power amp 2325

calman46

AK Subscriber
Subscriber
Should a 2sc496 transistor test open when hot (out of circuit)? .I can't control the idle current on the Left power amp board (marantz 2325) and the trim pot is good. The 2SC496 is part of a dynamic bias circuit with temperature sensitive components so it maybe working correctly . This board was repair and changed before and I want to get it back to original state. Anybody familiar with this ?
 
Transistors should never test open (if you're talking about the base-emitter junction). If this is the VBE multiplier transistor, then it is indeed there for thermal tracking, but it certainly doesn't 'open up' at temperature.
 
Put your DC voltmeter between the bases of the class-AB driver transistors. The bias circuit is designed to control the voltage between these two points.

If this voltage gets bigger or smaller as you adjust the bias pot, then you can conclude that the "uncontrolled" current is a function of the drivers or outputs. If the voltage does not get bigger or smaller, then the problem is in the bias or class-A stages.

If the 2SC496 is open, you would probably FRY your output transistors and very like injure your drivers as well.

Now . . . here is an important question. What is the bias current right now, as determined by measuring the DC millivolts across one or both emitter resistors?

Fred Longworth
 
I can't take any readings now. The outputs get way to hot to quick and R744 glowed red. I removed the outputs and they test ok . Can I run the board without the outputs? Could it be the + and - rails could be shorting together . H708 and H709 (drivers) shorted at the BE junction.I replaced H714.
 
Based on what you're saying, Calman -- that it gets super, super fast -- I would replace the 2SC496 immediately.

Fred Longworth
 
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