• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

SA-8800 with Bias Splitter problem

@markthefixer: Hi Mark, I understand you are dealing with a lot - please get well soon! In any case I have not posted in a while because of multiple reasons, not the least of which was trying not to impose on you. So I had started writing a post yesterday based on what seems to be a whole paradigm shift in my understanding of what the actual problem might be...

A New Start:

After troubleshooting this Right amp channel for mystery DC bias splitter problems, and shotgunning a whole bunch of parts to no avail, I put it aside for a week or so. I finally came back to it and said "wait, wait, wait, wait, I'm being a dope, what about AC"? And damned if there isn't a significant 60Hz waveform floating around on this Right channel. I'm still beating my head against the wall trying to figure out where it's coming from, here's where I think I am...
  • Grounding problem? - every ground I can think of to check, all measure 0 ohms to chassis
  • Amp board or power supply? - The Left channel power amp, and the preamp for that matter, are dead quiet and are working fine. So it seems to me it has to be something located on the Right channel half of the power amp board. Both channels share the regulated DC rails so they seems to be ruled out, plus they measure clean, no AC, no 60Hz. The Right channel unregulated supply also measures quiet... right? Right? Except, when I put the scope on it... - no, holy crap. The Left channel main filters C5-C6 only have 120Hz ripple, pretty typical sawtooth, in general seem "good enough", fine, and besides the Left power amp channel is working well. And the Right channel +50V rail C7 looks pretty much the same as the left. But Right channel -50V rail C8 looks a hot mess. The DC measures fine and it holds charge just fine when power is off. But it's putting out all kinds of noise and hash on the rail and while the 120Hz sawtooth is discernible, it's much smaller (+/-4mV) and the scope has a hard time even capturing a frequency because everything is jumping around so much. Anyway, it may or may not explain where 60Hz is coming from in the Right channel, but it CLEARLY needs to be addressed and I am putting this whole thing on hold until I get new caps and rectifier diodes installed on the Power Supply board. So all 4 main filters and their associated 3A rectifiers will be replaced.
  • Input? - All the testing is being done with the Pre Out/Power Amp In jumpers pulled, and 600 ohm shorting plugs in the Power Amp In jacks. No AC detected within the input network (R52, C34, R54, C36, Q8, C38, D2, VR6, R60)
  • So where is the AC waveform? 60Hz Sine wave (300mV Pk-Pk) after the diff amp (but not before), and within the VAS stage, then becoming a larger 60Hz Square wave after the VAS stage and through the Drivers. Measures around 2.7VACrms (using both Fluke 87V and Agilent 34401). Been pulling the film and ceramic caps, haven't found anything that measures badly (Peak Atlas LCR45).
So more to come - will report back sometime next week after refurbing the unregulated supplies.
 
GlasgowGrip said:
@markthefixer: Hi Mark, I understand you are dealing with a lot - please get well soon! In any case I have not posted in a while because of multiple reasons, not the least of which was trying not to impose on you. So I had started writing a post yesterday based on what seems to be a whole paradigm shift in my understanding of what the actual problem might be...

A New Start:

After troubleshooting this Right amp channel for mystery DC bias splitter problems, and shotgunning a whole bunch of parts to no avail, I put it aside for a week or so. I finally came back to it and said "wait, wait, wait, wait, I'm being a dope, what about AC"? And damned if there isn't a significant 60Hz waveform floating around on this Right channel. I'm still beating my head against the wall trying to figure out where it's coming from, here's where I think I am...
  • Grounding problem? - every ground I can think of to check, all measure 0 ohms to chassis
  • Amp board or power supply? - The Left channel power amp, and the preamp for that matter, are dead quiet and are working fine. So it seems to me it has to be something located on the Right channel half of the power amp board. Both channels share the regulated DC rails so they seems to be ruled out, plus they measure clean, no AC, no 60Hz. The Right channel unregulated supply also measures quiet... right? Right? Except, when I put the scope on it... - no, holy crap. The Left channel main filters C5-C6 only have 120Hz ripple, pretty typical sawtooth, in general seem "good enough", fine, and besides the Left power amp channel is working well. And the Right channel +50V rail C7 looks pretty much the same as the left. But Right channel -50V rail C8 looks a hot mess. The DC measures fine and it holds charge just fine when power is off. But it's putting out all kinds of noise and hash on the rail and while the 120Hz sawtooth is discernible, it's much smaller (+/-4mV) and the scope has a hard time even capturing a frequency because everything is jumping around so much. Anyway, it may or may not explain where 60Hz is coming from in the Right channel, but it CLEARLY needs to be addressed and I am putting this whole thing on hold until I get new caps and rectifier diodes installed on the Power Supply board. So all 4 main filters and their associated 3A rectifiers will be replaced.
  • Input? - All the testing is being done with the Pre Out/Power Amp In jumpers pulled, and 600 ohm shorting plugs in the Power Amp In jacks. No AC detected within the input network (R52, C34, R54, C36, Q8, C38, D2, VR6, R60)
  • So where is the AC waveform? 60Hz Sine wave (300mV Pk-Pk) after the diff amp (but not before), and within the VAS stage, then becoming a larger 60Hz Square wave after the VAS stage and through the Drivers. Measures around 2.7VACrms (using both Fluke 87V and Agilent 34401). Been pulling the film and ceramic caps, haven't found anything that measures badly (Peak Atlas LCR45).
So more to come - will report back sometime next week after refurbing the unregulated supplies.
Click to expand...
It's been a long time, but did you manage to solve the issue?
 
fede65 said:
It's been a long time, but did you manage to solve the issue?
Click to expand...
Hey @fede65, thanks for asking. So this little bugger is still sitting on the shelf looking at me. I had to buy parts to refurbish the unregulated DC supplies, which put it on hold. I eventually did purchase the parts, but I never got back around to working on it again. One of these days, hopefully soon...

Anyway, sorry I don't have better news for you, but I am still pretty optimistic that I had finally identified the real problem. Of course I still need to fit the new Right channel output devices to the heatsink, and I want go back and re-read through all of my notes and all the NSA posts I have bookmarked here on the AK Pioneer forum.

I'm in the middle of completely tearing apart my shop and building a custom bench (with a side desk and shelves and power distribution and fume extraction, etc). And then I have a couple of Marantz receivers to knock out first, but then I would LOVE to get back into this darned Pio and see if the bad power supply was the last remaining problem.
 
You must log in or register to reply here.
Back
Top Bottom