Pioneer SX-850 Power Amp Board Problems?

[sdrobbins]

I finally got around to working on my SX-850 that has some bad resistors on the power supply board resulting in no relay click. I recapped the power supply, Power Relay and phono boards, also replacing most transistors, installed a new relay, and that issue seems to be repaired.

However, when attempting to adjust the bias, I could only get VR1 and VR2 down to around 16 mV, and the readings would fluctuate (jumping up high after every time I touched the trim pot). I also noticed that the DC offset is fluctuating quite rapidly. Since it was still low enough to be considered safe for speakers, I briefly hooked some up, but was met with a considerable hum from the speaker, that seemed affected if I touched the volume control.

I have already replaced the 4 2SA726 differential transistors on the Power Amp Board with matched KSA992s prior to attempting to adjust bias.

I was going to recap the Power Amp Board and replace the remaining transistors (probably not output transistors?) and hope for the best, but I was wondering if anyone had any suggestions for potential problem areas? Any help would be appreciated.

 

[tsd71]

While replacing anything on the power amp board you should have gone ahead and did the entire board. The trimmers usually are ok but sometimes not, you need to desolder them and check them with a multimeter on ohms. I have attached a guide for transistors and diodes that I use on 850 & 950 restorations and i've done many of them. I posted the left side of the amp, the other side is exactly the same. Also make sure you check the big 5 watt resistors near the top of the board. Good luck!

 

[zebulon1]

I like to replace the trim pots. This way I don't have to worry if they don't test. Cheap insurance. The multi turns are sweet when making your adjustments.
Be really careful with the compensating diode and make sure its secure to the heatsink (you don't need to horse down it tight, just so its snug to the heatsink). The fine leads are delectate and can break easily. Also re-flow the traces on the amp boards.

Tsd71, what ohms does the 850 use?

 

[39cross]

I have attached a guide for transistors and diodes that I use on 850 & 950 restorations and i've done many of them.

just wanted to say, awesome graphic - thanks!

 

[tsd71]

I like to replace the trim pots. This way I don't have to worry if they don't test. Cheap insurance. The multi turns are sweet when making your adjustments.
Be really careful with the compensating diode and make sure its secure to the heatsink (you don't need to horse down it tight, just so its snug to the heatsink). The fine leads are delectate and can break easily. Also re-flow the traces on the amp boards.

Tsd71, what ohms does the 850 use?

100ohm & 10Kohm

 

[zebulon1]

What nice is, its right side RIGHT! You don't need a mirror to figure it out.

 

[sdrobbins]

Thanks TSD71 for the schematic and advice.

I replaced the remaining transistors and caps in the power amp board along with the trimmers, tested the 5W resistors and diodes, and everything looked fine. I plugged the receiver into my dim bulb tester and again everything looked as it should. And then a big however, I plugged the Pioneer in the proper outlet and some smoke immediately came from the power amp board (I think).

After that excitement, I noticed that the heat sink on the one side was quite warm, and that was the end of my corrected speaker click (pins on the power supply board still check out). I did some checking and the output transistor at Q15 was shot. I looked for the source of the smoke and all I can see is burn marks on the yellow wires coming from pin 3 and 18 (don't think they were like that before). I was thinking that perhaps two wires got shorted when moving the board back and forth, but I am plugged back into the dim bulb tester and it seems like the heat sink on that side is still warming up more than the other. I am hesitant to plug in into the wall again.

Well, now I am a bit up the creek. I rechecked a few of the transistors, but cannot fathom where to go from here, I rechecked and it appears everything is installed as it should be.

If anyone has any quick thoughts that may be a big help. For whatever reason this receiver has been beating me up whenever it gets a chance...

 

[sdrobbins]

Well, I went back and checked the Power Supply and Protection circuit boards to try to trace the reason for the no relay click. On the Power Supply board the readings of Pins 2 to 11 are a little above spec. For example, for Pin 6, the voltage is 58.3 (rather than 51.5). These seem slightly above what I would want, as I believe markthefixer mentioned in other posts that within 10% is ok. The one reading that is trouble seems to be pin 12 on the Protection circuit board is practically 0, where I think it should be 7.5V (according to the SX-950 service manual).

I have checked all of the transistors on the protection board and nearby caps and can't find a problem.

Back to the Power Amp board, now I am thinking that instead of the heatsink on the one side being too warm, it is rather that the other side is too cool and the output transistors are not activated. On that side I have pulled and checked every transistor and electrolytic cap and still cannot find a problem.

