Pioneer SA-8800 - high voltage readings

[Bob123456]

Hello, newbie here hoping for some advice. I was recently given an SA-8800 in not-great-condition to refurb and sell on behalf of an elderly relative. I've got a little experience with electrical repairs and I guess I've had some luck in the past, but I have limited equipment (and space) to work with- I don't even have real speakers to test it with but I figured modern headphones would probably do for the moment.

Initially the amp was stuck in protection mode, so after research here and a lot of careful probing I found that Q1 wasn't switching despite good voltage input, so I replaced that with a KSC2383YTA (1A, 160V NPN) as recommended by someone on here. The result was the amp now comes out of protection mode after a few seconds, and it hasn't blown again since. So that's good.

When I started trying to get sound through the amp however it quickly became apparent that something was very wrong, from the dust burning off of R141 and R142 (headphone jack power resistors). Some quick probing confirmed that I now have 100VAC and ~50VDC (left channels are -49.9, right channels are +46.6 relative to ground) on the audio outputs.

These are the readings for the terminals on the Edit: Power Supply board (VAC, VDC), all relative to chassis ground, with the preamp links removed and no speakers etc connected:

1. 0, 0
2. 101, +46.1
3. 0, -49.9
4. 0, 0
5. 0, 0
6. 0, 0
7. 241, 0
8. 241, 0
9. 1.8, 0
10. 1.8, 0
11. 48.9, 0
12. 49, 0
13. 48.9, 0
14. 0, -49.6
15. 0, -33.5
16. 0, 0
17. 70.2, +32.1
18. 107.5, +49.0
19. 104.2, +47.5
20. 109.4, +49.8
21. 0, 0
22. 0, -50.7
23. 0, -50.0
24. 0, 0
25. 110.6,+50.5
26. 36.2,0
27. 36.2,0
28. 101,+46.1
29. 0,-49.9
30. 0,0
31. 0,0
32. 36.1,0
33. 36.1,0

Apart from the voltages being scary, the power indicator lamp doesn't come on (it was like that before I changed Q1), and I don't think I've seen the right channel of the display light up although that could well be a bad input.

Thank you for reading this far, and thanks in advance if you've any suggestions on what could be wrong/what to check next.

 

[markthefixer]

Could you do these same measurements to the AWR-182 power supply board pins?
I know some are repeats, IF the wire isn't broken.

My first inclination is that if the headphone resistors got hot, this unit has no business coming out of protection.

The various power supplies have a particular grounding scheme. I think you have a floating ground somewhere.
DO NOT do anything that changes the factory grounding to try to get this working.

 

[Bob123456]

Could you do these same measurements to the AWR-182 power supply board pins?
I know some are repeats, IF the wire isn't broken.

My first inclination is that if the headphone resistors got hot, this unit has no business coming out of protection.

The various power supplies have a particular grounding scheme. I think you have a floating ground somewhere.
DO NOT do anything that changes the factory grounding to try to get this working.

Apologies, those readings are all from the AWR-182 power supply board and not the power amp board as I initially wrote, sorry, I've edited that post now.
Thanks for looking though; I'll go through the GWH-115 power amp terminals after work tomorrow and post them here. I did have a close look at everything initially and couldn't see anything obviously blown or disconnected, but there's a layer of old dusty gunk on everything that could be hiding almost anything.

 

[markthefixer]

 

[Bob123456]

Ok so that makes the negative DC supplies look pretty suspect; maybe blown capacitors on the outputs? I note both the schematic and board layout in the service manual have two C16's; I'll give them a good probing later and see if they're still in one piece and let you know.

