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Pioneer SX-780 receiver problem

Discussion in 'Electronic Repair' started by CHale, Apr 25, 2004.

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  1. CHale

    CHale Guest

    Hello--I've got a "vintage" Pioneer SX-780 stereo receiver that has
    bitten the dust. My son said it just faded out over a couple of
    seconds and quit; we found the 4A slo-blo fuse out when we opened it
    up. (this unit has a bunch of fuses on a board, but I think this is
    the biggest one?) I replaced it and turned it on, and the fuse blew
    again and smoke appeared around, I'm pretty sure, one of the two
    "Darlington Power Pack STK-0050" devices heat sunk to the large main
    sink in the center of the unit. I disconnected the two large orange
    and red wires leading from the fuse board to the Darlington area, and
    saw 78 V DC there, with polarity nominally correct for the Darlington
    per the silkscreen symbols on the PCB. With those two wires still
    disconnected, the fuse no longer blows and things all power up (all
    the lights and such; didn't check to see if I had a "single channel
    stereo" situation now, but I could do that).

    What thinkest thou, electronics guys? Think I just need a new one of
    those Darlington blocks? (are those things pretty easily found?)
    Or...? Thanks and I look forward to your advice.

    C. Hale, Lafayette CO
     
  2. David

    David Guest

    If the STK-0050 is really bad, good luck finding one. They are very scarce
    and as a result will cost you when you find one.
     
  3. They can still be had. I get them from East Coast Transistor Parts.

    800-645-3516

    If there was smoke, there may be another part in that area bad now -
    probably a resistor.

    Mark Z.
     
  4. Tim Schwartz

    Tim Schwartz Guest

    Mr. Hale,

    I'd agree with the other poster's that one of the STK IC's has probably
    shorted. While (as suggested in another reply) the voltage regulator
    almost certainly could use a resoldering, I've never seen these bad
    connections cause the fuse to blow.

    An easy way to see if one of the STK's failed is to put your voltmeter
    into the lowest OHM range (x1 ideally) and measure adjacent pins like
    1-2, 2-3, 8-9, etc. and compare the 2 IC's. If one of them has a dead
    short where the other one does not, then you've likely figured out which
    one is bad. Don't forget to order some heat sink compound (heat sink
    grease) to put between the replacement IC and the heat sink, or the new
    one will fail too. It might not be a bad idea to carefully add some
    more compound under the remaining IC as well, as sometimes the factory
    was too sparing with the stuff.

    When installing the new IC torque the 2 mounting screws firmly and
    evenly, but not too tight, as if you manage to flex the metal plate of
    the IC, you are likely to damage it.

    Regards,
    Tim Schwartz
    Bristol Electronics
     
  5. El Meda

    El Meda Guest


    If you can't find the STK0050, you can use the STK0080 instead. Beware
    of the fake STK's: if it's too cheap, it's probably fake.
     
  6. CHale

    CHale Guest

    Thanks for everyone's feedback so far; it's invaluable. I'll start
    looking into repair accordingly. I've looked up the SX-780 on eBay
    and entire, functional units are going for about $65-70...so, I guess
    it would be pretty foolish to spend more than that, frankly. It's
    such a pretty box, tho, I hate to abandon it. At any rate, I'll
    proceed with looking for the Darlingtons, and let you all know how
    this goes. Thanks again--Charley
     
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