In article ,
Readily Visible wrote:
The left channel bugged out again and I double confirmed that it is
*not* the speaker relay. I also double confirmed that switching right
and left preamp outputs to left and right power amp inputs respectively
*does not* change which channel drops out, confirming that the problem
is downstream of these connections. Then, with the left channel out, I
sprayed some freeze onto the tranny I suspected of being at fault, with
no results. I then sprayed every other tranny on the amp board,
including the output trannies. No change. Then I sprayed all the
resistors and caps. Then I gave the whole amp board a general spraying.
I got absolutely no reaction.
The channel continues to respond to an increase in volume, that is, it
kicks back in with a crackle when the volume pot is turned just past the
halfway point.
I guess now is the time to begin probing with a voltage meter on the
right and left sections of the amp board to see where I might find
discrepancies.
Another possible candidate, under these conditions, would be a bad
inter-stage coupling capacitor - an electrolytic located in the signal
path. These are used in some designs to block DC offset voltages in
one stage from affecting the next (or previous) stage.
At this point I think you really need to compare signals on the two
sides of the circuit... but with an oscilloscope rather than with a
voltmeter. You're likely to find the problem faster that way.
If you don't have a scope, you should definitely acquire one. Even a
slow, old scope will have plenty of bandwidth for looking at audio
circuits. As a crude example, I picked up an old-style Tektronix
scope (bandwidth is around 5 MHz) at a local ham-radio flea market for
about $15 or $20, as a "learning and practice" scope for a friend.
Works fine within its limits... that sort of scope would be adequate
for your needs.
--
Dave Platt AE6EO
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