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NAD 3125 left channel weak

Discussion in 'Electronic Repair' started by TDWesty, Jun 14, 2005.

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  1. TDWesty

    TDWesty Guest

    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
     
    Jikester likes this.
  2. Tim Mitchell

    Tim Mitchell Guest

    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.
     
  3. Bob Shuman

    Bob Shuman Guest

    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
     
  4. Eric

    Eric Guest

    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
     
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