Electronics question

I have another that's a closer match to the original on the way . In the meanwhile I can still use the machine if I need to . While I have the pedal apart for I think I'll check the resistance of the leads in the cable .

You didn't get a logarithmic pont instead of a linear, did you??

Linear taper , not audio .

It was unplugged from the welder . Checked at the plug per the diagram and instructions I downloaded and printed last night . I have a closer match to the original part on the way thanks to David Billington . Snag

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I received a lot of training and experience in accurate electrical measurement while building precision industrial automated test equipment. DVMs are handy but not highly dependable because of their uncertain connector, lead and probe resistances. I check by shorting the probes together before making a measurement.

The slower, better way is to pass a known and observed current through the Device Under Test (DUT) and measure the voltage drop across it with separate leads. Since less than a microamp passes through the voltage leads their resistance doesn't matter, if it's 10 K the error is 0.1% on a 10 Meg input. The cheap HF DVM is different, it's input resistance is 100 K.

A controlled constant current is better though the current source could be a power supply or battery and resistance as long as you measure it. A light bulb is a pretty good current limiter because its current draw changes considerably less than a resistor's as voltage varies, the filament resistance increases with temperature. The test light I made in the 70's to carry on my motorcycle from a wired tail/stop bulb socket gives around 1/2A through the tail filament and 2A for the stop. The current varies only a few tenths from 12 to 14V. The combined current is still 1A down at 2V. It was all I needed for the common repair of the time, filing and resetting the contact points.

For example 2 Amps causes an easily measured voltage drop of 2 milliVolts when passed through 1 milliOhm, which is an inch of 10 AWG wire.

Male pins recessed in a connector are difficult to probe without adding uncertain resistance. When it matters I make connectors to plug onto them from hobby brass tubing or crimp connectors with the insulation removed and the sleeve compressed or expanded to fit.

Your strange pot may have internal damage, possibly a worn, corroded or burned wiper contact that adds resistance unpredictably. I recently disassembled an inexpensive in-line car audio circuit breaker that had that problem, the brass(?) contacts were burned and blackened from one test of opening at rated current. It was little better than a fuse, it tripped correctly once, then heating from the burned contact made it trip at varying lower currents. I used it as a switch for battery load testing until it failed completely, then performed an autopsy.

One of my large Variacs was available cheap because it had a bad spot, the brush had worn to its stop and no longer reached the winding there. The replacement would have cost over $100 if available (not) so I machined new brass holders for carbons from a Diesel repair shop.

Carbon brushes can be milled as long as the cut is inward from an edge, outward will chip the carbon off. The composition (resistivity) is different between brushes for 12V and 120V. They need to drop a few tenths of a volt to reduce circulating currents through the windings they bridge short.

I have another that's a closer match to the original on the way . In the meanwhile I can still use the machine if I need to . While I have the pedal apart for I think I'll check the resistance of the leads in the cable . Snag

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High resistance is most likely to be in connections, though wire can corrode if water gets in through insulation damage.

The current limited by a headlight bulb is enough to measure wire resistance. It may be between 5 and 10 Amps so be careful how you measure it. The 10A range on some DVMs has a time limit.

Electric water heater elements have resistance in the low tens of Ohms and can serve as current limiters, with the advantage of screw terminals. Like light bulb filaments, heating element resistance changes with temperature so you can't measure the cold resistance and then compute current from voltage drop. The resistance change for Nichrome is low but you can't assume it's used.

I make homebrew current shunts to repair old ammeters with Constantan from Type T thermocouple wire.