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Balance woes with an old receiver

Discussion in 'Electronic Repair' started by Tuner Watson, Oct 4, 2007.

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  1. Tuner Watson

    Tuner Watson Guest

    OK I'm trying to bridge the contacts where the balance knob used to be on a
    Mitsubishi R11 receiver. I've completed this operation successfully on a Sanyo
    and it was quick and simple, but the Mitsubishi receiver is doing nothing but
    confusing me. The balance knobs on these old receivers always go bad and I
    prefer removing them rather than trying to fix or replace them. With the Sanyo
    I just removed the balance knob and bridged contacts on either side of the
    wiper where the pot was - so simple! But when I try this on the Mitsubishi, I
    get the channel I bridge coming over both Left and Right channels as mono, and
    the level is somehow decoupled from the volume knob! I managed to poke around
    and find a way to bridge the channels so that the volume knob works, and the
    channels are 'weighted' in their proper directions, but the left still 'bleeds'
    slightly into the right channel and the right channel bleeds slightly into the
    left. This is not acceptable, and seems pretty weird to me. Could it be that
    balance knob (which was two stacked 250k pots) used the 250k resistence of the
    pot turned all the way in either direction to silence the bleed of one channel
    into the next? Has anyone ever seen an amplifier wired up like that? Seems
    like a really poor way to wire an amp, if that is indeed how its supposed to
    work. The Sanyo had the balance pot grafted to another pot beside it in order
    for the one knob to turn two channels, whereas the Mitsubishi has the second
    channel pot stacked behind the first. Perhaps one was wired in series and the
    other parallel? I even found a pdf of the service manual for the Mitsubishi
    and it doesn't help with this at all.

    Any comments whatsoever would be appreciated...
  2. This probably doesn't help much, but can't you check the old balance pot
    with a meter and see how it works? That should solve any mysteries.

  3. Tuner Watson

    Tuner Watson Guest

    Thanks for the reply, Mike. But it's not so much how the pot works that is the
    problem, the problem is how the circuit works in conjunction with the pot. The
    pot is fairly simple, (though I will check it with a meter to be certain, that
    was good advice) either the back pot is wired in reverse to the front pot or
    the circuit is plotted in reverse on the pcb (so that turning the knob causes
    one pot to wipe one direction, while the other pot wipes the opposite direction
    , so that as you turn it to the left the left channel gets louder and the right
    channel gets quieter, and vice-versa). What I don't understand is, where the
    hell are the 'clean' or 'isolated' right and left channels? Like I said, if I
    bridge the pot contacts I either get the channel I bridge coming over both left
    and right headphones/speakers, or a louder left or right channel but with some
    of it in the corresponding channel at a much lower level.

    If anyone really wants to waste some time helping me, take a look at the
    service manual to give yourself some idea of what I'm dealing with:

    As you can see, if you look at the service manual, the balance knob, loudness
    knob, treble, bass, and tone defeat are all on the same little PCB. If I can't
    get the balance bridging sorted out, I'll probably try to bypass this board
    entirely, so any tips on doing that would be appreciated as well.
  4. mark

    mark Guest

    Your link doesn't work
  5. Tuner Watson

    Tuner Watson Guest

  6. PhattyMo

    PhattyMo Guest

    Connect a short wire jumper between the "center" pin of each pot,and one
    of the outer pins.
    The other outer pin is ground,if you have a Multimeter,check which one
    of the outer pins reads 0-ohms to ground.Connect the short jumper from
    the center to the opposite (un-grounded) pin.
  7. Arfa Daily

    Arfa Daily Guest

    It looks as though there might be a discrepency in the manual between the
    schematics, and the layouts. The schematics quite clearly show that the
    balance arrangement is absolutely standard for a dual pot system. They are
    connected exactly as if they were two additional volume controls ahead of
    the main volume controls. The difference is that they are connected
    differentially, so as one turns up, the other turns down. To accomplish this
    in a 'smooth' way, one is almost certainly log taper, and the other reverse
    log. Both will have their wipers at the same resistance from one end, when
    they are the centre, but it will be the *opposite* end for each pot.

    Again, if you look at the layout diagram, you can see quite clearly that the
    left and right inputs to the tone and volume circuits come into the board on
    points 24 and 26, and with the vol / balance assembly facing you and to your
    left, point 26 goes to the *left* end of VR401's track, whilst point 24 goes
    to the *right* end of VR301's track. Then it all goes pear shaped. According
    to the schematics, signal passes from the balance pot wipers, through the
    loudness arrangement and off the tone board at points 27 and 29, and next
    fetches up at the attenuator board to become the signals for the top ends of
    dual pot VR303 / 403. I'm assuming that is the main 'volume' control. The
    signals then apparently leave this board on points 30 and 32, to return to
    the tone board, where they continue on via R355 and R455. All seems good so
    far. The layout diagrams show an 'attenuator' board with the appropriate pot
    and wiring point numbers on it.

