Balance control problem on Panasonic SG-999

Discussion in 'The Projects Forum' started by jimkarl, Oct 10, 2014.

  1. jimkarl

    Thread Starter New Member

    Oct 10, 2014
    21
    2
    Have a vintage Panasonic SG-999 integrated stereo, pop up turntable experiencing a strange balance control problem. On one channel, the balance control works fine, meaning if you start center and go to one end it attenuates and fades the signal on that channel down to zero. On the other channel, the control has no effect. It is always on, regardless of the balance control setting. I disassembled and removed the balance pot (R4 on the schematic, 100K with three terminals). Was certain that had to be the problem but using a resistance meter, center pin to end pin(s), it seems to work fine. Reads about 61K to zero as you sweep the control, on both pin combinations. So if the pot is good, there must be something in the circuit bypassing the signal through, ala short someplace, but only in the one direction. Where in the signal path should I be looking? Link to a portion of the schematic is here:
    http://home.comcast.net/~jimkarl/Pan...schematic1.jpg

    Both channels sound fine and equal volume-wise but this balance control circuit isn't working properly.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    Look for a bad connection in the wiring or PCB from the pot outside arm to C26 or C38, depending on which channel is not attenuating.
     
  3. jimkarl

    Thread Starter New Member

    Oct 10, 2014
    21
    2
    Problem turned out to be a broken trace on the circuit board. One of the pots leads that came back to the board made it ok, but the trace was broken along the way to the the other components. Looked like corrosion had eaten through. I had to jumper over that spot,. All works perfect now.

    Was strange that 6.4 V came into center (was actually measuring about 4.8) but anyway first time I've seen a circuit design like like. In any case - all is working fine now.
     
  4. Experimentonomen

    Member

    Feb 16, 2011
    331
    46
    This is a very early solid state design before they figured out the current and considered proper way of using transistors, note the similarity to tube circuits, and compare to a modern day solid state circuit.
     
  5. RichardO

    Well-Known Member

    May 4, 2013
    1,228
    382
    MrChips: Do you have X-ray vision too? :cool:
     
    #12 likes this.
  6. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    I amaze myself sometimes.
     
    djsfantasi likes this.
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