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Left channel dead till crank

Discussion in 'Electronic Repair' started by Kermit, May 25, 2004.

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  1. Kermit

    Kermit Guest

    I have an old Vector Research VRX 9100 receiver that will (usually)not
    play the left channel until you really turn it up. Once you do, you
    can back the volume down to a much lower level and it will work fine.
    Sometimes I turn it on and it's fine out of the chute. I assumed that
    the protection relay was causing the problem so I opened a little
    access port in the side of it since it was too well sealed to get open
    and cleaned up the contacts for that side. Still same problem. I can
    actually get in there with a pick and screw around with the contacts
    when it doing this and still no dice. Thats exhausts this rookie's
    limited diagnostic ability. I assume it could be an output transistor
    but i don't know if they fail this way.

    Thanks,

    Robb
     
  2. The best way to check the relay is with an oscilloscope. Either the signal
    is there at the input side when your problem occurs or it's not. The problem
    may be almost anywhere in the circuit, but likely at some point after the
    volume control, since as you noted, increasing the volume fixes the problem
    momentarily.


    Mark Z.
     
  3. Bob Shuman

    Bob Shuman Guest

    I'd have guessed, like you that this symptom was being caused by a defective
    relay contact. If you are certain it is not the relay, I'd trace the
    channel's output to look for a cold solder joint or a high resistance
    connection that gets overcome when the voltage is increased. Make sure you
    also check the speaker wire and the connections in the speaker as well.
    This can be done by temporarily swapping the left and right channels at the
    back of the amp and seeing if the problem follows the speaker or stays with
    the left channel.

    Good Luck!

    Bob
     
  4. Norm Dresner

    Norm Dresner Guest

    If you're willing to open the unit up, look for "cold solder joint(s)"
    in and around the protection circuit.
     
  5. Kermit

    Kermit Guest

    Thanks Bob and Mark,

    I neglected to mention that the signal disappears at the input side of
    the protection relay when this happens so I know it's not the relay
    itself. I've visually checked the circuit path back a fair distance
    from there and have poked and prodded a fair amount to see if I could
    get it to come alive when this happens. I've also done the standard
    percussive maintenance of slamming the entire unit on the counter but
    still nothing :)

    Can an ouput transistor fail like this? I can't imagine any of the
    discretes acting this way as these are not SMT parts, so intermittents
    don't seem likely but then again I don't have enough experience to
    make this call. Any additional help would be greatly appreciated.

    Thanks,

    Robb
     
  6. You could try manipulating various controls and switches and see if any of
    them affect your problem, aside from the volume control that is, since we
    already know it does.

    It's pretty unlikely that the outputs are bad on that channel., but not
    impossible. Flat-pack transistors sometimes go open-circuit on the
    emitter-base junctions. Goosing the volume might make them work temporarily,
    though in my experience it's usually the opposite - increasing the load
    makes the transistor fail.

    You say you've done "percussive maintenance", but does this include
    specifically tapping around on the various boards, etc?


    Mark Z.
     
  7. Robb,

    I have seen this happen when dust or 'fuzz' has gotten inside the volume
    control 'POT'. It will not work till the control is turned well up, and
    then works when down to the regular volume, often intermittently. Dust can
    also cause a crackling sound when the volume or balance is adjusted. Then
    the affected control must be cleaned.

    There are contact and tuner cleaners that can be used. Just find a open
    hole on the control, usually where it's crimped together. Also, I came
    across a service kit for one of the well known oscilloscope brands and they
    said to use WD-40 to do the job. You just need to use what you think will
    work best, IF this is the cause.

    If you determine that it is the POT control, if all else fails, replace the
    pot.

    Rob H
     
  8. Kermit

    Kermit Guest

    Hi Mark,

    I don't see any correlation between manipulating controls other than,
    as you mentioned, turning the volume way up, but I will check again.
    I have poked around different components in the area to see if I could
    get anything to change but no luck. With your suggestion though, and
    the fact that you doubt that the transistors could be causing this
    problem, I will check this again also. BTW, is it bad etiquette to
    repost this if it becomes a little dated and I still don't get it
    resolved?