    However, according to the layout diagram for the tone board, this is not the
    way it actually is. If you follow the print around, the wipers of the
    balance pots (correctly) connect to the loudness pots, but then, instead of
    the signals going off-board to the attenuator board, and then returning to
    the tone board, they actually stay on the tone board, and go to VR302 and
    VR402, which appear to be mounted behind the balance pots, making this a
    quad-coaxial control. These two pots must be the volume control, but do not
    appear on the schematic under those ref numbers. The wipers of these two
    controls then pass (correctly) to R355 and R455.

    So, does the version you are working on have the balance and volume all on
    one pair of concentric knobs, or does it have the volume control separate ?
    Either way, you should be able to defeat the balance control by shorting the
    centre and left pins on the front-most pot, and the centre and right pins on
    the next pot back. This should not affect channel separation figures at all,
    as you are making no connections between channels. It might not be a bad
    idea to disconnect the ground-y ends of the balance pots.


  8. Put long URLs inside <> like this:
    so they don't break.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  9. Tuner Watson

    Tuner Watson Guest

    Boy you really took a close look at things, didn't you! Thanks for doing so.
    I also studied the schematics a lot last night so I think I know what you're
    getting at. The attenuator is actually 'volume', and you're right, one drawing
    shows it being on the tone board, while the other shows it going off to its own
    board and coming back on. The latter is the actual case - the 'attenuator'
    (volume) knob loops out from right behind the balance pot. VR302 and VR402 are
    the volume knob, and behind the balance knobs are two sets of three wires to
    loop out to the volume knob and right back again. Now, since the signal
    eventually leaves the tone board at points 41/37 (approx, I'm not looking at
    the schematic right now), what I'm going to do is bypass the tone board
    altogether, by taking the input to the tone board, at 24, 25, and 26, and
    splice those wires to the wires going out to the volume knob, then I'll take
    the wires coming back from the volume knob and splice them to the wires that
    I'll lift off of points 41/37. That way the signal will go straight from the
    selector knob to the volume knob, then to the amp, bypassing the tone board
    altogether, which is definitely the best fix possible, as far as my needs go.

    Anyway thanks to you and everyone else who looked into it for me, I managed to
    figure it out on my own but if I hadn't I'm sure I would have with your help.

    Yeah that might just work, but like I said I've decided to bypass the tone
    board altogether, because even with the tone defeat switch, the loudness
    control remains in circuit and I would prefer as clean an amplifier as
    possible, and it's kind of lucky that this particular model is built in a way
    in which bypassing the entire tone board is very easy.
  10. Tuner Watson

    Tuner Watson Guest

  11. JW

    JW Guest

  12. Franc Zabkar

    Franc Zabkar Guest

    Set the balance control to its midpoint, measure the resistances
    between the wiper and each end, and then replace the pot with discrete

    - Franc Zabkar
  13. Tuner Watson

    Tuner Watson Guest

    At the midpoint there is no resistence. Do you understand what a balance pot
  14. OK, so given that it's a matter of which two of the three legs you are
    measuring, but the "output" side of each channel of the balance control
    would be near zero ohms to ground only when the control is turned over to
    the other channel. If for example the wiper of the balance pot is connected
    to the input coupling cap of the next stage, you would measure near the full
    value of the pot from that point to ground when the control is centered.

    Mark Z.
  15. Tuner Watson

    Tuner Watson Guest

    The balance control consisted of two pots stacked together, which I mentioned
    in my original post. At the center position, both pots are supposed to be wide

    Anyway I solved the problem weeks ago, and I appreciate the responders who
    responded back then who were actually quite helpful, unlike you
    johnny-come-latelies throwing in belated obfuscation due to not reading the
    first post of the thread...
  16. That was unnecessary. My post was accurate and relates equally well to a
    dual pot, in fact that is exactly what I was talking about. Further, I was
    only responding to a recent post - not to your original. I have been on this
    group for years and have earned the respect of those whose shoes you
    probably aren't fit to shine.


    Mark Z.
  17. I would hardly call Mark a johnny-come-lately. Anyone who reads this group
    very much knows his skill and knowledge are far beyond all but a few of the
    posters that you will see. He is straightforward and helpful. You have to
    be reading from a very strange perspective to see obfuscation in he attempt
    to help.

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