Amplifier Hegel H80 noise problem


2014-08-28 11:03 am
Hi guys!

Bought this beautiful Hegel H80 amplifier and I am satisfied with it. But there is one problem - the amplifier has cracking&popping type noise in the one of the channels, something like we meet in the old receivers due to old and not good transistors (shot/static noise type).

Been checking with an oscilloscope for a while and came to the sweet spot where applied charge temporary disables the noise (by applied charge I mean when touched with a piece of metal, eg., oscilloscope probe, tweezers, screw driver). It is a pair of PF5102 FETs (seems like a differential pair part of the PA? Applied signal is present on Gate and Source pins). When touched one's Drain (goes to the other's Source) it is a pop noise heard and the channel is free of noise for several or tens of seconds. Please see photo below. Then it comes again. By the way, audio signal is present and keeps playing all the time despite the noise.

Screenshot by Lightshot

Tried cooling down with an pressurized air and heat with hot air station these FETs. When heated, it seems like the noise got bigger, but needs more evidence here. Sometimes when amplifier is cold no noise is present though.

Maybe someone was in the same situation and has an advice. Any ideas are appreciated! Can't find any information on that mysterious custom made Hegel IC SE3090. Seems something like a custom driver for power transistors. Going to order these FETs from Ebay as they are discontinued now. But not sure whether it is possible for relatively new transistors have this shot noise like the old ones BJTs.