"N_Cook" wrote in message
...
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.






Yes, the timing certainly suggests this. At the end of the muting cycle, a
large DC bias is applied to the base or gate. With the drain or collector
sitting across the output to ground, this leaky transistor will then pass DC
to the output. It has been a relatively common failure on Pioneer and Yamaha
receivers, and I've seen it on Hafler preamps.

Mark Z.