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amateur amp repair

Discussion in 'Electronic Repair' started by [email protected], Jan 5, 2005.

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  1. Guest

    Hi Group: Any advice for the following? I'm a guitar player, so bear
    with me! (and a very amateur amp repairman) I'm repairing a Behringer
    Ultatwin amp---without a schematic. I'm pretty sure the output amp--a
    National Semiconductor LM3886T--is shot. I'm going to replace it, but
    I'm wondering why it totalled out in the first place. Everything seems
    to check ok---any suggestions as to what to look for?
    Thanks in advance.
  2. sofie

    sofie Guest

    I would replace the output amp chip and that will probably be all that is
    wrong..... obviously you should first do some component testing and look for
    other faulty parts, but unlike most audio power output stage discrete
    designs with individual transistors, the "power pack" chip IC modules
    usually don't have catastrophic failures because of faults in preceding
    amplifier stages.... they do sometimes but not usually. It could have just
    been it's time to fail or maybe too much volume well into distortion, too
    many speakers, etc. Make certain that you use plenty of heat sink compound
    when mounting the replacement chip.
  3. Some reasons why power devices fail:
    1/ Voltage transients caused by lightning, etc.
    2/ Too much signal (volume turned all the way up).
    3/ Inadequate thermal design of the heatsink.
    4/ Improper installation (no thermal compound).
    5/ Environment (unit was operated in an enclosure and thus overheated)
    6/ Shorted output.
    7/ Murphy's Law
  4. Could have been a normal failiure or poor thermal connection to the
    heatsink. Some Behringer Eurodesk PSU's, for example, suffered from poorly
    machined heatsinks which meant the voltage regulators didn't have full
    contact with it and so could not dissipate the heat quick enough, and

    When you buy the new device, buy also a good quality thermal pad if there
    was one fitted originally. The more expensive ones (grey or white rubber
    sheets) conduct heat better and don't require heatsink compound. Also make
    sure the mounting area is clean and there are no burrs or any other debris
    between the heatsink and the device, and tighten the mounting nut well.
    Make sure there is no possibility of speaker wiring faults.

    Having said all the above, I would expect there to be at least rudimentary
    protection built in to this device (check the manufacturers data sheet) to
    prevent pverheating or damage by short circuits. But reality and theory are
    sometimes two different beasts.

  5. Guest

    Thank you!
  6. Guest

    Thank you!
  7. Guest

    Thank you!
  8. Guest

    I am presently repairing a guitar amp of much older vintage, in which
    the previous repair person tore up some of the printed circuit traces
    by removing parts the wrong way. It's worth being very careful pulling
    a multi-pin part. One method that sometimes works is to clip all of the
    leads, throw the part away, and then you can work on one lead at a

    My technique, once the pins are cut away from the chip, is to melt the
    solder and pull the lead out with a tweezers, then once the leads are
    gone, clean out the holes with the desoldering pump. This seems to
    inflict minimal damage on the board.
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