Pioneer SX-950 No Audio Output

[TechGuru87]

Hi, I acquired an sx950 with no audio output. I am an veteran electronic technician for 25+ years but can't figure out what the problem is? I have checked all outputs stages and found no shorted, burnt components or blown fuses. Unit powers on, all lights work. The pre-out stage works fine when connected to an external amp. However, the main output relay won't turn on. I tested the relay out of circuit and works good. Not sure if something is tripping the protect circuit or if there is a problem in the protect circuit itself? I took some voltage measurements from the Protect Circuit Board (AWM-062) as shown in red on the schematic I have attached. Can someone take a look and let me know what might be the problem here or where to look next? Thanks...

 

[merlynski]

Looks like you may have a DC offset problem. What are the voltages on pins 1 and 3 of the Protection board? They should be close to 0 vdc, less than +/- 0.05 v is considered ok. I suspect that pin 1 is going to be about -2.5 vdc from looking at the voltages around Q1 on the protection board. The volume should be minimum, no input. Check pin 10 on the Power Amp Assembly AWH-050, it is where Prot pin 1 is connected.
Do you have the SX-950 on a DBT?

 

[merlynski]

BTW welcome to Audiokarma

 

[ivandezande]

Just redid a 950. Same issue, but I had to replace every transistor and diode on the protector board for it to stop being wonky. So basically a lot of 1N4148's and a 1N4004.
Follow here: http://hudsonvalleyhifi.com/pioneer-sx-950-restoration

 

[tsd71]

please post power supply voltages

 

[TechGuru87]

Hi All, I want to say thanks in advance to everyone for helping out. Here are the voltage readings as requested:

AWH-050 (From these readings looks like a problem in Left Channel even though all transistors passed preliminary testing)
Pin#
1. -49.6
2. -49.6
3. 0
4. 36.4
5. 36.4
6. 0
7. 50.8
8. 50.8
9. 0
10. -2.7
11. -2.7
12. -2.7
13. -2.7
14. 55.5
15. -55.5
16. -49.6
17. -49.6
18. 0
19. 36.4
20. 36.4
21. 0
22. 50.8
23. 50.8
24. 0
25. 0
26. 0
27. 0
28. 0
29. 55.5
30. -55.5
31. -55.5
32. 0
33. -0.5
34. 0
35. -0.5
36. 55.5
37. 0
38. 0.5
39. 0
40. 0.5
41. -55.5
42. -2.8
43. -3.4
44. -2.8
45. -3.4
46. 55.5
47. -2.8
48. -2.3
49. -2.8
50. -2.3

 

[TechGuru87]

Here are the power supply voltages
AWR-101
Pin#
1. 0
2. 5.5
3. 5.5
4. 13.8
5. 13.8
6. 50.8
7. 50.8
8. 36.3
9. 36.3
10. 36.3
11. 28
12. 0
13. -12.9
14. -49.6
15. -18
16. 40.8 VAC
17. -55.5
18. 55.5
19. 40.8 VAC
20. 0
21. 0
22. 7.9 VAC
23. 7.9 VAC
24. 8.4 VAC
25. 53 VAC
26. 53 VAC
27. 21.9 VAC
28. 21.9 VAC
29. 7.9 VAC

 

[TechGuru87]

Looks like you may have a DC offset problem. What are the voltages on pins 1 and 3 of the Protection board? They should be close to 0 vdc, less than +/- 0.05 v is considered ok. I suspect that pin 1 is going to be about -2.5 vdc from looking at the voltages around Q1 on the protection board. The volume should be minimum, no input. Check pin 10 on the Power Amp Assembly AWH-050, it is where Prot pin 1 is connected.
Do you have the SX-950 on a DBT?

I don't have a DBT only a variac was looking into to building a DBT though

I checked DC Offset adjustments and was able to set Right Channel spot on but the Left Channel best I can get is -1.9v between terminals 9 & 10 with VR1 at max. I checked the pots and are good. Also, the relay clicked on and stayed on after Left dc offset reached -1.9v. But no output since I pulled the shorting pins. Don't dare to put pins back in and see if there is output with offset that far off. What could be the problem? Thanks...

Oh, I posted my voltage readings above

 

[kevzep]

I would be rebuilding this amplifier, no point in wasting time trying to figure out whats going on, its most certainly going to be due to ageing components failing, out of spec, leaky etc etc.
Download one of the BOM's from here, rebuild the amp from top to bottom and it will be a mint runner.
Trimmers need to be replaced, a lot of silicon needs to be replaced, caps etc etc.

