Troubleshooting Pioneer SX-838 receiver - second posting, mistakes corrected

[Jack]

This is a great piece with one little problem. Often, when switching the
unit on, the left channel is out, no sound. Turning the volume up always
kicks it in. Often the volume has to be turned up severely to accomplish
this so I tried doing it with the speakers switched off. Switch off
speakers, turn volume up to max, turn volume down to min, switch
speakers on, turn volume up a bit and the sound is back in both
channels. In addition, after getting both channels to work, after 10 to
20 minutes, give or take 5, the left channel goes out again.

This behavior lead me to suspect the volume switch, so I flooded it with
switch cleaner and worked it, but to no avail. Problem persisted. Next I
checked all the cold solder joints and found one that was loose. I hot
soldered ALL the cold solder joints. Problem continued. Today I used
some cables to switch the PRE OUT channels into the POWER IN channels,
left to right and right to left. The next time the problem cropped up it
was still in the left channel. This is telling me that the problem is in
the left main amp section.

Given the behavior described above I can only guess that a rise in
voltage in the main amp produced by turning up the volume with the
speakers turned off somehow corrects the condition causing this problem,
but at the moment I don't know what this condition might be. I assume
that with the speakers turned off there is not a lot of current flowing
in the main amp when the volume is turned up, but that the voltage is
increased. As I write this, a dawning suspicion presents itself and that
is: the problem might be with a transistor, as they are voltage regulated.

Next step is to open up the unit and do a visual inspection, hook it up
to a pair of speakers and turn it on and attempt to compare voltages
along the signal path of right and left channels of the main amp when
the problem crops up.

[Dave Platt]

In article <jsj0vq$ev$1...@dont-email.me>, Jack <@noway.com> wrote:

>Given the behavior described above I can only guess that a rise in
>voltage in the main amp produced by turning up the volume with the
>speakers turned off somehow corrects the condition causing this problem,
>but at the moment I don't know what this condition might be.

One thing to suspect in cases like this, is a bad electrolytic
coupling capacitor between one stage and the next.

Bad output-protection relays can also cause this sort of "turn it up
far enough and it starts working" problem.

Usual next step is to use a signal tracer / oscilloscope, and see how
far through the output circuitry you can detect audio when it's in the
"not working" condition.

--
Dave Platt <dpl...@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

[Tim Schwartz]

My vote is for bad relay contacts on the speaker protection relay. Try
tapping on the relay with an insulated rod, like a small stick while the
unit is on and the channel is out. If tapping on the relay gets it to
come on, then clean or replace the relay. You have to take the relay
apart to clean it properly.

On the other hand, it could be other switches and controls being dirty.
Have you tried moving the speaker switches and tape monitor buttons
while the problem is there?

Regards,
Tim Schwartz
Bristol Electronics

[Jack]

Dave Platt wrote:
> In article <jsj0vq$ev$1...@dont-email.me>, Jack <@noway.com> wrote:
>
>> Given the behavior described above I can only guess that a rise in
>> voltage in the main amp produced by turning up the volume with the
>> speakers turned off somehow corrects the condition causing this problem,
>> but at the moment I don't know what this condition might be.
>
> One thing to suspect in cases like this, is a bad electrolytic
> coupling capacitor between one stage and the next.
>
> Bad output-protection relays can also cause this sort of "turn it up
> far enough and it starts working" problem.
>
> Usual next step is to use a signal tracer / oscilloscope, and see how
> far through the output circuitry you can detect audio when it's in the
> "not working" condition.
>

Hi, Dave, thanks for the suggestions... I just opened up the unit and
have it sitting here in front of me and I was just about ready to do as
you suggest, see how far through the circuit the signal goes, except
that I don't have a signal generator. I plan to just turn the unit
ontune in to an FM station and probe for voltages. On the schematics I
see where the output from the power transistors goes through the relay
before going to the speakers. I plan to probe both sides of the relay to
see what I get. I will post the results.

[Jack]

Hi Tim, thanks for the feedback... there is no reaction from operating
the speaker and tape monitor switches while the problem is present and
tapping the relay gets no reaction, so I am going to probe for voltages
now. Will posts results.

