I have an NAD 3125 integrated amp (1986 vintage) with a strange left
channel problem. With a lower level input, such as my 4125 tuner, or a
VCR, the left amp channel drops out to almost nothing. Switching the
amp to mono equalizes both channels. I have swapped speakers, wires,
patch cables, and tried all of the line level inputs on the amp, with
the same results. I have swapped left&right, etc, but the problem is
always the left channel of the amp. The tuner works fine on both
channels with another amp.

With a higher level input such as CD or DVD, both channels work and
sound fine. The left channel behaviour was inconsistent for a while,
but is now consistent, and has been for months.

I have done some reading about replacing caps on older amps, along with
checking for bad solder joints, but have found no visible problems in
either case. I have had the amp for 20 years, and it has never been
abused.

I would consider replacing the caps myself, if I had some idea where to
start (ie: which ones are the mostly likely culprits, or which ones
should be replaced due to age), but I would prefer to have a schematic
to work with. I have found the schematic for the NAD 3120, which has
test voltage values, etc., but so far no luck on the 3125.

Any ideas appreciated.

Thanks, Shawn Wright