Working on an Elka AC1280R guitar amp. Speaker coil has gone open cct due to intermittent spikes, almost -ve rail voltage, at the O/P.
Since starting work on the amp the spikes have subsided but O/P rests at -2V DC. On a cold startup, when filter caps are flat, O/P will briefly peak to around +15V for a few secs then sits steady again -2V.
Hooking up an external speaker a really harsh crackling sound is heard right through the program signal or when sitting idle and the -2V persists.
Disconnecting the preamp from the power amp the O/P will drop to around -65mV and the crackling gets quieter.
Switching the amp off the O/P will slowly rise from -2V to around +15V unless the speaker load is connected which dissipates this and the O/P comes to rest at 0V.
The O/P measures 0V AC at all times while idle.
All the transistors are measuring 500-650 mV across their junctions as are the diodes. Supply rails are a constant +/-48V.
I've been right accross the power amp board twice and resoldered every nipple, this is what has brought some stability to the amp.
I'm considering replacing the 2 output and 2 driver transistors. Can power transistors like that get stressed and even though they measure ok still not function correctly?
Any experienced advice with this would be greatly appreciated. Cheers.