Another Carver PM 1.5 question.

[ElectroAcoustic]

Hello,
I own a small repair business, and a client just brought me 3 Carver PM 1.5's in exchange for some speaker service I did for him. All three are in various states of (un) serviceability. The worst of the three has visible burns at many points on one amp channel, including the driver transistor, the bias circuit, one of the output transistors shorted, various burned resistors, and who knows what else...
The second has no visible burns anywhere, and all outputs test good. However, this one stays in protect mode upon power-up. I have yet to go through the troubleshooting procedures outlined in the service manual on this one, but since everything 'looks' ok, it might not be THAT bad (yeah right...wishful thinking).
Now to the third one, which brings me to this post. I figured I would start with the most hopeful of the trio.

This amp powers up and operates correctly, for the most part. I have already gone through the calibration procedure; adjusted and verified the rail voltages and the +- 12V supply at idle, no signal, no load. All are within tolerance and stable. The bias was a little off on one channel, but after adjustment, has been stable. No early clipping, balanced L-R output, and audio with load sounds great. I have been operating this amp now for about 12 hours and all is stable. There is, however, one SMALL problem that it exhibits.
Upon shutdown, just about the time when the protection LED dims out, an uneven squeal fades in and out through the speakers, both channels, but not ‘mono’. I noticed that the squeal was audible when the input attenuators were 'on' (i.e. NOT fully ccw). I made the assumption that the sound was passing through the input op-amp, and switched out the input module with one of the others. This had no effect. I then suspected the +- 12V supply. There is also a small turn-off thump, though not 'Excessive', it is audible, and the manual suggest that it could be caused by an, 'uneven collapse of the +-12V supply'. I hooked up my Simpson 260 to the + and - 12V supplies, alternately, during power-down, hoping to observe any irregularities. The -12V supply remains steady until the fault LED just about goes out, then it drops slowly to 0V. The +12V, however, immediately drops to 0V at shutdown, then jumps up to +6V when the fault LED dims out, then slowly drops to 0V. I took this as an irregularity, and, since observing that the +- 12V supply is regulated via the +-36V supply, I repeated the same procedure on that supply. I observed an erratic drop from the +36V rail as well, but not directly proportional to the erratic nature of the +12V supply. The +-76V and +-124V supplies both drop fairly smoothly on power-down.
So far, I have checked the two diodes in the +-12V regulator circuit (D13, D14), and both test ok with a dmm. I am about to start going through component by component, which I suspect may be a tedious task.
I am here looking for any insight before I leave my loved ones indefinitely and debark on a quest for a proverbial needle in a haystack.
I know there are several 'experts' on this forum when it comes to Carver PM's, and I graciously beg for any assistance (Anatech, where are you!).
I have been intrigued with the circuitry of these amps since I got them, and they have made for an interesting project and learning experience.
Any advice would be greatly appreciated!
Thanks,
ElectroAcoustic.

[sakis]

e mail bob carver .....they are very helpfull guys try www.sunfire.com or his secretery is a very nice miss sarai fulton
saraif@sunfire.com

[ElectroAcoustic]

Perhaps I was being to specific to the model I'm working on.

If I can ask a more general question about the problem this amp is showing.

To the point, when the amp is powered down, there is a leakage into the input op amp from the draining of the power supply.

I imagine this could happen to any amplifier.

So if this were happening to your amp, what would you be looking for, where would you start?

I could really use a second opinion on this one, and I know this is the place to get the best ones.

ANY ideas would be appreciated.

Thanks,
ElectroAcoustic.