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NAD 3125 left channel weak

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

Reply to
TDWesty
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In article , TDWesty writes

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.

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Tim Mitchell
Reply to
Tim Mitchell

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

Reply to
Bob Shuman

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

Have

Many

help

so,

Reply to
Eric

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