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Touch sensitive antique radio volume knob

Discussion in 'Troubleshooting and Repair' started by Eric Wells, Aug 9, 2017.

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  1. Eric Wells

    Eric Wells

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    Aug 9, 2017
    Equipment: Philco 20A, 7-tube radio

    I've begun to restore this old Philco radio and I discovered that when I touch the bare metal of the volume control knob the sound volume dramatically increases and the sound becomes a bit clearer. I have cleaned the pot which eliminated the pops & crackles. Everything seems to be working great otherwise.

    Circuit diagram
    [​IMG]

    Underside of chassis with volume knob indicated w/ red arrow
    [​IMG]
     
  2. Alec_t

    Alec_t

    2,982
    807
    Jul 7, 2015
    Your body is probably acting as an antenna extension or enhanced ground, improving the signal reception.
     
  3. Eric Wells

    Eric Wells

    13
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    Aug 9, 2017
    You can see the effect on a video I uploaded here:
     
  4. Eric Wells

    Eric Wells

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    Aug 9, 2017
    Ok, what's the fix?
     
  5. 73's de Edd

    73's de Edd

    3,143
    1,322
    Aug 21, 2015
    Sir Eric Wells . . . . .

    Back in that units days, they expected the use of both a ground and a long wire aerial / antenna wire.
    Your touching of ANOTHER controls shaft should also be producing a like affect.

    If it has the old original power plug with it's identical sized blades, you reverse the plug in the AC socket 180 degrees and test again to see if that resolves your grounding situation and its present touchee-touchee body contact to shaft situation..

    That is being one immaculately clean underchassis !

    If more info needed . . .just ask . . as I go back even further than that unit.

    Don't go severely " messin' " with that unit, as its worth hundreds of dollars.

    73's de Edd
     
  6. Eric Wells

    Eric Wells

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    Aug 9, 2017
    73's de Edd

    Thanks for complimenting my underchassis!

    Flipping the prongs around had not effect. I think I'll need to clean up the antenna connection terminal as it's dusty & has a heavy layer of oxidation. It's one of those spring type which just holds the wire a loop and spring steel ribbon.

    I also think I'll replace the power cord (temporarily) with a modern day 3 prong that'll ground the chassis too.
     
  7. 73's de Edd

    73's de Edd

    3,143
    1,322
    Aug 21, 2015
    Sir Eric Wells . . . . .

    OK . . .then, can you confirm that the same, volume shift effect, is also produced when you touch another control shaft. or even when you press a wet thumb into the chassis proper.
    Also . . . do you happen to have a paper or poly or disc ceramic capacitor that is somewhere in the .05 ufd range and rated at 400 volts or more ?

    73's de Edd
     
  8. Eric Wells

    Eric Wells

    13
    0
    Aug 9, 2017
    73's de Edd

    The issue does not occur when any other knob or the chassis is touched. In fact something I noticed when poking around and looking at the diagram is that the knob is actually adjusting the "volume" received by the RF tubes. That may be a hint that there's something fishy in the volume control. Perhaps it needs to be cleaned out some more.

    As to your cap question. I do not have any caps on hand. I just started this project am beginning to order replacement ones. I'm wondering if replacing the 80 year old caps will actually fix the issue.

    Since I am so new, what is your suggestion with the .05 uF cap and the theory behind it? Oddly enough there are four .05 uF caps throughout the radio circuitry itself.
     
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