2325 Bias/idle current issues

msabol

Newbie/Just learning
Hello all! I did some searching through the threads here for my problem to see if this question has been raised before but I didn't find it, so here goes.

I've got a 2325 with new output transistors, they appear to be On Semi mj15025g/mj15024g. I don't know why they were replaced or who did it but all eight devices were replaced. The power supply board was recapped and a new relay was installed. So far so good. While checking DC offset and idle current I found that I was unable to adjust the idle current at all, really. My multimeter would read the same, for the most part, through the entire range of the adjustment pot. It seemed that the pot only had a range of 10mV. The other 2325 I'm working on right now seemed to move a bit more when the pot was adjusted. So....

My question is do the On Semi transistors that were used to replace the original behave just the same as the originals? Should I be looking for the 15mV across the test points or do these transistors need a different bias setting? And if they do need a different bias how do I figure out what it needs to be.

I'm also seeing my readings jump all over the place, almost as if a little AC snuck in there. Both the DC offset readings and the idle current readings are hopping around. Input is on AUX with nothing connected and the jumpers are out. I'm wondering if there is typical resistor or transistor upstream that can start to fail and cause that kind of jumping around or if this behavior is more or less normal.

I apologize for the rookie questions and would appreciate any input or insight the community might have to offer. Thanks so much.
 
There's a lot of things that can cause inadequate bias current, but the replacement transistors installed in your amp are fine.

Because the output stage is a common emitter design, you can measure bias by connecting your meter directly to the collectors of the output transistors to make sure that you are getting a proper reading.
 
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