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Default Troubleshooting Pioneer SX-838 receiver - UPDATE

Jack wrote:
Mark Zacharias wrote:
"Jack" wrote in message ...
Jack wrote:
Mark Zacharias wrote:
"Jack" wrote in message ...
Mark Zacharias wrote:
"Jack" wrote in message
...
Jack wrote:
After a week of playing with no problems, the right channel went
out.
It
was the left channel previously. I was dismayed until I remembered
that,
when I reseated the output trannies, I switch them, left for right.
I
was further heartened when that same trick of switching off the
speakers
and turning the volume up to max brought the sound back. Since there
are
a pair of outputs in each channel, switching one of them again and
waiting to see which channel goes out next should tell me which
tranny
is bad.
After switching back one pair of the output transistors, the PNP's,
NEC
B539A's, the problem stayed in the right channel, so I replaced the
right channel NPN, an NEC D287A with an NTE284. It's playing fine at
the
moment. I will button it up and consider it fixed until it tells me
it
isn't.

Jack
If you have not cleaned the relay contacts yet - you are wasting your
time.

Mark Z.
I cleaned the relays a couple of years ago when I tried to tackle this
the first time. Buffed the contacts with 1200 grit wet/dry sandpaper.
This time I tested the relays by bridging them when the bad channel was
out. didn't help. It wasn't the relay. The problem followed the NPN
output transistor when a swapped them left to right and right to left.
OK, but 1200 grit...Wow. The coating on the contacts must not be too
good
anymore...

I've used 6000 grit in the past and even that was too much.

I'm not sure swapping the transistor really revealed the nature of the
problem. As I'm sure you have seen, often we do something, it seems to
fix
the problem, then re-occurs later.

You may very well have both channels cutting out at differnt times.

I'm getting in on this late... have you cleaned the front panel
push-button
controls? They were NOTORIOUS for cutting out on this model.

Mark Z.

Mark,

The problem returned while playing an FM station. I clicked the Phono-1
button and then the FM button again and the channel came right back.
Something new to delve into.
When the left channel went out again, applying slight pressure to the
push button circuit board brought it back. I also noticed that when the
channel was out, touching the metal prong in back that connects the
preamp to the main amp for that channel produced a 60hz hum.


Yeah - definitely spray some De-Oxit into those puchbutton switches.
Fortunately on that model, the rear of the switches are open and accessible
easily. Tilt up the rear of the unit and spray the stuff into the rear of
each switch, and work them in and out.

Mark Z.



I did as you suggested, Mark. It took a while, as I was convinced, due
to erroneous thinking on my part, that the problem was in the main amp
section. I expected the bad channel to switch from left to right on the
balance knob when I switched channels at the "pre amp out" "main amp in"
RCA jacks in the back. It took me a whaile to see things correctly. At
this point I switched channels between the push button input selector
board and the tape monitor board to further isolate where the problem
is, because after treating all the switches and pots, the problem
presented itself again as diminished volume in the same left channel. I
suspect that if the problem remains in the left channel, then the tape
monitor switches are suspect. If the problem changes channel then the
push button switches are suspect. This time I hope my thinking is right
on this.



Well... after switching channels between the push button input selector
board and the tape monitor board, the bad channel switches from left to
right. This indicates to me that push button switches are the problem.
However, when I use the tape playback inputs on the back of the unit and
flip the appropriate tape monitor flip switch, the problem continues.
The tape monitor switch switches on the tape playback jacks and switches
off all input from the pushbutton switches. I will have to look at the
schematics and try to puzzle out this behavior.
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Default Troubleshooting Pioneer SX-838 receiver - UPDATE

Jack wrote:
Well... after switching channels between the push button input selector
board and the tape monitor board, the bad channel switches from left to
right. This indicates to me that push button switches are the problem.
However, when I use the tape playback inputs on the back of the unit and
flip the appropriate tape monitor flip switch, the problem continues.
The tape monitor switch switches on the tape playback jacks and switches
off all input from the pushbutton switches. I will have to look at the
schematics and try to puzzle out this behavior.


Okay, I totally disconnected the signal wires going from the pushbutton
switch circuit board to the tape monitor switch circuit board and I am
playing from the computer through the tape play back jacks. Both
channels are playing fine at the moment, but being that the problem is
intermittent without any way to induce it (that I know of) I will just
have to wait a see again.
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Default Troubleshooting Pioneer SX-838 receiver - FOUND IT!

Jack wrote:
Jack wrote:
Well... after switching channels between the push button input selector
board and the tape monitor board, the bad channel switches from left to
right. This indicates to me that push button switches are the problem.
However, when I use the tape playback inputs on the back of the unit and
flip the appropriate tape monitor flip switch, the problem continues.
The tape monitor switch switches on the tape playback jacks and switches
off all input from the pushbutton switches. I will have to look at the
schematics and try to puzzle out this behavior.


Okay, I totally disconnected the signal wires going from the pushbutton
switch circuit board to the tape monitor switch circuit board and I am
playing from the computer through the tape play back jacks. Both
channels are playing fine at the moment, but being that the problem is
intermittent without any way to induce it (that I know of) I will just
have to wait a see again.


With the two signal wires from the push button switch board disconnected
from the tape monitor switch board, the right channel went out while in
tape monitor mode....(many minutes later) just found a likely suspect...
the -20 db circuit board and the pushbutton switches mounted on it. It
is right next to the push button input switch circuit board and is
dragged down with it when I push down on that board with a chopstick.
The bad channel is much more responsive to pushing down on the -20 db
circuit board than on the push button input switch circuit board, and
pushing the -20 db switch exactly mimics what happens when the channel
goes out... volume on that channel drops way down but is still
responsive to the volume control. I FOUND IT!!!
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