Jack

[Jack]

Dave Platt wrote:
> In article <jsj0vq$ev$1...@dont-email.me>, Jack <@noway.com> wrote:
>
>> Given the behavior described above I can only guess that a rise in
>> voltage in the main amp produced by turning up the volume with the
>> speakers turned off somehow corrects the condition causing this problem,
>> but at the moment I don't know what this condition might be.
>
> One thing to suspect in cases like this, is a bad electrolytic
> coupling capacitor between one stage and the next.
>
> Bad output-protection relays can also cause this sort of "turn it up
> far enough and it starts working" problem.
>
> Usual next step is to use a signal tracer / oscilloscope, and see how
> far through the output circuitry you can detect audio when it's in the
> "not working" condition.
>

I just bridged the left channel contacts on the relay and got nothing,
so the problem appears not to be with the relay itself. There is a
circuit on the schematics called the "Protection Circuit Assembly", the
sole purpose of which appears to me to be to activate the relay. Seeing
as how the relay activates I don't see that as being the source of the
proble so I will proceed to the left channel of the main amp section.

[Jack]

I tried probing DC voltages across speaker output and ground at each
channel with a digital meter with the speakers connected and unconnected
and the readings were essentially the same, under 10 mv, left and right,
speakers connected and unconnected. I suspect that this is not an
effective test.

[Jack]

I downloaded a 1000HZ wave file and this is proving useful for probing
voltages. What I have done so far is to measure the voltage across the
output legs of the paired output transistors. Balance is set to top dead
center, equal balance. With no signal present the right channel, the
good one, measures 16.3 mv. The left channel, the bad one, measures
16.6mv. With the 1000HZ wave file input into the tape monitor, the good
right channel measures 61.8 mv. The bad left channel measures 16.6 mv.

Measuring across the output terminals of each of the 4 output
transistors and ground, I get half the value of measuring across the
outputs of the paired right transistors on each of the the paired right
transistors and the same for the paired left channel output transistor.
So this is consistent.

My next step is to somewhere between the input and the output of the
main amp board and compare voltages between right and left. I'll post
the results.

[Dave Platt]

In article <jskupl$hn6$1...@dont-email.me>, Jack <@noway.com> wrote:

>I tried probing DC voltages across speaker output and ground at each
>channel with a digital meter with the speakers connected and unconnected
>and the readings were essentially the same, under 10 mv, left and right,
>speakers connected and unconnected. I suspect that this is not an
>effective test.

Well, it tells you that you don't have a serious DC fault in the
output stage, which is good news. Some sorts of failures can send the
full DC rail voltage (or near to it) to the output, and this can very
rapidly damage the woofers (high current flow, voice coil heats up,
magic smoke leaks out, bad smells occur, fire or burnt-out coil
results).

You're almost certainly going to need some way of actually looking at
the audio signal. You *might* be able to use an AC voltmeter, but an
oscilloscope will make the state of the signal a lot easier to
visualize. In either case, some sort of audio input signal will be
needed... FM tuner tuned to a station, or tuned in between stations
with the muting turned off, or a signal from a CD player, or etc.

Hint: if you have a CD burner, and a copy of Audacity (or a similar
audio-editing package) you can make your own "signal generator" for
cheap. Use Audacity to create a stereo 44.1 ksample/second 16-bit
audio file, containing a test tone (e.g. 1000 Hz). Store this on disk
and burn it as a one-track audio CD. Put it in your CD player, set
the player on "repeat", hit Play, and you've got a nice clean
consistent audio waveform that you can feed to the receiver and then
trace through the circuitry.

If you use a lower frequency (e.g. 100 Hz) it'll be within the
specification range of almost any simple AC voltmeter, and should read
out properly... you can trace the signal through both channels, and
see where the differences first appear by comparing the voltage
readings.

Try to borrow an o'scope, though... you'll see more about what's going
on, than you can deduce from voltmeter readings.

[Jack]

I downloaded a 30 second 1000HZ sine wave in 44100/16 format and I have
been playing that through the computer into the tape monitor playback of
the receiver.

>
> If you use a lower frequency (e.g. 100 Hz) it'll be within the
> specification range of almost any simple AC voltmeter, and should read
> out properly... you can trace the signal through both channels, and
> see where the differences first appear by comparing the voltage
> readings.

I'll download the 100Hz signal now.

