Rotel RA985 BX problems

Here is a Rotel RA 985 BX come to the shop on protection mode .
Quick look presented about 500mv of offset in the output , no bias , and no output and fuses fine .

first think to look given that fuses was OK was the small resistors of 100-220 R if any open to bring the amp out of balance ...no findings :mad:

Then was a quick look for open or shorted semis ...no findings :mad:

Then was voltage drop at various stages which plenty of unbalanced readings was there and parts swapping started from the good channel but with absolutely no results ....:mad:

Parts swapping is something that i don like to do one way or another had to swap everything a job that consumed 9 hours on the variac and after one part swap i had to repeat all measuring ...no results :mad:

When i was done and ready to throw the amp out of the lab i asked one of my boys to remove all miller capacitors ( electrolytics have been checked/replaced before ) and he founds a 330pf from bias to ground shorted ...

replaced the capacitor which actually if shorted shunts negative side drive to ground , shunts bias also , and effects the current source that supplies positive side of 1/2 of the LTP .... IMPOSSIBLE TO TRACE !!!!

here is a pic of the schematic and someone please explain to me WTF this capacitor does ...

Kind regards
Sakis

Attached Images

ra985bx.JPG (87.0 KB)