 

[Bob123456]

Ok I now have the numbers from the GWH-115 Power Amp board, again these are all AC,DC relative to chassis ground, with the pre/power amp links removed -

12. 0, 0
13. 48.9, 0
14. 110.8, +50.5
15. 0, -49.9
16. 0, -50.1
17. 0, 0
18. 0, -50.7
19. 0, 0
20. 101.2, +46.1
21. 109.4, +49.9
22. 0, 0
23. 0, 0
24. 0, 0
25. 107.3, +48.9
26. 0, -49.5

 

[Bob123456]

I've now checked all the capacitors on the Power Supply board, and found a couple of oddities but everything else looked to be intact -

• C3 - I think I'm getting a false reading from this one, as it reads a small resistance initially and quickly drops down to zero.
• C20 - does not exist on my power board. The trace linking it to to R1 isn't present either; unfortunately the service manual I have doesn't have any version numbers but I assume the amp is just a later revision, the rest of the board matches.

Capacitors were all checked in-situ, with the power off, using a multimeter set to measure resistance. If I see the resistance going up, or if I see a voltage going down when I change to measuring DC voltage, then I figure the cap is ok.

 

[markthefixer]

I did not see any problems with the AWR-182 readings that would point me somewhere... (well, other than those strange AC readings popping up)
I did that array and post to relax me from an arduous day, yet conked out before adding any commentary.

I am getting behind and able to steal less and less time for AK troubleshooting.
Do not mistake my delays for anything else.

You need to double check all those listed grounds. VISUALLY AND DMM resistance check.
Sometimes readings will fool you, going through a transformer winding to a center tapped ground.
There are reasons for different grounds on the board, all eventually connecting to the chassis ground.
One broken ground can have voltages adrift, floating at some fraction of the power line.
A bad diode in a grounding scheme can be even worse, conducting at times and isolated at others.

Soon I hope to look at your GWH-115 board readings.

Needless to say, the protect should NOT be connecting the amps to the speakers and headphones.

 

[Bob123456]

Ok, then tonight's task will be to inspect and measure resistance of all of the ground pins to chassis (and maybe check all the diodes too) and report back here, thank you.

I really appreciate your assistance with this, and of course I understand you have other things to do. I'm just keen to try to answer any questions promptly so that whenever you do have a look at my problem you're not stuck waiting for a time difference

 

[Bob123456]

I've now measured resistance to ground and inspected all of the wires for the terminals on the power supply board, I couldn't find anything obviously broken but maybe there's a clue in the readings?
My notes are as follows -

1. 0ohms both ends
2. Hi impedance
3. Hi impedance
4. 0ohms
5. 0ohms both ends
6. Hi impedance
7. Hi impedance
   
8. Hi impedance
   
9. 4ohms both ends
10. 4ohms both ends
11. 4ohms
12. 4ohms
13. 946ohms both ends
14. Hi impedance
    
15. Hi impedance
    
16. 0ohms both ends
17. Hi impedance
    
18. Hi impedance
    
19. Hi impedance
    
20. Hi impedance
    
21. 0ohms both ends
22. Hi impedance- looked to be charging while measuring
    
23. Hi impedance- looked to be charging while measuring
    
24. 0ohms both ends
25. Hi impedance
    
26. 2ohms
27. 2ohms
28. Hi impedance
    
29. 0ohms - looked to be charging while measuring
30. 0ohms both ends
31. 0ohms both ends
32. 2ohms
33. 2ohms

 

[Bob123456]

I have now gone through all the diodes on the power supply board, please see the results below.
I measured with a multimeter set to resistance mode (and supply voltage off), and if I saw very low resistance in one direction I figured it was ok. I then switched-on the supply to the amp and measured the DC voltage across the suspicious-looking diodes, let me know if that's helpful for the rest of the list and I'll get that done too.

 

[Bob123456]

After thinking and probing a bit more, I decided I wasn't confident about some of the rectifier diodes on the power supply board, so I replaced D5 and D7 with 1N5404's, but didn't go any further than that as the ones I removed tested fine in isolation (and changing them had no effect).
My next best idea is to start replacing everything shown with a triangle on the schematic. I know the power indicator lamp is definitely blown (and is one of those items), it seems unlikely that could that be the source of my 100vac but it's the only thing I can find? It seems like the replacement for the AEL-047 indicator is also now discontinued, what would be a suitable replacement these days?