>
> Try to borrow an o'scope, though... you'll see more about what's going
> on, than you can deduce from voltmeter readings.

My main task is to determine where to probe. I decided that the
transistors that feed the outputs transistors is where to go next but
until I can get hands on an o'scope... and learn to use it, it's been a
while, I will have to rely on the volt meter. The smallest scale for AC
is 200 volts. I'll see if the 100Hz tone can be read with it.

[Jack]

Dave Platt wrote:
>
> If you use a lower frequency (e.g. 100 Hz) it'll be within the
> specification range of almost any simple AC voltmeter, and should read
> out properly... you can trace the signal through both channels, and
> see where the differences first appear by comparing the voltage
> readings.

At the output stage, playing the 100Hz tone, measuring across a single
transistor and ground, using the 200v AC meter, with the speakers turned
off, I read about 38-39 volts at maximum volume, so this may be useful.
Thanks for that tip. But since I turned the volume to max, the problem
has disappeared so I have to power down the unit until it returns. In
the meantime I will map out my probe points.

[Jack]

I found where the value of the AC voltages diverge between the same
transistors on the right and left channel. There are a pair of 2SA726's
in each channel and the value for these is exactly the same across all
pins in both channels. The next transistor down the signal path is a
single 2SC1451 in each circuit. The AC voltages of all the pins reads
0.0 volts except for the collector on the good right channel and that
reads 2.6v

Next down the path are two each 2SD358 and 2SB528, one in each channel.
The voltages are as follows (these were read at a lower volume setting
than those above):

2SD358
Right Channel----- E 4.4 -- B 5.1 -- C 90.3
Left Channel----- E 0.6 -- B 1.8 -- C 90.4

2SB528
Right Channel----- E 3.2 -- B 2.6 -- C 0.0
Left Channel----- E 0.0 -- B 0.0 -- C 0.0

Now I have to puzzle out what to do next :-)

[Jack]

I just realized that when taking the AC voltage readings on the 2SA726's
I had to turn the volume up to accommodate the 200 v scale on the meter
and this caused the problem to disappear, so those readings that were so
perfect have to be thrown out because the problem did not exist when I
took them. (grrr) The readings for the 2SC1451's are good.

[Jack]

Last night I pulled heat sink for the last stage output transistors to
get a good look at the back of the power amp board. This involved
moderate flexing of wires coming to and from the board. All the solder
joints looked sound. Apart from taking a soldering iron to all of them,
I could see no obvious need for attention so I put everything back
together. After that the left channel, which was cutting out
consistently, either at startup or after ten minutes or so of play after
bringing it back by turning up the volume, was now playing without
cutting out. This morning both channels started up and both have been
playing for an hour or so.

Last night, after an hour or so of consistent play, I took more
measurements of the transistors of the second to last stages, playing a
100Hz tone, and found that the AC voltages of the emitters and bases of
the left channel were half or less than those of the collectors and
bases of the right channel. These transistors are 2SB528 and 2SD358


2SB528
L Channel -- E 4.0 -- B 3.4
R Channel -- E 9.2 -- B 8.6

2SD358
L Channel -- E 5.2 ---- B 5.8
R Channel -- E 10.5 -- B 11.1



This morning, playing a local jazz station in mono mode, I noticed that
the left channel is lower in volume and the high is missing, cymbals
disappear. I switched speakers to make sure they weren't the problem and
they weren't. I switched the preamp out and power in jacks on the back
to determine if this was a preamp or main amp problem. The problem
stayed in the left channel. My guess is that these problems are related.

NTE has replacements for 2SB528 and 2SD358, being NTE292 and NTE 291
respectively. Fortunately for me, I live close to Al Lasher's
Electronics and they have both in stock. Being that the transistors in
the next lower stage all measured the same when the cutting out problem
was not manifesting itself and the these particular transistors have
widely divergent readings, the problem would appear to be the
transistors themselves or some associated diodes, resistors or capacitors.

[Jack]

Since I felt I had narrowed down the problem area to between where the
transistor readings were consistent right to left, measuring both AC
voltage to ground while playing a 100Hz tone and measuring resistance
with the power down and to where the readings were inconsistent. Based
on advice from this group I replaced a couple of electrolytic caps with
no improvement in the loss of treble in the left channel, which I became
aware of while troubleshooting the cutting out of the left channel. Plus
the left channel was again not playing when powering on. This is when I
decided to change the 2SB528 and the 2SD358.

