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Need Realistic STA-200 service manual

In my old ECG cross reference book, the HV80 is NOT a Zener diode at all. Maybe the spec sheet on post #33 was a misprint for the ZV80 which is a 80V Zener diode. ECG substitute for the HV80 is their ECG601, a (0.6V) varistor. Looking at the application in the STA-200 power amplifier circuit, you can safely replace them by 1N400x or 1N4148

D1, D2 can be substituted by 1N400x. The thermal feedback in this amplifier is certainly very bad. As you can see, there are four b-e junctions between the base of Q4 and the base of Q3, but there are only two 0.6V diodes in the bias spreader. It would be important to mount both D1and D2 on the power transistor's heat sink, as close as possible to one of the power transistor. Actually, D1 is mounted on the heat sink and D2 is mounted on the board. For a better thermal tracking, I would move D2 in series with D1, on the heat sink. I would install the bias trim pot where D2 is located on the board, and I would install a 1N4148 diode where R12 is located. It would be easy to bent the legs of that diode so that diode would be in good thermal contact with Q4. With one more diode in the bias spreader, the bias trim pot should be a 100 Ohm pot.
 
Hi,

It was my experience with the first of these units I owned in the 1970s that they periodically consumed the output transistors. This one looks as though it has had a few come and go along with a driver. Your suggestions sound well thought out. I will hold off making any mods you suggest until you are thoroughly satisfied with your analysis and solution. I am not an engineer, merely a tech that patches up other's designs when parts fail. Thank you for your concern and help. The STA-200 was never a great sounding receiver IMHO. D-7 appears to be a 33 volt zener, do you agree? I have ordered what appear to be genuine 2SA680 transistors and some 2SC1080 I am not so sure of. The 680s looked like I remember and were expensive, the 1080s were shiny and though they were marked Toshiba, were about two bucks a piece. Do you recommend I should use these, or use the MJ21193/4 pairs?
 
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I changed the resistance of the bias trim pot (modification) because initially I thought that both D1 and D2 were mounted in the vicinity of the power transistors. After I consulted the parts layout I realised that only one diode was mounted on the heat sink. It became clear to me that something should be done to improve the thermal coupling between the transistors (driver and power output) and the bias spreader. With the addition of a third diode the resistance of the bias trim pot should be reduced, otherwise the bias adjustment would be too sensitive. The current in this trim pot will be approximately 7mA, and you don't need much more than 600mV drop in the bias trim pot. 100 Ohm looks to be the ideal value. There is no need for multi-turns trim pots when the values of components are correctly selected.

D7 must be a Zener diode of at least 30V. The value of 33V is perfect. The explanation of the shut down circuit, on page 9 of the service manual, is wrong on point 5. When the shut down circuit engages, Q5 becomes OFF (that is the purpose of the Zener diode D7), and that condition will maintain Q7 ON until the power is cut at the receiver.

I can not endorse components sold on the bay.
 
Would you please recommend subs for the transistors on the power amp board that will be consistant with the mods you have suggested. I intend to totally replace all the parts on the power amp boards because of the facts these STA-200 have never sounded that good and their propensity for destroying pre, driver, and output transistors. I have two of these presently, one in its box, packing, remote and manual which is all original and worked perfectly when I put it in the basement. I am sure it will need a complete overhaul when I take it out of storage. The one I am working on now was fine before it went into the attic. It comes down with shorted diodes and open emitter resistors. I very much appreciate your help and the time you have taken to read and analyze the circuit.

For some reason I cannot download some zip files with sub charts, I am told on the page I need to sign in, but it won't let me because of cookie problems, though I am already signed in. I messaged the contact page several days ago and pasted the messages I was getting but no one has contacted me. Perhaps I am doing something wrong? I am using XP and a Chromebook and have this situation on both.
 
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For the power transistors: 1st choice: MJ15003G/MJ15004G, 2nd choice: MJ15022G/MJ15023G. They are all available at Digikey at the moment.
For the drivers: TTA004B/TTC004B, 2nd choice: TTA006B/TTC011B
For Q2, Q5, Q6, Q7: 2SC2383Y
For Q1: 2SC1815

Increase the bootstrap capacitor C13, to 100uF/50V
Increase the voltage rating of C4, 100uV/35V
 
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