[Readily Visible]

Readily Visible wrote:
Arfa Daily wrote:
"mark" wrote in message ...
Readily Visible wrote:
I came into possession of this beautiful old Pioneer SX-838 receiver a
couple of years ago and I would like to fix this problem. After the unit
has been playing for a half hour or so, irrespective of source, the left
channel will drop out. When it drops out, it fades out over a period of
a second or two. It does not cut out instantaneously.

To isolate the problem I switched the preamp-to-main amp connections in
the back so that the left preamp channel feeds the right main amp
channel and vice versa. The left channel continued to drop out. This
tells me that the problem is in the main amp section (speakers are *not*
the problem). I was hoping that the problem was in the volume pot, this
behavior seems to rule that out and points to the left channel of the
main amp section.

If I turn the volume up high enough the dropped channel can be heard
faintly and if I turn it up even higher it will kick back in with a
crackle and play okay for a while before dropping out again. To avoid
blasting the volume out of the speakers when doing this I used the
speaker button on the front to disconnect the speakers. This technique
resurrects the channel as well as when I leave the speakers connected.
This seems significant as turning the volume up past a certain point
resurrects the channel whether or not current is flowing through the
circuit. Apparently, the higher voltage applied to the circuit is enough
to do the trick.

When the channel drops out, I can power down the unit for 10 or 15
seconds and when I turn it on again the channel is still out.

Can anyone suggest to me which type of component(s) might be causing
this behavior, resistor, capacitor or semiconductor? I have the
schematics and I don't see any coils. I don't suspect the power supply
because both channels seem to be powered by the same circuitry and if
one channel went out, both would go out.

I don't have a scope or a signal generator for probing the circuit, just
an analog and a digital meter.

Any help appreciated.
Thanks
Push in the tape monitor switches when the problem occurs, tape
monitor switches are a source of problems on a lot of old stereos they
often need to be cleaned.
Seconded. Next suspect after that has to be bad joints, particularly on any
devices screwed to the heatsink. You can look for bad joints (and to some
extent, dicky connectors) by pressing and tapping on the PCB with some kind
of insulated rod - a piece of wood dowelling for instance. After that, a can
of freezer spray, and a hair drier are your best friends ...

Arfa

Been busy at what is for me the bread and butter work. I picked up a can
of Super Cold 134 today and I am waiting for the left channel to bug out
again. Yesterday, when I didn't have stuff, it was bugging out fairly
regular. Poking at the main amp components didn't seem to have much
effect at this point like it seemed to have earlier.

I will post any significant developments.

Jack

The left channel bugged out again and I double confirmed that it is
*not* the speaker relay. I also double confirmed that switching right
and left preamp outputs to left and right power amp inputs respectively
*does not* change which channel drops out, confirming that the problem
is downstream of these connections. Then, with the left channel out, I
sprayed some freeze onto the tranny I suspected of being at fault, with
no results. I then sprayed every other tranny on the amp board,
including the output trannies. No change. Then I sprayed all the
resistors and caps. Then I gave the whole amp board a general spraying.

I got absolutely no reaction.

The channel continues to respond to an increase in volume, that is, it
kicks back in with a crackle when the volume pot is turned just past the
halfway point.

I guess now is the time to begin probing with a voltage meter on the
right and left sections of the amp board to see where I might find
discrepancies.