I first swapped out the 2SB528 for the equivalent NTE 292 and powered
up. The left channel was out, but turning up the volume brought it back
as, as usual. There seemed to be a marked improvement in the treble but
that might just be wishful hearing. At any rate I decided to change the
2SD358 since I was able to get both NTE equivalents at Al Lasher's
Electronics. I unscrewed the heatsink and pulled it, and with soldering
iron in one hand and the transistor firmly grasped between thumb and
forefinger of the other, getting ready to melt the first of three blobs
of solder, I noticed the base pin on the 2SD358 was wriggling slightly
in its blob of solder. The joint was cracked! And that is when I stopped
to record this event right here, in real time. I'm torn between
resoldering the joint and changing the transistor. I will resolder it.
That's quicker. And then we will see what we shall see. Here
goes.......... okay, done. Well, the left channel started right up at
power on, but it is obvious that the high end is still gone and the
volume is still still weak. So I will swap out the 2SD358 for the NTE
291. Done. Treble still gone in left channel, but recording software is
showing the two channels equal in strength, both being a fraction of a
decibel off peak while playing a stereo jazz station, so the what I
perceived as a lack of volume might have been just a lack of treble.

I am going to assume that the problem of the left channel cutting out
was caused by the broken solder joint of the base pin of the 2SD358 and
concentrate of getting the treble back in that channel.

[Jack]

Left channel just bugged out again... waaah

[Chieftain of the Carpet Crawlers]

On Sat, 30 Jun 2012 17:55:10 -0700, Jack <@noway.com> wrote:

>Left channel just bugged out again... waaah

TIGHT! speaker post connections. RCA plug? THAT is the problem.

Otherwise, you need to make SOLID connections.

Oxidation on mechanical connections is usually the culprit.

Then Caps and things. Look at the connections first.

Or you could just buy a modern Pioneer for cheap. Even their upper
crust stuff ain't all that expensive.

Visit some garage sales!

[Jack]

Hi Chief, the problem is in the main amp section, of that I'm fairly
certain. I ruled out speakers for that left-channel-cutting-out problem
long ago. However, speakers WERE the problem for the loss of treble I
posted about earlier, but it was not loose connections. I was using some
humongous White Van speakers I bought used for $20 a while back. I
mainly use them stacked to support one end of a pull-up bar. One seems
to be not as trebly as the other. But with the system hooked up to some
good Kliptsch speakers both channels are singing beautifully. While
playing a 100 Hz tone, all AC voltages read consistent from left to
right after changing some electrolytic caps and intermediate stage
transistors that had inconsistent readings earlier. I swapped the output
transistors from left to right. Maybe reseating them might have solved
the problem, only time will tell. At the moment both channels are
singing. The receiver is open, top and bottom and sitting on one end and
that is how it will stay for the next week or two. But I'm pretty sure
that no matter how long I wait, the problem won't occur again until I
button it up.

It's not so much that I need this receiver. It's that I feel challenged
by the problem.

Jack

[Jack]

After spending the weekend up to my elbows in this receiver it is
playing beautifully through both channels and the left channel hasn't
cut out since I reseated the output transistors, which I am hoping is
what was causing the problem, because that means I fixed it.

To Tim Schwartz, I can see how the speaker switch may have been causing
the problem since it is between the main amp and the speakers, and if it
crops up again I will go to that next by jumping around it. I know it
isn't the relay because I jumped that and the problem persisted. And
looking back at the last time I posted about this 3 years ago I pulled
the relay and buffed the contacts with exceedingly fine wet/dry sandpaper.

And thanks to Dave Platt for the tip about playing a 100Hz tone to probe
for AC voltages. That was a real point in the right direction for me.

All in all, I learned a lot this time around. I may even try to
resurrect that Harmon Kardon 930 a friend of mine gave me a while back.
It eats 3 amp speakers fuses in one channel.

[Jack]

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.

[gregz]

I have seen small transistors go intermit ant, but never outputs. Sounds
like you got it.

Greg

[Jack]

It does sound slam-dunk, doesn't it? Too bad these SX-838s don't command
more money on eBay. I'm selling some stereo stuff that I don't use
anymore. There is a thriving market there for some stuff. I saw an old
McIntosh tube preamp that was being sold for parts go for over $400.

