[Jack[_11_]]

gregz wrote:
Jack wrote:
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.

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

Greg

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