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Problem with Pioneer SX-850 receiver

Discussion in 'Troubleshooting and Repair' started by horsetrdr, Oct 13, 2010.

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  1. horsetrdr

    horsetrdr

    3
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    Oct 13, 2010
    Vintage rec'r works fine except for one issue. Virtually no sound comes out of the right channel until I increase the volume to at least a moderate volume (2 lines on the scale). Then once it's on, it stays on fine. Then, if I power the unit off, it will happen again if the volume is low to start. Left channel is fine. If I tap the heat sink with all the power transistors, the sound will cut out of the rt channel. Happens on all sources. Any thoughts?

    All the wires going to the power amp board are wire-wrapped to the transistors, so I was planning on soldering them all to try and eliminate the problem. I tried cleaning the volume control pot wit contact cleaner but no change. Both A and B speakers do the same thing.

    Thx in advance,
    Bob
     
  2. davenn

    davenn Moderator

    13,674
    1,892
    Sep 5, 2009
    hey Bob,
    yeah sounds like there are some dry joints there somewhere. less likely on the wirewraps more likely on the PCB itself particularly around components are are drawing the most current.
    look on the PCB where it has darkened (browned ) due to heat ... Hi watt resistors, power transistors etc


    Dave
     
  3. shrtrnd

    shrtrnd

    3,773
    493
    Jan 15, 2010
    I've got a couple Pioneer receivers from the 1970's.
    Biggest problem is the contacts on the push-button switches. Often not even always the switches for speaker selection. pushbuttons are to multi-contact slide switches. And each circuit has individual left/right selection. You can clean the slide switches themselves, or try to just find replacements if the contacts are just too worn. I know it sounds unlikely, but that's the problem. When you increase the volume, you reach some kind of break-over current through the worn/dirty slide switch contact, and the volume works fine (until you greatly reduce the volume again, or turn the unit off).
     
  4. horsetrdr

    horsetrdr

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    0
    Oct 13, 2010
    Thx for the replies guys. I think I've isolated it to the amp section by removing the jumpers between the pre-outs and amp-ins and hooking a separate amp to the pre outs. The volume increases smoothly in this case, so I think all the switches and preamp section are ok and I'm gonna focus on looking for the power devices in the power amp section. Wish it was clear which devices go to the right channel. Just FYI I tried soldering many of the wire wrapped joints to no avail.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,419
    2,790
    Jan 21, 2010
    Oddly enough, wire wrapping can form a better joint than soldering. There's a lot more to it than just wrapping some wire around a post.

    edit: Reference
     
    Last edited: Oct 18, 2010
  6. horsetrdr

    horsetrdr

    3
    0
    Oct 13, 2010
    True that. I remember reading a long time ago that the sharp edges on the wrapping posts dig into the wire to form a gas-tight connection, at least in theory, if the wrapping is tight enough.
     
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