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[email protected] captainvideo462002@yahoo.com is offline
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Default Vintage Pioneer SX-838 receiver loses one channel after warmup

On May 25, 1:08*pm, Readily Visible wrote:
Mark Zacharias wrote:
"Arfa Daily" wrote in message
...
"mark" wrote in ....
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


The OP just got through saying he had narrowed the problem to the main amp
section...


Granted the controls and switches should be cleaned on general principles,
but inn this case I'd strongly suspect tarnished or pitted speaker relay
contacts. A knowledgeable tech can usually revive these safely; a DIY
attempt may just ruin the relay.


The speaker select switch(s) should be cleaned as well.


Mark Z.


Thanks for the careful reading, Mark.

I hadn't even considered the speaker relays. If that *is* the problem, I
should be able to confirm with a simple continuity test when the
condition prevails. Power down and test the left side for continuity.

But upon further thought, the fact that the condition persists through a
power down and power up cycle makes me doubtful that it is the relay,
which physically opens and closes during power down/up.

Also, the fact that the sound does not cut out immediately, but fades
over a period of a second or two would definitely rule out the speaker
relay. Plus the fact that when the volume is turned way up, the bad
channel can be heard faintly.

What intrigues me about this problem is that the I can cause the channel
to kick back in by turning up the volume with the speakers muted. That
indicates that the problem can be overcome by merely increasing the
voltages to the amp circuit by way of increasing the volume, without the
flow of current through the circuit.

What is interesting is what is happening as I type!

The volume is fading out more slowly now, over a period of 3 or 4
seconds and then fading back in. It did it twice since beginning to type
about what intrigues me. Now it just faded back in over a period of
about 15 to 20 seconds!

Real time troubleshooting... now it's playing okay.

One of the fade-outs was so gradual that I thought it was part of the
music on the jazz station, but then it faded back in just as gradually
and the same song was playing. It did that long fade out/in twice.

Still playing okay, but those fade-outs and fade-ins were unmistakably
long and drawn out. Now it occurs to me that it might have been the
station fading in and out for some reason. I have the receiver set to
mono and the balance set all the way to the left channel so I can
monitor it while hearing both channels of music. I don't know what was
happening with the right channel during those drawn out fades. But, as I
now recall, I have noticed this same behavior once before with the CD
player driving the unit.

Now the left channel has done a quick fade out. I will wait to see if if
it revives on it's own.

Ten minutes and the channel is still out. Turning the volume up a bit
past half way kicked it back in. It then kicked out more three times in
well within a minute and kicked back in when the volume was turned up to
the same point, just a bit past halfway.

It seems to be cutting out more frequently after about 30 to 45 minutes
of playing. The last time the cut out was abrupt, as if a switch were
opened.

Time to put it on the desk and bust the case open.- Hide quoted text -

- Show quoted text -


Bad FET's were a problem with some of the older receivers too.
Sometimes a can of cold spray used in small areas at a time can reveal
much. Then reheating the suspect component with a soldering iron can
help nail down the culprit. But a word of caurtion here. Don't
overheat. Lenny