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Mystery part

Westside

Jun 22, 2016
3
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Jun 22, 2016
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Can you help me identify the name or function of this old part? It looks like a regular potentiometer, with the 3 tabs I will call 1, 2 and 3 (left to right, looking at knob face). But there's an extra 3/8" thick disk behind it with two heavy (14-gauge?) wires labeled A and C. The knob clicks "off" at full CCW. Here's what it does, as far as I can tell with an ohmmeter:

Knob "off": A and C are open, 2 and 3 are closed/short
Knob clicked "on" (any amount): A and C instantly closed/short, 2 and 3 instantly open
Knob turned fully CW, about 300 degrees: 1 and 2 closed/short

There's no gradual change in any of these characteristics, as the knob turns.

Thank you!
 

Bluejets

Oct 5, 2014
5,881
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Oct 5, 2014
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5,881
Sounds like a standard switched pot with an open circuit pot section. Any markings on the body of the unit anywhere?
 

davenn

Moderator
Sep 5, 2009
14,040
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Sep 5, 2009
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14,040
a photo or 2 would be VERY helpful
 

Westside

Jun 22, 2016
3
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Jun 22, 2016
Messages
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Thank you for your replies! I went back and looked at it again as if it would be a standard pot. Now I see that the seemingly unusual binary behavior (open and closed) was due to my ohmmeter being set too low. Turns out to be a 880K pot, with the three tabs acting normally.

But, I am still wondering about the function of the add-on disk behind the pot. The disk body labels one wire as A and the other as C. Wikipedia says "Older potentiometers may be marked with an "A" for linear taper, a "C" for logarithmic taper." So I was looking for some kind of variable reading from those wires as the knob turns. Only get the binary result from my original post: open when knob clicked off, closed/short when knob clicked on (any amount).

So for now I will assume that the A and C markings have nothing to do with taper. It's simply an on-off switch to control something else, of higher voltage, perhaps even 110V (AC!).

What would be an electronic device that uses a single knob to turn something on and off, and also acts as an 880K pot? I guess maybe a stereo amp: turn the power on and then control the volume?

The only other marking is on the side of the pot body: F in a circle, followed by BIM.
 

73's de Edd

Aug 21, 2015
3,366
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Aug 21, 2015
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3,366
Alright you can now see that it is a given that the rear portion is being a single pole single throw switch.
Now, If you want to further differentiate on the potentiometers type , you will be finding that it has a 270°
rotational range, now find the center of that range at 135.
Now measure from the center of that pot to each side terminal, and see if they are being approximately equal in value .
If you find that to be the case , then you have yourself a linear pot .
If you find yourself finding approximately 50 K on one center to side terminal, compared to finding approximately 800 K on the opposite half of the pot.
You have yourself a logarithmic potentiometer .
If you confirm that type of potentiometer, further check at the one third and one half positions of the section which was giving you the low readings .
Then, kick your brain into full logic and reasoning mode.
Then you wonder, "how dey do dat " ?
If you find yourself having a logarithmic pot I think they've done quite well in approaching that overall one megohm assigned value, considering the intermediate values of resistance tapering that they had to accomplish .


73's de Edd


.
 
Last edited:

Westside

Jun 22, 2016
3
Joined
Jun 22, 2016
Messages
3
OK yes it seems to be a linear pot, or nearly so. Thank you!

Item is now labeled and in the Pots drawer.
 
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