MJ21193/94 with new micas and paste.

 

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Chassis and re-assembly

To try to reduce transformer hum, the rubber washers used to mount the transformer were replaced with new washers, and the end bell screws were tightened as much as possible.

The chassis was cleaned, and input and output terminals cleaned and tightened as needed. The foam backing on the faceplate was replaced.

 

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Final results

Bias set to about 1.2v. DC offset: ChA = 8mV, ChB = 11mV

Clipping with 1kHz input at: 29.25v/106.9 ChA, 29.50w/108.8w ChB

Distortion measured at 0.13% at 1 watt, 0.29% at 5 watts at 0.48% at 10 watts to 101.5w, just before clipping.

These distortion measurements are slightly worse than before the amp was serviced, which is a bit surprising, but could be due to measurement equipment or technique, neither of which are state of the art.

The amp sounds very lively and clean, but the transformer hum is still there, and clearly audible from three feet away. There is no hum through the speakers.

Does anyone have any ideas on how to get rid of the hum coming directly from the transformer?