[Jack[_11_]]

Tim Schwartz wrote:
On 6/28/2012 9:39 PM, 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.



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

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