Marantz 1070 repair

I've recently acquired one of these amps and it has almost been
working.. It will often go for a few minutes and then the protection
relay will cut out, although sometimes it doesn't cut in to start
with. I have traced it through to the protection circuit kicking in
due to DC being present on the output of the left channel. Haven't
been able to trace it further than that yet... When it does work, it
sounds fine.
Any clues on where to start?

 

I'd lean towards coupling capacitors, or the differential pair of pre-amp
type transistors at the amp input stage. A can of freeze - mist might help
narrow it down.
I'm trying to remember - there was a particular type of transistor or two
they had problems with back in those days. I think it was 2SD458, or maybe
2SC1312.

Mark Z.

 

One other component to check is a glass diode on the power amp board.
These would fail fairly often and the symptom was D. C. offset. Chuck

 

Got it sorted... it was mostly a whole lot of dry joints in the power
amp stage, but there was a pair of transistors in a darlington
configuration driving the relay that was a bit dodgy too... The main
one was a 2sc1318