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Need help identifying the value of a potentiometer

Started by henry909, August 13, 2012, 12:56:28 PM

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henry909

I have an old RMI 140 solid state amp with a broken pot (the shaft pulls out of the body).  I am trying to find a replacement  but the pot does not have typical marks such as  "A100K".  I tested the pot with a multimeter (dial set to  200k ohms) and got a reading of 101.1.  The only markings on the pot are  "CTS", "USA",  "204-1040",  and "1376927".  Can anyone help identify the value of this pot?  Thanks in advance for any help with this problem.

joecool85

Quote from: henry909 on August 13, 2012, 12:56:28 PM
I have an old RMI 140 solid state amp with a broken pot (the shaft pulls out of the body).  I am trying to find a replacement  but the pot does not have typical marks such as  "A100K".  I tested the pot with a multimeter (dial set to  200k ohms) and got a reading of 101.1.  The only markings on the pot are  "CTS", "USA",  "204-1040",  and "1376927".  Can anyone help identify the value of this pot?  Thanks in advance for any help with this problem.

Sounds to me like a 100k pot, which would make sense as it is a very common value.
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Enzo

1376927 means CTS made it (137)in 1969 in the 27th week of that year.

204-1040 is the part number for whoever they made the pot for.


Your mete would seem to measure 100k, but if in doubt, take a 100k resistor and see if you get the same sort of reading.

If you want to know taper, then set the control at its half-way point, then measure from the wiper to each end.  If it is roughly 50k to each end, then it is a linear pot.  If it is something like 20k to one end and 80k to the other, you have some log taper.

henry909

Thanks for the replies.

Enzo:

I tried the tests you suggested.  When I put the meter on a100k resistor I get 99.9 (200k ohm setting); when I tried it on the pot again I get 103.3.

Re: The taper. This test is more problematic because the problem with this pot is that the shaft pulls out of the body and I am not sure I am getting the half way point. But at the half way point as nearly as I can find it I am getting 68.4 on one side and 35.9 on the other.

what do you think the value of the pot is based on this info?   Thanks again for your help.

bobhill

Considering you are guessing that you have it turned halfway, it could be either linear or analog. What is it controlling? If it is a volume control, get an A100k replacement. If it is for a tone control, a B100k will work just fine.

Roly

Your meter is almost certainly no better than 5% accurate, so this is nominally a 100k pot (and it's exact value is very unlikely to be critical anyway).

If the pot is so damaged you don't know what the position of the wiper is, remove the back cover and measure the resistance from each end to a point directly on the track half way around (12-o'clock position).  If the resistance from the middle to each end is roughly equal it's a linear pot, if quite different it's a log or "audio" taper pot.
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

Roly beat me to it, since the pot is destroyed anyway, take it apart and measure the resistive stripe directly from its center point.

So the resistor test verifies your meter reading was 100k and not 100 something elses.


Aside from meter accuracy, pots are notoriously wide in tolerance.  A 100k pot could measure all over the map.  I agree its value likely doesn't matter, this is a guitar amp, not a NASA space probe computer.

DOn;t even worry about the taper. the thing will work regardless of taper.  If you get the "wrong" taper, all it means is the action of the control will be a bit bunched up near one end.   If it bothers you at that point, then we can look for a different taper.

Roly

Quote from:
...and not 100 something elses.

...this is a guitar amp, not a NASA space probe computer.

:lmao:

You're in good form.  8|
If you say theory and practice don't agree you haven't applied enough theory.