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 RCA 9K3 "Funky" Volume Control

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feldmakg



Join date : 2014-09-07

PostSubject: RCA 9K3 "Funky" Volume Control   2017-08-30, 12:38 pm

Hi,
     I just started restoring an old RCA 9K3 Tombstone Radio.   I gave the electrical chassis a good cleaning.   I'm now checking out the potentiometers, transformers and coils with an Ohm meter.   I like to get a heads up if any are bad as these take some time to track down.

     Anyway, the Volume Control Potentiometer is bad.  It flunked the Ohm meter testing before and after Deoxit. I took off the cover there were lots of broken pieces inside.  

     The potentiometer has 5 lugs plus a ground.   The schematic shows the potentiometer as having 3 sections, all in series. 
- 2.5 Meg Ohm
- 250K Ohm
- 250K Ohm

One "tap" goes to a resistor and Capacitor in series and then ground.
The other "tap" goes to a resistor and capacitor in series, then a Music - Speech Switch, then ground.
This reminds me of some more modern radios that use a tap as a "bass boost" when listening at low volume.

Anyway, questions:
- Does the "wiper" only "wipe" the 2.5 Meg Ohm Section, or does it "wipe" all 3 sections.
- This part looks like an "Unobtainium".  

I'm thinking of using a 2.5 Meg Audio Taper Potentiometer and two separate 250K resistors in series with the Potentiometer.  I will then install the resistors and capacitors per the schematic.  I might lose the ability to play the radio at low volumes.   If so, I can always reduce the size of the 250k resistors.   If I'm really lucky, I might find a 2.5 or 3  Meg Audio Taper Potentiometer with a tap.  Then I would hook the tap up to the resistor and Capacitor going to the Music - Voice Switch. 

Thoughts?

Ken
KB3MDT
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Cliff Jones
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Join date : 2010-11-22

PostSubject: Re: RCA 9K3 "Funky" Volume Control   2017-08-30, 1:34 pm

Try ohming each tab whilst physically moving the shaft, or is that impossible?
LAY OUT the carbon path and see if there are wiper marks on both ends of the other connections that are there.

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feldmakg



Join date : 2014-09-07

PostSubject: Re: RCA 9K3 "Funky" Volume Control   2017-08-30, 4:48 pm

Hi,
    I was able to measure all combinations 5 terminals with an ohm meter.   The measurements did not change when I turned the shaft.  The measurements are consistent with my theory that the 2.5M Segment, the first 250K segment and the second 250K segment are all in series.  

    The wiper mechanism is broken.   I believe the first wiper that touches the carbon track is okay.  I believe there was a ring of thin metal just inside the bakelite housing connected to one of the lugs.   A second wiper would rub against this ring.   The gotta is that the metal ring was in small fragments throughout the entire assembly.  I may be able to use a small piece of flexible wire to emulate the second wiper and ring to take some measurements.   

    Thanks.

Ken
KB3MDT
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chas

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Join date : 2017-04-09

PostSubject: Re: RCA 9K3 "Funky" Volume Control   2017-08-31, 1:09 pm

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