A couple of simple vintage (60 year old) potentiometer questions

Discussion in 'The Projects Forum' started by bigalthethird, Feb 17, 2010.

  1. bigalthethird

    Thread Starter New Member

    Feb 10, 2010
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    What's up guys?

    So I've been working on a 1950 TV console, and I've got a question about the audio pot. It has two problems:

    1. It makes the audio much quieter than if you bypass it, the speaker only attains at most 1/4 the volume when attached to the pot

    2. It has a nearly instantaneous taper. If you turn the knob all the way up, there is volume. Turn it about 1/8" counterclockwise, and the volume is more than halved. Any more than that, and there is no volume at all.

    Now, I would (in any other application) simply replace the pot. But, it's a fairly specialized one. The shaft for the knob is fused to the pot somehow, and I wouldn't be able to replicate that without great difficulty.

    Is there any way to get this pot working the way it originally would have? Or is replacing it my sole option? Thank you!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    It's probably an audio (log) taper pot of about 50 - 100K ohms value. You can desolder the wires to check the resistance - measure between the outer lugs. Any log taper pot of the same overall resistance should replace it.

    A lot of those knobs fit on a D section shaft with a tight spring. Try prying the knob straight off with a couple of screwdrivers.
     
  3. bigalthethird

    Thread Starter New Member

    Feb 10, 2010
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    Well I have gotten the knobs off and removed the chassis a number of times. The pot is actually a three function pot. The inner shaft is on/off and then volume, and the outer shaft shield controls brightness. So there's essentially two knobs.

    Furthermore, the main shaft (on/off, volume, where the main knob attatches to) is about 4 inches long - in order to poke through the wooden console.

    The reason I'm supplying these details is that I think replacing it might be nearly impossible. I've fixed scratchy pots before, any possible way to fix this? Sounds like no...
     
  4. bigalthethird

    Thread Starter New Member

    Feb 10, 2010
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    So in this picture, the red wire is input to the pot, the black wire is output to speaker. If you can see, there are three more lugs above these three lugs. Those three control brightness.

    Is there any other explanation for the problems I mentioned than a faulty (old/worn) pot? Or could this be related to the wiring? I highly doubt that.

    And if it is purely the pot's fault, this one will be nearly impossible to replace since it's all one piece. Any way to fix it?

    [​IMG]
     
  5. ifixit

    Distinguished Member

    Nov 20, 2008
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    If the pot has been used alot the resistive material is likely worn off or cracked or dirty. Try pot contact cleaner first.

    If you can find a replacement that matches everything except the unique shaft, then maybe you can disassemble the pot and replace only the worn part and keep the shaft.

    Good Luck
     
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  6. bigalthethird

    Thread Starter New Member

    Feb 10, 2010
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    Thank you very much! I'll use that cleaner on it and tell you how it goes.

    If I were to replace it, I don't need the third function (brightness). I only need on/off and volume. I'd imagine that those are fairly common, but I can't seem to find one, at least in my local shops. How common are they, and where can you get one?

    Thank you for the help
     
  7. ifixit

    Distinguished Member

    Nov 20, 2008
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    Where do you live? You could try an electronic waste recycle center. Look through all the TVs that have been turned in to them if they will let you do that.

    Maybe look on-line... google television repair parts.
     
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  8. bigalthethird

    Thread Starter New Member

    Feb 10, 2010
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    I live in Los Angeles, so I'd presume there's something like that around. I'll try it out, thank you!

    It just seems like somebody would still manufacture combination on/off-volume pots
     
  9. someonesdad

    Senior Member

    Jul 7, 2009
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    Finding the pot shouldn't be too hard; finding one with the needed shaft length might be a bit harder. If you find a suitable pot, you might be able to convince a machinist to make a longer shaft for you. If it was me, I'd look at making a shaft extension, then pinning it on with e.g. a 1/16" spring pin or a small taper pin. I'd guesstimate it would be 1 to 2 hours of work with files; less with a lathe.
     
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  10. bigalthethird

    Thread Starter New Member

    Feb 10, 2010
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    Appreciate the help! I'd bet it'd be possible to graft the old shaft onto the new one somehow.

    Just wish this were simpler
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
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  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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