Well, now I play the waiting game. Sometimes it goes out every 15
minutes, sometimes it takes weeks.

Jack

[Jack]

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

[gregz]

Looking at pictures of it, I don't see anything that would command a high
price.

Greg

[Mark Zacharias]

"Jack" <@noway.com> wrote in message news:jtnp3c$dcf$1...@dont-email.me...

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

Mark Z.

[Jack]

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.

[Mark Zacharias]

"Jack" <@noway.com> wrote in message news:jtqirc$g96$1...@dont-email.me...

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.

[Jack]

Oh, well...

>
> 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.

The right channel never cut out until I swapped transistors. After I
swapped transistors, ONLY the right channel cut out. You have to go on
something and that seemed fairly compelling.

>
> 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.
>

I bypassed all that by using a standalone preamp to drive the power amp.
Problem persisted.

Jack

[Jack]

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.

[Jack]

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.

[Giganews]

"Jack" <@noway.com> wrote in message news:jsj0vq$ev$1...@dont-email.me...

> This is a great piece with one little problem. Often, when switching the
> unit on, the left channel is out, no sound. Turning the volume up always
> kicks it in. Often the volume has to be turned up severely to accomplish
> this so I tried doing it with the speakers switched off. Switch off
> speakers, turn volume up to max, turn volume down to min, switch
> speakers on, turn volume up a bit and the sound is back in both
> channels. In addition, after getting both channels to work, after 10 to
> 20 minutes, give or take 5, the left channel goes out again.

The SX 838 was a great machine in its day, which if memory serves was about
40 years ago.

Today you can get an equivalent 100 wpc stereo receiver for under $100.

I recently investigated local summer clearance sales and obtained a
full-fledged 100 wpc 5.1 receiver from a top manufacturer for only a little
more than that. It was easy to configure as a proper 100 wpc 2.1 channel
system control center with A/V source selection, as well as digital and
analog audio inputs with separately adjustable delay, gain and a 7 band
equalizer for each speaker. The internal AM and FM tuners were among the
best I have ever had.

[chuck]

As Mark said, the switches on the front are a major problem and need
to be cleaned. I would buy a can of Deoxit 5 and spray it into all
the switches and work the push buttons in and out. Chuck

[Jack]

Jack wrote:
> This is a great piece with one little problem. Often, when switching the
> unit on, the left channel is out, no sound. Turning the volume up always
> kicks it in. Often the volume has to be turned up severely to accomplish
> this so I tried doing it with the speakers switched off. Switch off
> speakers, turn volume up to max, turn volume down to min, switch
> speakers on, turn volume up a bit and the sound is back in both
> channels. In addition, after getting both channels to work, after 10 to
> 20 minutes, give or take 5, the left channel goes out again.
>

> This behavior lead me to suspect the volume switch, so I flooded it with
> switch cleaner and worked it, but to no avail. Problem persisted. Next I
> checked all the cold solder joints and found one that was loose. I hot
> soldered ALL the cold solder joints. Problem continued. Today I used
> some cables to switch the PRE OUT channels into the POWER IN channels,
> left to right and right to left. The next time the problem cropped up it
> was still in the left channel. This is telling me that the problem is in
> the left main amp section.
>
> Given the behavior described above I can only guess that a rise in
> voltage in the main amp produced by turning up the volume with the
> speakers turned off somehow corrects the condition causing this problem,
> but at the moment I don't know what this condition might be. I assume
> that with the speakers turned off there is not a lot of current flowing
> in the main amp when the volume is turned up, but that the voltage is
> increased. As I write this, a dawning suspicion presents itself and that
> is: the problem might be with a transistor, as they are voltage regulated.
>
> Next step is to open up the unit and do a visual inspection, hook it up
> to a pair of speakers and turn it on and attempt to compare voltages
> along the signal path of right and left channels of the main amp when
> the problem crops up.

