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Lenco MC-4000 mini-stereo, weard Volume control

Discussion in 'Electronic Repair' started by BRASTO, Feb 10, 2007.

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    BRASTO Guest

    This LENCO MC-4000 stereo receiver when powered on will read Volume
    position 16, when rotating the volume control CCW the output volume
    increases rather than decreasing. It will stop at MAX 40 and cannot be
    reduced. Pulling the powercord for at least one hour will reset the
    initial VOL to 16 and the problem can be duplicated at will.

    I have inspected the VOL control, which appears to be a sort of
    digital shaft encoder rather than the old days potentiometer.
    Presumably something is wrong with the logics in the set, can anybody
    help to find the cause of the problem or knows its schematics ?

  2. Arfa Daily

    Arfa Daily Guest

    This is a *very* common problem on hifi's using a rotary encoder for control
    of volume. It is caused by the grease that is put in the shaft bearing to
    give it that "stirring treacle" feel, migrating down into the control body,
    and contaminating the encoder disc and its pickup fingers.

    It is usually a straightforward job to remove the control (two metal tabs
    and three pins), bend open the four metal tangs that hold the control
    closed, and clean it out. If you use electronics grade isopropyl alcohol on
    a cotton tip to deal with the contact fingers, it will usually cause the
    grease to 'ball', which makes it easy to remove from them with tweezers or a
    pin. Take care with the contacts, as they are quite delicate.The encoder
    disc can be cleaned with a cotton tip and alcohol too.

    When it's all clean, //slightly// re-bend the fingers to restore their
    original contact pressure, put a small amount of switch cleaner / lubricant
    on the disc, and reassemble. When refitted to the hifi, you should then have
    a working control again. I do probably a couple a month.

  3. HE

    HE Guest

    Arfa Daily schreef:
    This was exactly my experience with my high end Pioneer receiver, which
    was driving me mad because its sound volume was jumping from normal
    level to maximum , almost blowing up the speakers, by just moving the
    volume knop slightly.
    Simply cleaning all vaseline from the contacts in the rotary encoder
    with some alcohol did indeed the job ... I guess all "digital volume
    controls" with grease on the shaft will suffer from this sooner or
    later... unless they are constructed in a different way, preventing the
    leaking of lubricant to the interior.
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