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Keyboard amp died

Discussion in 'Electronic Repair' started by Hamad bin Turki Salami, Dec 17, 2005.

  1. I have a Roland KC100 keyboard amp that died on me. First,
    I started noticing an annoying low level ring from the
    speaker when my keyboard was hooked up. I thought at first
    it was a ground loop, but I couldn't get it to go away.
    Then, the amp started cutting out for brief stretches of
    2 or 3 seconds. The sound would die out and the power light
    would fade off. But it would usually come back on by itself.
    When it didn't come back, I could sometimes get it back by
    jostling the unit. Finally, it died altogether. When I power
    on, I hear a faint "power on" sound from the speakers, but
    I get no sound from my inputs anymore.

    A guy in a local music store told me, "Sounds like you blew
    a capacitor in your amp."

    Does anyone here have a good idea of what might have happened?

    I have no experience with such things, but I'm hoping I
    might be able to do the repair myself, to save the $100 or
    so a shop would charge me. Does anyone have any comment on
    how advisable that would be or how I'd go about figuring out
    how to fix it?
     
  2. Arfa Daily

    Arfa Daily Guest

    Your problem is almost certainly one of simple dry joints. On group
    equipment, these commonly occur on the front preamp board, where the power
    supply from the rear power amp board comes in, and is zenered down by two
    damned great wirewound resistors, and two zener diodes, to provide the (
    typically ) +/- 15v supplies for the preamp ICs. The power on LED is also
    usually fed from one of these rails.

    I would suggest that you open the thing up, and run it open, providing you
    are confident enough to do so without risking electric shock, and then
    gently tap and poke components using a long plastic stick, such as a
    ballpoint pen. Go for any large components, as these suffer transport
    vibration when you carry the thing to gigs, which can result in fractured
    solder joints, and anything which looks as though it runs hot, and may have
    discoloured the board material in the area.

    Chances are that you will rapidly find the area or exact component that
    causes it to go on and off, when disturbed. If you don't then know how to
    locate the exact component or bad joint, you can at least then take it to
    someone who does, and point them to where the problem is.

    Don't be afraid to at least have a go, it's the only way to learn. The only
    area that you should be careful to steer away from for your personal safety
    with this type of equipment, is any exposed wiring where the power cable
    comes in, or on the back of the power switch. Avoid attempting repairs to
    tubed kit though, as there can, and usually will, be very high and dangerous
    voltages present. Good luck with it

    Arfa
     
  3. Asimov

    Asimov Guest

    "Hamad bin Turki Salami" bravely wrote to "All" (17 Dec 05 00:49:13)
    --- on the heady topic of "Keyboard amp died"

    HbTS> From: Hamad bin Turki Salami <>
    HbTS> Xref: core-easynews sci.electronics.repair:351712

    HbTS> I have a Roland KC100 keyboard amp that died on me. First,
    HbTS> I started noticing an annoying low level ring from the
    HbTS> speaker when my keyboard was hooked up. I thought at first
    HbTS> it was a ground loop, but I couldn't get it to go away.
    HbTS> Then, the amp started cutting out for brief stretches of
    HbTS> 2 or 3 seconds. The sound would die out and the power light
    HbTS> would fade off. But it would usually come back on by itself.
    HbTS> When it didn't come back, I could sometimes get it back by
    HbTS> jostling the unit. Finally, it died altogether. When I power
    HbTS> on, I hear a faint "power on" sound from the speakers, but
    HbTS> I get no sound from my inputs anymore.

    HbTS> A guy in a local music store told me, "Sounds like you blew
    HbTS> a capacitor in your amp."

    HbTS> Does anyone here have a good idea of what might have happened?

    HbTS> I have no experience with such things, but I'm hoping I
    HbTS> might be able to do the repair myself, to save the $100 or
    HbTS> so a shop would charge me. Does anyone have any comment on
    HbTS> how advisable that would be or how I'd go about figuring out
    HbTS> how to fix it?


    Just a small analogy to outline the problem:
    If you had no experience of brain surgery would you really even
    consider attempting it to save money?

    Similarly, sophisticated electronics require knowledge to diagnose and
    tactile skills to make a repair. There is a steep learning curve to
    each too. It isn't brain surgery however but it can't be done without
    any experience at all. Forget DIY, send it to the shop.

    A*s*i*m*o*v

    .... [] <- Please write your complaint legibly in that box.
     
  4. Guest

    If you know how to solder already, it doesn't take a great deal of
    additional skill to open up a piece of gear and replace all the big
    electrolytic caps. Caps are usually a good bet for some reason if
    you're stumped, especially if the noise you're talking about is of the
    60Hz variety. It's kind of like replacing the plugs and wires before
    you try to troubleshoot your ignition system. I wouldn't hesitate to do
    this kind of job with a $5 iron from RadioShack.

    If you're not experienced at all with this kind of work, though, $100
    sounds cheap for an easy and guaranteed solution. If you have to buy
    two caps and a soldering iron, and a roll of solder and a braid, you
    might spend half that before you know it'll fix the problem.
     
  5. Success! I gathered some tools and my 4 year old son and
    I opened the amp up and took a look. I figured at the
    very least I could teach him the little I know about
    electronics. But following your advice, I quickly
    located a transistor with two cracked solder joints.
    After resealing those, the amp came back to life.

    I saved $100 and had some fun with my son, so it turned
    out to be a good gamble.

    Thanks for the advice.
     
  6. Guest


    Your son is convinced you're an electronics sorceror now, too. :)
     
  7. CJT

    CJT Guest

    Soon all his son's friends will be coming to him for repairs. :)
     
  8. Arfa Daily

    Arfa Daily Guest


    Glad to have been of help. What the group's all about ...

    Arfa
     
  9. I think he's ready to take his share of credit. The problem is,
    he can't wait for things to break now.
     
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