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Antique Radio Technical Forum
Potentiometer Replacement

Posted by Edd on 04/27/2009 14:15

I also would typically question your pots type, in respect to it being a LINEAR type, if being used in a
conventional volume control application towards the rear of the set in an audio amp grid section.

NEVER-NO-HOW-SOME-EVER . . .IF its being used in an RF gain type situation up in the FRONTAL section of
an old TRF receiver or even some first generation superhets, it could be a linear unit. . . .but taking
issue with its ohmic value even being anywhere near the high value of a I meg unit.

In that situation, the “volume” control pot was utilized in a combinational action, with it shunting across (shorting out)the sets RF input the first portion of its rotation, and then at some transitional point, it then kicked in and became a lower resistance, variable cathode control /resistor in a frontal RF tube(s) stage , thereby , I can see a
linear unit being used in that particular application, BUT with the resistance values typically being
waaaay down in a typical 5-10 K ohm value.

Come forth with the maker and model of your set and we can research and see what the true situation

Quickee-Qiuckee Testee-Testee:

To analyze that pot which you have, out of circuit, or else with center terminal and one side terminal
disconnected from radio circuitry, place a larger knob on it, preferably with a pointer or add on a
wire to act as a “pseudo” pointer and then play with it and find its rotational capability to typically be
limited to ~270 degrees of rotation. . . .mechanical end to end

Use your pointer index referencing to then to find MID / CENTER rotation position of the pot.

Now, take ohmmeter in hand and measure across the units extreme end terminals to find the resistance being in the order of ~ your specified 1 meg unit . If your unit truly was a linear unit, expect ~ ½ of that 1 meg (500k)as your
ohmic reading when doing a re measuring from its center terminal to either side terminal.

Next, further that 135 degree positioning of the shaft in half and then expect the then position of ~67.5 degrees to be creating a new reading of ~250k ohms for one legs reading and ~750k ohms on the other leg.

In that testing manner, your readings would have now fully identified a linear pots resistive response.

NOW, should you have a LOGARITHMIC, or audio taper. . . .the common type used for a volume control. . . .
now let’s take a look at it:

On the log pot, the resistance change from max CCW position is so condensed, that it seems to take for-ev-ah to make a pronounced change of the resistance value of the portion that is deposited upon the pots resistance element in the portion between the max CCW rotor /wiper position and its mid

Specifically. . . .a 1 meg pot would have only transitioned to half of its resistive value until the pot shaft
had been fully rotated ~ 90 percent of its rotational range.

Reference the supplied chart below and interpolate for different mechanical rotational positions of the
rotor positioning, versus the expected resistance value and something becomes readily apparent.

The idea being, for the potentially most used portion of the control, then to be able to give a slow
gradual adjustment of the sets volume, with a very precise and exacting degree of control.

Now, compare the insertion of a LINEAR type of control into that volume control application and see the
FAST transition of value within that audio “sweet spot ” area of adjustment. . . .expect it to have one
extreme “hair trigger” effect on your, now very limited, primary volume adjustment range.

REFERENCING. . . . .Linear Versus Log Pot. . . . . . Resistance Characteristics:

73's de Edd

Potentiometer Replacement 
Steve Bento 04/24/2009 13:17 
Marv Nuce 04/24/2009 20:08 
Dave A. 04/24/2009 20:56 
Steve Bento 04/25/2009 13:25 
Marv Nuce 04/25/2009 17:50 
Lewis L 04/25/2009 18:47 
Dave A. 04/25/2009 19:27 
Dave A. 04/25/2009 19:34 
planigan 04/25/2009 22:48 
Marv Nuce 04/26/2009 01:03 
Edd 04/27/2009 14:15 
Steve Bento 04/27/2009 15:29 
Lewis L 04/27/2009 16:54 
Lewis L 04/27/2009 16:56 
planigan 04/27/2009 18:05 

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