Classe CA-150 troubleshooting help

[shawnra]

My first post on this forum and I say Hi to all from Canada. I am working on a Classe CA-150 where on one channel all the output transistors conduct too much current when I turn it on and current ramps up, if I leave it on the output short detection is tripped. Not enough to blow the main fuse but enough to cause the slow start resistor to smoke and heat up good. I thought this has got to be the output is being biased to high from a problem in the base drive bias circuit but I've compared voltage drops across several components with the good channel and they are very close. Could something have blown all the output transistors. Wondering where to start with this. Guess I should pull one of the output transistors and test to see if they are blown.

I have the service manual with schematic.

Any tips and pointers would help.
Thanks

 

[WOPL Sniffer]

The troubleshooting section shows what the outputs should ohm out like. Start with ohming them out. Welcome aboard.

 

[WOPL Sniffer]

You should be able to ohm them out IN circuit

 

[laatsch55]

You've checked bias and DC offset?

 

[WOPL Sniffer]

Should you find a bad one, the book says to replace them all since they are matched sets.

 

[WOPL Sniffer]

I'd Off-Line it first with a multi-meter before putting the power to it again. At least check for major issues. After all, the Output Short light is lit

 

[shawnra]

thank you all for your insight and suggestions. I think I found the issue and it was the tiniest solder bridge between two legs on the 2Sj109 fet pack. they are just so close together, took me forever to see it. I'll fire it up tomorrow and see if changes anything. I think what I was seeing with what was happening with the output was the amp was oscillating.

 

[WOPL Sniffer]

So the amp WAS working until somebody worked on it??

 

[nakdoc]

Short circuit across C110 to force the bias off. Outputs are MOSFETS so they are not shorted. Look for solder connections around the bias trimpot, and the leads to the heat sink mounted bias transistor.