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I'd appreciate some help to identify a potentiometer. It's from a small handheld drill and there is no reading on an avo across the two outer terminals so I'd like to replace it if I can find something the right size. There is no name on the pot which was directly mounted on a small pcb. On the underside of the pot are three separate marks B 50 4. Potentiomet after removal from pcb

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  • \$\begingroup\$ I'm not sure what 'avo' is, but the two outer terminals are mounting points. You can see that they're both connected to the metal shield that goes over the potentiometer. \$\endgroup\$ Oct 26, 2015 at 16:05
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    \$\begingroup\$ AVO (OP, note capitals) is short for Avometer, a very popular analogue multimeter and first launched in 1923 - primarily for the British Post Office. AVO stands for amps, volts, ohms. See en.wikipedia.org/wiki/Avometer. \$\endgroup\$
    – Transistor
    Oct 26, 2015 at 16:13

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This looks like an ALPS RK09 type potentiometer. To get the total resistance, measure across the outer two of the three in-line contacts coming out one end of the potentiometer.

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Guess: "B" means linear taper; "50 4" means "50*10^4" or 500k.

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Can you tell from the PCB if the device is used as a variable resistor or a potentiometer?

schematic

simulate this circuit – Schematic created using CircuitLab

If it's a variable resistor as shown here the problem is fairly simple. Connect in any mid-range potentiomer - say 1k to 10k - and try it out. At minimum resistance it will give one speed (maybe zero) and then adjust the pot until the speed stops changing. Measure that value (between the top and the wiper) and put in the nearest value pot you can find with a higher resistance. If the speed is still changing at the end of travel on the test pot you may need to use a higher value.

For the potentiometer you're in a little more difficulty. You may have to try a few values. You can use standard pots and when you find the right value buy the correct part.

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  • \$\begingroup\$ If it's a potentiometer, it's probably being used as a voltage divider, in which case the exact value of the pot doesn't matter much. \$\endgroup\$ Oct 26, 2015 at 17:08

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