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Pioneer SA-7500 - Protection issues - dumb mistake!

F

forsgren

AK Subscriber
Subscriber
Good afternoon, hope everyone is being careful, taking necessary precautions and is doing well! Like many of you, I have some extra time to catch up on my stereo projects (so far the wife has been quite patient)! I am in the process of restoring my SA-7500 integrated amp. I have replaced the capacitors and bad transistors on the Control Amp AWG-033 as well as the Equalizer Amp AWF-018A (manual states AWF-015B, board is marked AWF-018A). I was admiring my work and loving the sound (and not being too careful) when the Control Amp board got too close to the 10,000uf capacitor mounting bracket. Sparks flew, sound cut out and the protection relay kicked in. I think pin #7 was the offender.

I have checked the output transistors, they are all fine, I have checked the driver transistors (pulled W7 the D357 and it tests fine). I measured the other D357 as well as B527 and get voltage on them all. Some voltages are spot on, others vary. Here is what I am getting:

AWH-040 Power Amp

Pin 33 40.4V should be 38V
Pin 28 -40.3V should be -38V
Pin 22 40.3V should be 38V
Pin 9 5.38V should be -40.5V
Pin 6 36.5V should be 37V
Pin 10 0.74V should be -15V
Q7 base 6.99V should be 1.2V
Q3 base 4.6V should be -1.2V
Q10 base 4.7 V
Q8 base 6.9V

I will measure voltages on Control Amp, EQ Amp and Power Supply when my wife allows!

I have not looked at the relay as of yet. Any early indicators or suggestions?

I thank you all in advance!
 
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Not on DBT, no issues with fuses either, I checked those first. Getting about 10.4V on base of Q5. I have not pulled it yet.
 
I followed the wire from pin #7 on the Control Amp board, it runs directly to the emitter of Q3 on the Switch Circuit Assembly board AWM-072C. I measured the voltage on the emitter, should be 40.5V, actual is 11.08V. I measured the collector, should be -51.5V, actual is -51.5V. I pulled the transistor, it is toast. I have a new one on order. I also measured Q4 which the emitter should be -13V, actual was 0.7V. I have also ordered some new 2SA733 transistors. Really not more I can do until the replacements arrive. The obvious question is, what else could be damaged? I appreciate the help so far, I will keep you posted in a few days.
 
Got the new 2SA816 and 2SA733 transistors, I pulled the 733 and it checked good. I installed the new 2SA816 and I am back in business. I really lucked out, I have heard horror stories with far worse results. Thank you and be safe!
 
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