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Pioneer SA-8500 Recap/Rebuild/Restoration/BOM

Okay, it's been 2 years and I'm reviving this thread. I've made zero progress since June 2018. I moved to a new house that summer and this project ended up on the shelf until now. I've got more free time since I'm always at home due to COVID so I'm catching up with all my restoration projects that have built up.

Tonight I turned this thing back on through my dim bulb tester and it seems to power up okay. I haven't connected it to any speakers for fear that it will damage them so I listened thru headphones using the aux input and all I get is a periodic popping/clicking in the left channel and nothing in the right channel. I measured the DC offset at the speaker outputs and I'm getting <10mV on each side but I also see the multimeter picking up the popping signal in the left channel.

My plan is to measure all power supply voltages and make sure they all look good. Then I'll play a 1kHz test tone into the aux input and try to see where it stops getting through. I'll update when I know more.
 
I began measuring voltages on the switch assembly and I'm starting to feel out of my comfort zone here so hopefully some people smarter than me can chime in...

Now that I have the bottom off I can clearly hear the relay clicking periodically, that must be the pop that's making it into the left channel output. Seems like the protection circuit is engaged but I can't figure out what's causing it yet. I don't have any input or speakers connected which I believe leaves me with either a broken component on the switch assembly OR a broken component on the power amp assembly causing a DC imbalance at the output of the power stage...

I'm looking for Q20 and Q21 that are described on pages 8 and 9 of the SM so I can probe them but I can't figure out which ones they are on the switch assembly (why do they re-number each board starting at Q1???). I'll try digging in more but can anyone help me start looking in the right direction?

Is it stupid to remove each transistor one-by-one and test them to see if any have gone bad?
 
To debug further I sent a 1kHz test tone into the AUX inputs and was able to trace the signal thru to the power amp (AWH-042) where I found a problem. The right channel has signal coming out of three pins -- 9, 15, and 16. The left channel outputs nothing from the power amp assembly. I took measurements at each power supply pin and across each transistor that the SM had expected values for. My results are:

Code: 
            LEFT CHANNEL                           RIGHT CHANNEL
 ------------------------------------    -------------------------------------
| Device | Pin | Expected | Measured |  | Device  | Pin | Expected | Measured |
|------------------------------------|  |-------------------------------------|
| Q1 PNP |  B  |   0.0 V  |   0.0 V  |  | Q2  PNP |  B  |   0.0 V  |   0.0 V  |
|        |  E  |   0.6 V  |  -6.4 V  |  |         |  E  |   0.6 V  |   0.6 V  |
|        |  C  |          | -44.5 V  |  |         |  C  |          | -43.3 V  |
|------------------------------------|  |-------------------------------------|
| Q3 PNP |  B  |   0.0 V  |  -7.0 V  |  | Q4  PNP |  B  |   0.0 V  |   0.0 V  |
|        |  E  |   0.6 V  |  -6.4 V  |  |         |  E  |   0.6 V  |   0.6 V  |
|        |  C  |          | -44.6 V  |  |         |  C  |          | -44.6 V  |
|------------------------------------|  |-------------------------------------|
| Q5 NPN |  B  | -42.7 V  | -44.5 V  |  | Q6  NPN |  B  | -42.7 V  | -43.3 V  |
|        |  C  |          | -27.6 V  |  |         |  C  |          |  -1.2 V  |
|        |  E  | -43.3 V  | -44.5 V  |  |         |  E  | -43.3 V  | -43.8 V  |
|------------------------------------|  |-------------------------------------|
| Q7 NPN |  B  |   1.2 V  | -25.1 V  |  | Q8  NPN |  B  |   1.2 V  |   1.2 V  |
|        |  C  |  45.0 V  |  47.6 V  |  |         |  C  |  45.0 V  |  47.1 V  |
|        |  E  |   0.6 V  | -21.2 V  |  |         |  E  |   0.6 V  |   0.6 V  |
|------------------------------------|  |-------------------------------------|
| Q9 PNP |  B  |  -1.2 V  | -27.6 V  |  | Q10 PNP |  B  |  -1.2 V  |  -1.2 V  |
|        |  C  |  -0.6 V  | -38.6 V  |  |         |  C  |  -0.6 V  |  -0.6 V  |
|        |  E  | -45.0 V  | -47.4 V  |  |         |  E  | -45.0 V  | -47.3 V  |
 ------------------------------------    -------------------------------------


    POWER SUPPLY JUMPERS
 --------------------------
|  J | Expected | Measured |
|--------------------------|
|  4 | -45.0 V  | -44.6 V  |
|  7 |  35.0 V  |  37.3 V  |
| 10 |  35.0 V  |  37.3 V  |
| 13 | -45.0 V  | -44.6 V  |
| 17 |  35.0 V  |  37.3 V  |
| 18 |  45.0 V  |  48.1 V  |
| 31 | -45.0 V  | -48.2 V  |
| 44 |  45.0 V  |  47.9 V  |
| 45 |  35.0 V  |  37.3 V  |
 --------------------------
 
Thanks everyone for the work on this list. Your hard work benefits those of us going after refurbishing one and is sorely appreciated.
 
