Carver M-500T Bias adjust question

[flyingtele]

Hi there,
I have just finished replacing the power transistors in one channel of a Carver M-550T power amp. I replaced both driver and all four output transistors, making sure to leave everything as I found it, keeping track of all screws etc.
That turned out to be my mistake, and as a result I got fire! (thankfully not much, just a single 1/2W resistor)

Turns out I had forgotten to install the plastic screw insulators on the two driver transistors, and as a result I burned the resistor from the power to the collector of one of the drivers as it was shorted to heatsink which is of course earthed. The reason I overlooked this in the first place was the original transistors were fully encased while the new transistors had exposed back plates. I rectified the problem, changed the resistor, and carried on.

Next time I powered the amp up, no sparks or burning, so I proceeded with the alignment. I set up the power supply voltage bias as per the service manual, and then went to adjust the individual bias controls for each channel. The original good channel was fine, hooked my meter up to the test points and no problems.
However, when I connected my meter to the test points on the repaired channel, I got a reading of 0 mV. Specified idle voltage is 6 mV across the test points, but I was reading nil. I tried adjusting the trim-pot but it made no difference.

I put some audio through the amp and both channels appear to be functioning correctly, in both stereo and bridge mode. I fed a 1K sine wave in and compared the output voltages across both speaker terminals and they were within 0.02 V of each other at 1.6 V output voltage.

Something odd I noted was the side that was good got barely noticeably warm but still noticeable, whereas the repaired side had no noticeable change in temp. Could it just be a coincidence that the transistors I installed were so closely matched that there was no idle voltage across the test points, and therefore the bias adjust pot had no real effect, or would my mistake of not having initially insulated the driver transistors had some sort of trickle-down effect?
I'm not overly concerned as the amp seems to be functioning just fine, but it would put my mind at ease if I knew why I couldn't adjust the bias on one channel.

Sorry for making such a long story out of this, I just want to make sure everything is described as well as possible to avoid (or at least reduce) confusion. I can email the service manual to anyone who wishes to have a look, has full schematics, parts list, alignment procedures etc but is too large to add as an attachment. Any thoughts on the topic would be greatly appreciated.
Thanks in advance,
Josh

[Tajzmaj]

There must be something burnt ..... that's 100%
I have some Carver amps and bias adjustment works well......
Check it with scope and dummy load.....maybe you will discover that repaired channel doesn't work properly.
Regards, Taj

[RichP714]

There is something wrong; whether or not it is in the bias generator remains to be seen (sometimes the bias generator adjust has no effect because the problem (elsewhere) is causing an imbalance that's out of the generator's range.

The adjustment should have an effect, even if the polairty balance is perfect; you should be able to set bias from 6mv to something so large that the current will destroy the outputs in a few seconds.

I have a troubleshooting guide for the M-500t here:

http://thecarversite.com/yetanotherf...8961#post18961