I have a Sansui 777A on the bench right now for restoration, EVERY electro cap is reading 100% higher than what they were when they were new, I am not kidding, ESR is high etc etc, drifted resistors, crappy old trimmers, leaky old small GP transistors....
All this stuff needs to be replaced.

Your SX950 is newer than the Sansui here, but the same rules apply, you are going to go around in circles trying to diagnose the problem and if you fix it, something else will go wrong in the not too distant future, so just rebuild it and have the satisfaction of knowing it will fire up and give years of trouble free service..

 

[merlynski]

I strongly recommend a DBT, it helps keep the magic smoke inside the $$ components.
The Protection circuit looks to be doing its job correctly. Since the right channel offset adjusts correctly the power supplies are probably OK, the voltages look OK to me. There is a DC offset problem in the left channel, pin 10 is -2.7vdc. Because of the negative feedback loop in the power amp the whole circuit will appear unbalanced while powered up. Power off and unplug.
Recheck every semiconductor in the power amp left channel, there are only 15 of them. Unsolder if necessary. Lift one end and measure each resistor. Check each electrolytic cap for leakage. This is the tedious part of the process that I dislike the most, but . . .
Note that Q1 and Q3 (2SC726 differential pair) are on the 'replace hit list' here on AK. See: http://audiokarma.org/forums/index....ilure-prone-whatever-and-replacements.731653/
I like to solve the problems first before rebuilding, but kevzep's point has value.
Good luck!

EDIT: content

 

[kevzep]

I like to solve the problems first before rebuilding

I used to, but I am finding it ends up most times being a waste of time anyway, it is the right thing to do, fault find first, but lately al my faults are related to components that will be replaced anyway, so I have changed my approach a bit these days.
Time on the bench needs to be in the name of efficiency when customers are paying good money.
If you are doing it for yourself, it is a little bit different though.

 

[merlynski]

I used to, but I am finding it ends up most times being a waste of time anyway, it is the right thing to do, fault find first, but lately al my faults are related to components that will be replaced anyway, so I have changed my approach a bit these days.
Time on the bench needs to be in the name of efficiency when customers are paying good money.
If you are doing it for yourself, it is a little bit different though.

I agree, I don't usually do paid electronics repair for others, just myself. I get great joy from it, but my OCD/perfectionism is definitely a factor.

 

[zebulon1]

Agree, fix first but:
It seems like an amp issue on a 950.
Rebuild it.
The heatsink for the outputs get removed and you can pass the whole amp through the chassis slot. Easy fix'ins.

 

[TechGuru87]

I agree as well. Since I tend to be a perfectionist too I will look into parts cost for a full restore. The tech in me always wants to know what's wrong and if it's worth repairing first. This looks to be a rather easy unit to work on unlike the Sansui or Marantz units. I closed my computer/electronic repair shop over 10 years ago and took a job as an IT Network Engineer but due to changes in the industry I decided to leave the corporate world and start my repair business back up. I always enjoy a challenge. There are some great folks here and I hope I can return the favor some day. Thanks everyone for your help...

 

[merlynski]

I agree as well. Since I tend to be a perfectionist too I will look into parts cost for a full restore. The tech in me always wants to know what's wrong and if it's worth repairing first. This looks to be a rather easy unit to work on unlike the Sansui or Marantz units. I closed my computer/electronic repair shop over 10 years ago and took a job as an IT Network Engineer but due to changes in the industry I decided to leave the corporate world and start my repair business back up. I always enjoy a challenge. There are some great folks here and I hope I can return the favor some day. Thanks everyone for your help...

Been there, Done that, highly respect your choices! I have been burned for choosing integrity over submission to improper authority. But HONESTLY I don't regret it!
EDIT: update

 

[kevzep]

I agree as well. Since I tend to be a perfectionist too I will look into parts cost for a full restore. The tech in me always wants to know what's wrong and if it's worth repairing first. This looks to be a rather easy unit to work on unlike the Sansui or Marantz units. I closed my computer/electronic repair shop over 10 years ago and took a job as an IT Network Engineer but due to changes in the industry I decided to leave the corporate world and start my repair business back up. I always enjoy a challenge. There are some great folks here and I hope I can return the favor some day. Thanks everyone for your help...