When the left channel drops out I can bring it back by pushing down from
the top with a chopstick on the circuit board that contains the input
selector buttons. I can then make it go out again by pushing up from the
bottom on the same circuit board. After doing this several times, the
channel comes back to stay... until it goes out again, which could be in
the next ten minutes or ten hours. Also, pressing the input selector
buttons at random also brings back the left channel. The puzzling
behavior comes in when I switch channels at the PRE OUT/POWER IN jacks.
If the problem is in the push button switches, the dropped channel
should change from left to right, since the switches are upstream from
the aforementioned jacks, but this does not happen. The dropped channel
continues to be the left channel. I just don't understand how this could be.

[gregz]

I hope you got all switches sprayed by now. Vibration, pushing, heat, cold.
You use all and try to narrow it down.

Greg

[Jack]

I have to run out and buy a can of deoxit before I can clean the
switches, which I'll do tomorrow, but I can't get past the fact that
switching channels at the PRE OUT/POWER IN jacks has no effect on which
channel drops out. It is ALWAYS the left channel. If the problem is
anywhere in the circuit upstream of the PRE OUT/POWER IN jacks,
switching channels at those jacks should cause the problem to switch
from the left channel to the right channel, but that doesn't happen. It
is ALWAYS the left channel that drops out.

Also, when the left channel goes out while the FM tuner is selected,
flipping a tape monitor switch that has a signal being input to it does
not bring back the channel. Flipping either tape monitor switch
effectively takes the push button switches out of the picture. They are
no longer a part of the signal path. However, the tape monitor switches
are always in the signal path, but they are mounted on a different
circuit board that remains immobile when I wiggle the board with the
push button switches that DOES have an effect on the dropped channel.

From the obvious reaction of the dropped channel to the wiggling of the
board with the push button switches on it, I can't escape the conclusion
that there must be something going on there, but it just doesn't make
sense, at least to me, that the problem does not switch channels when
the channels are switched at the PRE OUT/POWER IN jacks.

Oh, damn, the light bulb just went on. I was expecting the balance pot
to reflect a switch at the PRE OUT/MAIN IN jacks. In other words I
expected that, when I switched channels at the PRE OUT/MAIN IN, the
balance pot would reflect that change by playing the good channel when
it was turned left. When the channels are not switched at the PRE
OUT/MAIN IN jacks, the good channel plays when the balance pot is turned
right. What was happening was the bad channel was switching from left to
right at the SPEAKERS. But being deaf in one ear and oblivious to
stereo, I couldn't tell that this was happening. After pouring over the
schematics, I realized that switching channels at the PRE OUT/MAIN IN
jacks has no bearing on the balance pot.

Not bad for a carpenter.

Also, just for the record, the left channel does not always drop out
completely, but at times merely drops precipitously in volume, but not
stone cold dead. When that happens it responds to the volume pot, but
still way way lower than normal.

All indications point to the circuit board with the push button
switches, but when they are taken out of the signal path by switching to
TAPE MONITOR, the problem persists. The board with the push button
switches and the board with the tape monitor flip switches are
physically separated.

Anyway, I can definitely see that the problem is either in the push
button switches and/or the tape monitor flip switches and I will have a
go at them tomorrow

[Jack]

I know that most of you who replied suggested that I do this before
anything and you have my apologies. After studying the schematics and
realizing my mistake in expecting the balance control to reflect the
switching of channels at the PRE OUT/MAIN IN RCA jacks (duh...), I
bought some switch cleaner from a nearby electronics supply place,
Electrosolve Contact Cleaner for Electronics, made by MG Chemicals, and
used it on the push button and flip switches and on all the pots. Now
it's just a matter of wait and see.

[gregz]

Hope you worked them when wetted. You can reapply.

Greg

[Jack]

I worked 'em... I worked 'em GOOOOD

After all I been through? Damn straight I worked 'em :)

I have the local Jazz station playing and so far so good, but it will
take some time to be absolutely kinda sure. These old Japanese beasts
are such great machines I just hated to let this one languish. In the
meantime I fixed the reluctant tray on a NAD C-451 CD player. Apparently
the factory installed belt on these machines didn't quite cut it. This
one was reluctant to open fresh out of the box. A new 69¢ O-ring did
the trick. Next I have an Advent 300 and a Harman Kardon 930 that need
attention.

[Mark Zacharias]

"Jack" <@noway.com> wrote in message news:ju1pmu$l85$1...@dont-email.me...

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.

[Jack]

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.

[Jack]

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.

[Jack]

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.

[Jack]

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!!!