Any thoughts on the lack of voltage on pin 12, or the what may have gone on the power amp board? Anything would be appreciated...

 

[tsd71]

Well, I went back and checked the Power Supply and Protection circuit boards to try to trace the reason for the no relay click. On the Power Supply board the readings of Pins 2 to 11 are a little above spec. For example, for Pin 6, the voltage is 58.3 (rather than 51.5). These seem slightly above what I would want, as I believe markthefixer mentioned in other posts that within 10% is ok. The one reading that is trouble seems to be pin 12 on the Protection circuit board is practically 0, where I think it should be 7.5V (according to the SX-950 service manual).

I have checked all of the transistors on the protection board and nearby caps and can't find a problem.

Back to the Power Amp board, now I am thinking that instead of the heatsink on the one side being too warm, it is rather that the other side is too cool and the output transistors are not activated. On that side I have pulled and checked every transistor and electrolytic cap and still cannot find a problem.

Any thoughts on the lack of voltage on pin 12, or the what may have gone on the power amp board? Anything would be appreciated...

Pin 12 on the protection board is measured in AC not DC

You also need to check your transistor leg placements, new Fairchild transistors are usually E-C-B, different from the originals. The two transistors on each side attached to the heatsinks are B-C-E, the Fairchild replacements KSA940 & KSC2073 are also B-C-E, the others that are non heatsinked are all E-C-B

 

[sdrobbins]

Right, thanks. Pin 12 does have a good reading.

Now where to go? My only guess is back to the power board to pull and check resistors and diodes, assuming a problem there will result in no relay click and whatever else is going on. Perhaps I should recheck the output transistors again, each tested fine except the one that shorted during the smoke show.

I have been checking and re-checking caps and transistor pin placements as this set of issues really acts like I put something in at the wrong position or value. It is frustrating not to have much to go on for symptoms...

 

[sdrobbins]

Well, I am back for more

I finally found that R72 and R33 were burnt on the power amp board (at the same time?). I replaced each and finally had the relay click back, things were looking up again. However, when attempting to adjust the bias, I noticed the readings are still fluctuating rapidly (caps, transistors, trimmers replaced), but VR1 and VR2 will go below 15mV now. VR3 and VR4 will set to 20mV nicely.

Since things looked ok despite the fluctuation, so I braved plugging it into an old set of speakers. There was noticeable hum on each channel, but music played fine otherwise. I thought I would check for a bad ground connection, tightened some screws again, inspected ground wires, then tried it again with power. This time, same hum, but with a pop as R36 on the power amp board lit. So, it looks like it's back to ye old dim bulb tester (for the pioneer and perhaps myself).

Wondering what route cause is I am thinking is external to the power amp board as each side is burning resistors. Does this sound familiar to anyone who has worked on Pioneer? it is interesting that blown R33 and its match on the other side R36, both could be linked back to the power supply board if I read the schematics correctly. Also I did have a small problem with the power switch earlier where I had to deoxit to get it to turn the unit off. Also, I have not yet changed the main filtering caps, but I am not sure of that could cause this type of problem.

In summary: DC offset in fluctuation, hum, and burning resistors on power amp board, experience needed and appreciated...

 

[sdrobbins]

I took my own advice and rechecked the power supply board. Q1 was shorted and was apparently the cause of all the problems. I found out from another forum that Q1 is a regulator transistor, which seemed to explain why the the dc offset/bias was fluctuating. While I thought I rechecked this transistor previously, but it is possible I overlooked it. Lesson learned: check parts before you install them, even if they are new.

Glad this SX-850 was saved. It actually sounds far better than I was prepared for, I think it will be a keeper!

 

[tsd71]

I took my own advice and rechecked the power supply board. Q1 was shorted and was apparently the cause of all the problems. I found out from another forum that Q1 is a regulator transistor, which seemed to explain why the the dc offset/bias was fluctuating. While I thought I rechecked this transistor previously, but it is possible I overlooked it. Lesson learned: check parts before you install them, even if they are new.

Glad this SX-850 was saved. It actually sounds far better than I was prepared for, I think it will be a keeper!

Hey congrats, nice work! What did you use at Q1?

 

[sdrobbins]

I used KSC2073TU both times. The first one must have been a dud, unless something got shorted along the way without me knowing it.

Thanks, I forgot to add the dancing banana on my last post!