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Touch sensitive antique radio volume knob

Eric Wells

Aug 9, 2017
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Aug 9, 2017
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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
Model%2020%20circuit%20diagram.gif


Underside of chassis with volume knob indicated w/ red arrow
2017-08-09%2011.40.36%202.jpg
 

Alec_t

Jul 7, 2015
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Your body is probably acting as an antenna extension or enhanced ground, improving the signal reception.
 

73's de Edd

Aug 21, 2015
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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
 

Eric Wells

Aug 9, 2017
13
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Aug 9, 2017
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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.
 

73's de Edd

Aug 21, 2015
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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
 

Eric Wells

Aug 9, 2017
13
Joined
Aug 9, 2017
Messages
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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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