Marantz 250...too much DC on output and no signal

[phatterplatter]

Hello techies

I have a clients 250 here. One side is fine and adjusts per spec

The other side has over 4.00 DC on it and I've replaced a number of the smaller lytic caps and the first pre-driver transistor. It still has over 4.00 vdc out . The power transistors are ok with the exception that one has quite a bit diff gain than the others. Would THAT be the culprit?

Thanks

 

[catrafter]

There are many variations of the 250, here's somethings to check using the data I have.
CR501 and CR502 are 17V zener diodes which supply, among other things, the proper voltage to R502, the Zero Offset adjustment pot. If one of those are off, you won't be able to set the DC offset. Check or replace C503 and C504, too. The differential pair Q501 and Q502 can also affect the DC balance, as can many other components in the rest of the amp. Remember, the upper and lower amp 'halves' (schematic wise) must balance around the center point (R531, R532 junction) for there to be zero volts on the output terminal.
I doubt that the output transistor is the problem unless it is leaky. As a test, you can test for offset with just one NPN and one PNP output transistor installed and no heat sink for a short period of time, just don't drive it with an audio signal.

Tom

 

[wlhd1610]

Excellent analysis as usual Tom!!!

Bob

 

[phatterplatter]

Thanks so much...I'll be getting diodes in short order