NAD 3125 left channel weak

[TDWesty]

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

 

[Tim Mitchell]

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.

I'm not familiar with the particular amp, but your testing so far shows
that the problem is in the input stages of the amp not the power stages
(as if you turn it to mono, you get the same volume). So you are looking
for a problem on the left channel somewhere between the input terminals
and the mono switch. This narrows it down quite a lot.

It sounds like a bad connection which has gradually oxidised or
something. Jacks with switch terminals can do what you describe, if you
put in a small signal it doesn't get through but if you put in a larger
signal then it does get through. I would visually trace through the
signal path from the input connector and see if you can see anything
amiss.

 

[Bob Shuman]

At that age, I'd also suspect oxidized/dirty contacts on the controls. Have
you tried playing with all the knobs, buttons and switches (speakers A/B,
tone defeat, loudness, bass, treble, balance, etc) on the front panel? Many
times this will help to isolate the bad contact since the behavior will
change as you move the control between different positions. Once you
determine the cause, a good cleaning with a good contact cleaner should help
a lot.

Also, as a thought, is there a speaker relay near the final output? If so,
it is possible that the left channel contacts are pitted or worn and need
some attention. Many times you can remove the plastic cover on the relay
and assert some pressure to see if the behavior changes. If it does, then
you can use a small file or some emery cloth and contact cleaner to try to
clean up the contacts.

Good luck!

Bob

 

[Eric]

Tim,

Normally for me to repair an old amplifier with this similar problem is take
out and disassembly all the switches for a deep clean. Mostly the problem
gone.

Good Luck

Eric