    Thanks,

    Robb
     
  9. Not bad etiquette as long as nobody acts like a jerk along the way. So far
    we're in good shape there :)

    You need to try to break this down into blocks. Is the signal reaching the
    main amp input, for example. Does this piece have pre / main jumpers at the
    rear panel?

    Often one can king of drag a finger across coupling caps etc and induce a
    "buzz" that would tell you if signal is getting through from that part of
    the circuit. The nice thing about stereo is there's an identical channel
    there, usually laid out in a very similar manner. This can be most helpful
    for comparison.


    Mark Z.
     
  10. Kermit

    Kermit Guest

    Thanks guys,

    I've checked the volume and balance pots and nothing there
    contributing to the problem. They were not crackly as dirty ones
    usually are but I cleaned them anyway and no improvement. I may give
    up on this but I have one more question. In a receiver are the volume
    pots located in the pre-amp or amp circuit. I should know this but
    don't.

    Thanks,

    Robb
     
  11. sbell

    sbell Guest

    Rob:

    I, too, have this exact same problem with my VRX-9100. If you solve this
    issue, please inform me.

    sbell
     
  12. TCS

    TCS Guest

    sounds like a worn out volume potentiometer. When the wiper gets past
    a certain point it makes contact again.
     
  13. No, the symptoms don't match. If it were a bad control, going back down
    would cause the audio to drop out again.

    Could be a bad speaker relay, some other switch or bad connection
    in the left audio.

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  14. Norm Dresner

    Norm Dresner Guest

    We have a similar problem with a separate Stereo amplifier whose left
    channel occasionally goes missing until I crank up the volume on the PreAmp.
    I believe it's a relay in the amp's overload protection circuit that's the
    culprit but I've never taken the time to take it apart since the cure is so
    simple.

    Norm
     
  15. Asimov

    Asimov Guest

    "Norm Dresner" bravely wrote to "All" (04 Jun 04 20:41:53)
    --- on the heady topic of "Re: Left channel dead till crank"

    ND> From: "Norm Dresner" <>
    ND> We have a similar problem with a separate Stereo amplifier whose left
    ND> channel occasionally goes missing until I crank up the volume on the
    ND> PreAmp. I believe it's a relay in the amp's overload protection circuit
    ND> that's the culprit but I've never taken the time to take it apart since
    ND> the cure is so simple.

    ND> Norm

    It's not exactly a cure if it happens again, now is it? ;-)

    A*s*i*m*o*v

    .... Just a little force field zap.
     
  16. Norm Dresner

    Norm Dresner Guest

    I guess I have too much of the medical view of "cure" in mind and not enough
    of the engineering view, huh?

    ;-) Norm
     
  17. Kermit

    Kermit Guest

    I think Norm is right. I finally got this unit to work by cleaning
    the volume pot. I steered away from this as a possible culprit
    because, as some others have suggested, it did not exhibit any of the
    characteristics of this type of problem. The volume control was not
    scratchy in the least and it would play in the full range of volume
    once the circuit was established by cranking it up to 8-10 or so.
    Anyway, must have been some sort of contaminant that was causing this,
    as cleaning fixed it.

    Robb
     
  18. Asimov

    Asimov Guest

    "Kermit" bravely wrote to "All" (22 Jun 04 14:07:20)
    --- on the heady topic of "Re: Left channel dead till crank"

    That is a first for me too. Like the rest I would have suspected an
    electro or a semi with a bad internal connection. Live and learn!

    A*s*i*m*o*v


    Ke> From: (Kermit)

    Ke> I think Norm is right. I finally got this unit to work by cleaning
    Ke> the volume pot. I steered away from this as a possible culprit
    Ke> because, as some others have suggested, it did not exhibit any of the
    Ke> characteristics of this type of problem. The volume control was not
    Ke> scratchy in the least and it would play in the full range of volume
    Ke> once the circuit was established by cranking it up to 8-10 or so.
    Ke> Anyway, must have been some sort of contaminant that was causing this,
    Ke> as cleaning fixed it.

    Ke> Robb

    .... I worked hard to attach the electrodes to it.
     
  19. gothika

    gothika Guest

    Oft' times the grease used to lubricate the shaft of the pot "sweats"
    out onto the graphite of the potentiometer causing a loss of
    conduction.
    Cleaning with a mild contact cleaner will remove this waxy buildup.
     
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