Mark Zacharias wrote in message
...
"N_Cook" wrote in message
...
Hate the things, pa schematic out there but not prea it would seem.
Preamp
problem t'would seem. At switch on a pair of 47uF electros fed via 120K
from
27V rail give increasing voltage that at about 8V , after 4 seconds,
ends
up
putting a 3 to 5V pulse on the preamp outputs , straight into the power
amp,
both channels. This timed hold-off would seem deliberate, presumably
muting
off, but the DC surge?.
Anyway this of course upsets the amp which then has to re-settle,
otherwise
seetles in first second , but now outside its own time-off period and
delays
relay turn on ,or worse , makes the relay chatter , via its DC
loudspeaker
protection cct activating.
What to look for that the preamps , both channels oddly, come out of
mute
with a surge ?




Does the unit use muting transisors or FET's? These can get leaky, and are
often found right at the output.

Mark Z.


I will look into that. The eservice schema, cannot even see whether FETs
anywhere in the C version
As 2 separate "timers" I will add another 120K to one and see if the effect
is the same 2 sec in and 4 sec in.
As it stands they time in near enough together but only one may be at fault
and cross coupled into the other channel, otherwise 2 separate but identical
faults seems iffy. It may be a hf oscillation problem .
When I was scoping the amp output there was short bursts of 150KHz or so at
this 4 second point. Did not think of scoping the preamp then to see if the
hf was there also or just the DC pulse.