miniman82
Go Navy!
Yes, another one of these amps with DC offset. Actually, it’s hitting the positive rail and won’t come off it. I’m not adept at transistor amps having only messed with tubes most of my life, but just having repaired a Peavey CS-800 has me feeling lucky.
What’s been done so far: soaked the board with alcohol multiple times to get off all the old cap goo, then replaced the bad Elna caps. Replaced all bad output transistors and associated fusible resistors. Found a few more bad resistors (R122 & R135 going to the output stage bases), and replaced them as well.
From what I am able to tell, there’s a problem with Q118(neg driver?). It’s getting quite hot, and I measured +70vdc at its collector, which I’m sure can’t be right. Sure seems like there ought to be about -70vdc or so, but not too sure. I’m man enough to admit when I’m licked, any ideas out there?
My head runs in circles with DC coupled amps, because of the whole chicken and egg situation. Is it hitting the rail because the input stage is telling it to, or is there some other fault causing it to happen and the input stage is fine- know what I mean?
What’s been done so far: soaked the board with alcohol multiple times to get off all the old cap goo, then replaced the bad Elna caps. Replaced all bad output transistors and associated fusible resistors. Found a few more bad resistors (R122 & R135 going to the output stage bases), and replaced them as well.
From what I am able to tell, there’s a problem with Q118(neg driver?). It’s getting quite hot, and I measured +70vdc at its collector, which I’m sure can’t be right. Sure seems like there ought to be about -70vdc or so, but not too sure. I’m man enough to admit when I’m licked, any ideas out there?
My head runs in circles with DC coupled amps, because of the whole chicken and egg situation. Is it hitting the rail because the input stage is telling it to, or is there some other fault causing it to happen and the input stage is fine- know what I mean?
Last edited:
