[Jack[_11_]]

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.

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.

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.