Seems like every few years I pull this SA-8500 out of the basement and try again... I'm determined to finally get it working this time.

Hooked up to my dim bulb tester again I notice a faint glow and measure similar voltages to the ones I posted back in 2020. I have the power amp inputs terminated with resistors to GND per the bias adjustment procedure in the SM, isolating the problem to the power amp board and protection circuit on the switch circuit assy. Most significantly I still see Q5,C is close to the negative rail so it's not getting sufficient Vce. Pin 1 (L channel output before protection relay) is measuring -12V so something is pulling the L channel output down towards the negative rail. The R channel is fine and able to be biased and offset adjusted normally.

Taking a shotgun approach I started replacing known problem transistors and upgrading other parts, rechecking the bias/DC offset after each change. I replaced L&R input diff pairs (Q1-Q4) with gain matched ksa992s, Q5 & Q6 with ksc3503, and replaced VR1-VR4 with new Bourns trimmers. Same problem persists -- large negative offset seen at the power amp left channel output that I'm unable to tame. I'm measuring ~2.3V across D3 so I'm assuming it is fine, and the 100R trimmer is set to ~50R, which I can measure with my DMM so also assuming it's fine. I noticed Q5 is getting pretty hot so out of curiosity I swapped Q5 and Q6 between the Right and Left channels and the issue stayed with the Left channel, eliminating Q5 as the culprit. I lifted R25 to remove the negative feedback and the negative offset got even lower, meaning whatever is pulling the output low is downstream of the input diff pair. Edit: I also had Q1,B biased to 0V.

I suspected ceramic caps C13 (Q5,B -> Q5,C) or C17 (Q9,B -> Q9,C) could be leaky or shorting Q5,C to the negative rail so I lifted leads on both to no avail.

Continuing with my shotgun approach I started looking to the switch circuit assembly and replaced DC offset monitoring pair Q5-Q6, 2sc869 -> ksc1845. I also replaced the overcurrent monitors Q7-Q8, 2sc945 -> ksc2383. Still, same exact behavior.

I've done even more debugging but I don't want to continue rambling so can anyone help with ideas moving forward? I still can't confidently figure out if the issue is a short on the power amp assy or something external, like something loading the L ch output down on the switch circuit assy. Any advice moving forward would be great!
 
Last edited:
I believe I've isolated the issue to the power amp assembly (AWH-042). The left channel output leaves the board and attaches to the switch assembly (AWS-083-D) through 4 connections:

AWH-042 Pin 1 -> AWS-083 Pin 21 -> R27
AWH-042 Pin 3 -> AWS-083 Pin 27 -> R20
AWH-042 Pin 8 -> AWS-083 Pin 26 -> R19
AWS-083 Pin 22 -> AWS-083 Pin 3 -> C27

I lifted one leg of each component on the switch board, leaving the output of the amp board Left channel floating. After doing this I'm getting -14.6V DC bias at the Left channel output. It appears the overcurrent circuit around AWS-083 Q5 was loading the output down a bit because this is more negative than the -12V I was seeing before.

Still only a slight glow from my DBT so I don't have a dead short but something in the amp is definitely pulling down on Vout. I'm leaving the 4 aforementioned leads lifted while I continue to debug. I'll post amp board voltages later tonight.
 
Here are the voltages for all transistors on both channels:

Code: 
              LEFT CHANNEL                             RIGHT CHANNEL
 --------------------------------------    --------------------------------------
| Device  | Pin | Expected |  Measured |  | Device  | Pin | Expected | Measured  |
|--------------------------------------|  |--------------------------------------|
| Q1  PNP |  B  |   0.0 V  |   0.00 V  |  | Q2  PNP |  B  |   0.0 V  |  -0.01 V  |
|         |  C  |          | -43.45 V  |  |         |  C  |          | -43.48 V  |
|         |  E  |   0.6 V  |  -8.20 V  |  |         |  E  |   0.6 V  |   0.56 V  |
|--------------------------------------|  |--------------------------------------|
| Q3  PNP |  B  |   0.0 V  |  -8.91 V  |  | Q4  PNP |  B  |   0.0 V  |   0.00 V  |
|         |  C  |          | -43.42 V  |  |         |  C  |          | -42.14 V  |
|         |  E  |   0.6 V  |  -8.25 V  |  |         |  E  |   0.6 V  |   0.56 V  |
|--------------------------------------|  |--------------------------------------|
| Q5  NPN |  B  | -42.7 V  | -43.40 V  |  | Q6  NPN |  B  | -42.7 V  | -42.12 V  |
|         |  C  |          | -15.44 V  |  |         |  C  |          |  -1.27 V  |
|         |  E  | -43.3 V  | -43.43 V  |  |         |  E  | -43.3 V  | -42.75 V  |
|--------------------------------------|  |--------------------------------------|
| Q7  NPN |  B  |   1.2 V  | -12.75 V  |  | Q8  NPN |  B  |   1.2 V  |   1.22 V  |
|         |  C  |  45.0 V  |  33.90 V  |  |         |  C  |  45.0 V  |  34.16 V  |
|         |  E  |   0.6 V  | -13.26 V  |  |         |  E  |   0.6 V  |   0.63 V  |
|--------------------------------------|  |--------------------------------------|
| Q9  PNP |  B  |  -1.2 V  | -15.35 V  |  | Q10 PNP |  B  |  -1.2 V  |  -1.27 V  |
|         |  C  | -44.3 V  | -26.30 V  |  |         |  C  | -44.3 V  | -34.16 V  |
|         |  E  |  -0.6 V  | -34.25 V  |  |         |  E  |  -0.6 V  |  -0.69 V  |
|--------------------------------------|  |--------------------------------------|
| Q11 NPN |  B  |   0.6 V  | -13.43 V  |  | Q12 NPN |  B  |   0.6 V  |   0.63 V  |
|         |  C  |  45.0 V  |  34.72 V  |  |         |  E  |  45.0 V  |  34.66 V  |
|         |  E  |   0.0 V  | -13.99 V  |  |         |  C  |   0.0 V  |   0.02 V  |
|--------------------------------------|  |--------------------------------------|
| Q13 PNP |  B  |  -0.6 V  | -26.29 V  |  | Q14 PNP |  B  |  -0.6 V  |  -0.69 V  |
|         |  C  | -45.0 V  | -34.66 V  |  |         |  C  | -45.0 V  | -34.60 V  |
|         |  E  |   0.0 V  | -13.92 V  |  |         |  E  |   0.0 V  |  -0.06 V  |
|--------------------------------------|  |--------------------------------------|
| Q15 NPN |  B  |   0.6 V  | -13.29 V  |  | Q16 NPN |  B  |   0.6 V  |   0.63 V  |
|         |  C  |  45.0 V  |  34.61 V  |  |         |  C  |  45.0 V  |  34.84 V  |
|         |  E  |   0.0 V  | -13.78 V  |  |         |  E  |   0.0 V  |   0.01 V  |
|--------------------------------------|  |--------------------------------------|
| Q17 PNP |  B  |  -0.6 V  | -26.49 V  |  | Q18 PNP |  B  |  -0.6 V  |  -0.69 V  |
|         |  C  | -45.0 V  | -34.61 V  |  |         |  C  | -45.0 V  | -34.86 V  |
|         |  E  |   0.0 V  | -13.79 V  |  |         |  E  |   0.0 V  |  -0.06 V  |
 --------------------------------------    --------------------------------------


    POWER SUPPLY JUMPERS
 --------------------------
|  J | Expected | Measured |
|--------------------------|
|  4 | -45.0 V  | -43.42 V |
|  7 |  35.0 V  |  36.22 V |
| 10 |  35.0 V  |  36.22 V |
| 13 | -45.0 V  | -43.44 V |
| 17 |  35.0 V  |  36.25 V |
| 18 |  45.0 V  |  35.25 V | <--
| 31 | -45.0 V  | -35.07 V | <-- ~10V Low
| 44 |  45.0 V  |  34.90 V | <--
| 45 |  35.0 V  |  36.25 V |
 ---------------------------

The output stage power supplies are low compared to last time I measured the supplies on the bench. I'm assuming this is due to being powered in series with the dim bulb tester, but the Right channel output stage is operating normally so I'm ignoring it for now. I also verified all the voltages in the bias network at the input/base of Q1 and they all look fine. I also measured 6.3V dropping across R33 (Q7 emitter resistor) and 6.6V across R35 (Q9 emitter resistor). Strangely I'm measuring dead 0.0V across each of the Left-channel output transistor emitter resistors R47, R49, R55, and R57. On the right channel the output emitter resistors R48, R50, R56, and R58 all measure small but non-zero voltage drops in the range of 2-3mV.
 
I believe I've found the culprit(s): bad output PNP transistors Q13 & Q17. I removed all 4 output transistors from the left channel and the DC at the output dropped to 0V. Both NPNs test fine using my Peak Atlas DCA55 but both PNPs are measuring bad. One shows up as a 2-terminal diode and the other results in a "faulty device" reading. I still don't understand how this didn't show up on the dim bulb as a dead short but looks like I'm ordering new output drivers. I believe the following is correct:

AWH-042-0: Q11,Q12,Q15Q16: 2SD371A (NPN): 863-MJ21194G
AWH-042-0: Q13,Q14,Q17Q18: 2SB531A (PNP): 863-MJ21193G
 
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