Crest Pro series issue

[mmaki]

I am having a heck of a time trying to figure out a failure in a Crest 6001. The positive output is collapsing at about 30v :



Imgur: The magic of the Internet

If I lower the gain a bit it will clear up.



If I lower the supply voltage to 85v (120 volt model) it functions normally and actually produces more output.

It is the power trans board because if I sub the other channel board into the same position with the same preamp it works fine.

The limiting functions are all on the preamp board so that doesn't seem to be the problem

Whether the amp has a load on it or not doesn't make a difference. The 4 power supply rails are solid and don't move. The hi rail positive transistors are conducting and if they weren't I think I would see a flattened wave and not have half a positive wave.

I have measured voltage drop across most diodes and capacitors under normal and failure conditions and haven't found anything yet that is opening or shorting.

I am running out of ideas

 

[basreflex]

there are some miller pole splitting caps under suspicion. the collector base caps of the HV voltage gain stage could be the culprit.

 

[Enzo]

A schematic is more helpful, but if the transistors are OK< I would check ALL resistors for opens. On that power transistor board.

 

[turk 182]

check D32,33,34,35(FR204) also Z8,9(1N5927B) and C34,35(47uf 16v) of those the caps in my experience are the most likely culprits.

 

[mmaki]

A schematic is more helpful, but if the transistors are OK< I would check ALL resistors for opens. On that power transistor board.

CRESTAUDIO PRO 6001 Service Manual download, schematics, eeprom, repair info for electronics experts

Here is a link to the schematic. The same site also has a full service manual, which is thorough but hasn't help me in this case.