I am a full time tech too, and I lean towards this scenario believe it or not.
But the more you work on these old girls the more you will realise most issues are component age related. I am a bit OCD and a perfectionist too, but I also like to get the job done in a timely fashion, going around in circles with components which are going to get replaced just wastes time in my opinion.
Case and point, I fully restored a SX1250 recently, and when I went to turn it on, one of the 65V rails on the stabiliser board was only showing 18V, well, this was fairly easy to diagnose and repair because I had fully rebuilt it, the only thing that could pull down the voltage was this ceramic cap over the Base and Collector of the regulator. So I would never have suspected a ceramic cap, but since it was the only thing that could cause the problem I replaced it, and we were back in business, even so it had me scratching my head for a while.
Imagine going in there trying to diagnose this fault with the board unrestored, it could be anything?
There are arguments for both sides of the camp, but really, you are going to restore it anyway, so might as well get in there and do it, then if anything is amiss its going to be very obvious whats wrong.
Working on this gear all the time you get a feel for what those issues are.
But each to their own. We aren't all trying to make a living out of this so its different horses for courses...
What Sansui is difficult to work on? I work on a lot of Sansui's and cant think of a model which is hard to work on.
The Pioneers are well laid out, but those wire wraps can be annoying.
I think any brand has their good models and difficult models to be honest, I can't say I enjoyed the Yamaha CA2000 I did about a month ago...

Good luck with the SX950 anyway.....They are easy to work on and it should be pretty straight forward to get it running.

 

[TechGuru87]

I am a full time tech too, and I lean towards this scenario believe it or not.
But the more you work on these old girls the more you will realise most issues are component age related. I am a bit OCD and a perfectionist too, but I also like to get the job done in a timely fashion, going around in circles with components which are going to get replaced just wastes time in my opinion.
Case and point, I fully restored a SX1250 recently, and when I went to turn it on, one of the 65V rails on the stabiliser board was only showing 18V, well, this was fairly easy to diagnose and repair because I had fully rebuilt it, the only thing that could pull down the voltage was this ceramic cap over the Base and Collector of the regulator. So I would never have suspected a ceramic cap, but since it was the only thing that could cause the problem I replaced it, and we were back in business, even so it had me scratching my head for a while.
Imagine going in there trying to diagnose this fault with the board unrestored, it could be anything?
There are arguments for both sides of the camp, but really, you are going to restore it anyway, so might as well get in there and do it, then if anything is amiss its going to be very obvious whats wrong.
Working on this gear all the time you get a feel for what those issues are.
But each to their own. We aren't all trying to make a living out of this so its different horses for courses...
What Sansui is difficult to work on? I work on a lot of Sansui's and cant think of a model which is hard to work on.
The Pioneers are well laid out, but those wire wraps can be annoying.
I think any brand has their good models and difficult models to be honest, I can't say I enjoyed the Yamaha CA2000 I did about a month ago...

Good luck with the SX950 anyway.....They are easy to work on and it should be pretty straight forward to get it running.

I fully restored a Sansui 8080DB (which is my profile pix) several months ago including replacing all lamps and meter lamps with LED's it's not that it was a difficult receiver to work on but the Pioneer sx950 is well laid out and has ample space to work in compared to the cramped Sansui's and Marantz units. I also have a Realistic STA2100D on the bench which has a bad tuner pot and that has to be one of the more difficult sections to remove. I would prefer to work on the Pioneer compared to any of them...

 

[merlynski]

Look at the schematic, look at the responses for the reported measurements, reply accordingly, what else can you do? Dang gents, we do the best we can . . . I am responsible for my choices . . .

 

[TechGuru87]

Look at the schematic, look at the responses for the reported measurements, reply accordingly, what else can you do? Dang gents, we do the best we can . . . I am responsible for my choices . . .

I hope I did not offend anyone? I have been out of the loop for 10+ years and everyone's replies have steered me in the right direction and convinced me to restore the 950. As I progress back into the profession I'm sure I will have many more questions and problems. Thanks...

 

[kevzep]

I hope I did not offend anyone? I have been out of the loop for 10+ years and everyone's replies have steered me in the right direction and convinced me to restore the 950. As I progress back into the profession I'm sure I will have many more questions and problems. Thanks...

You're all good, no problems at all, there is a wealth of knowledge here, and as we are all individuals we do all work a little differently....

I fully restored a Sansui 8080DB (which is my profile pix) several months ago including replacing all lamps and meter lamps with LED's it's not that it was a difficult receiver to work on but the Pioneer sx950 is well laid out and has ample space to work in compared to the cramped Sansui's and Marantz units. I also have a Realistic STA2100D on the bench which has a bad tuner pot and that has to be one of the more difficult sections to remove. I would prefer to work on the Pioneer compared to any of them...

Yeah they can be a little cramped, there are a few models of the Sansui quad receivers which are like that too, QRX7001/8001/9001 all very difficult.....
But if you do enough of them, you find a way, and just get used to them.....