Here's the scoop on replacing the pot (potentiometer) on a Goodrich model 120 volume pedal. Having lost faith in the quality of the current Allen Bradley replacement pots and their supposed clones like the Clarostat I ran into what Tom Bradshaw was offering. See it here!. It's a 470K ohm, audio taper pot made by Dunlop. The old Allen Bradley type J pot was 500 kohms and the difference in resistance is a complete don't care. The 500k Allen Bradley pot (made in Mexico) I removed from this pedal tested at 550K ohms and the one I had before that (not made in Mexico and pictured below also) measured just under 500K ohms.
As you can see the carbon tracks in the old Allen Bradley pot were badly worn but the carbon wiper was still in great shape with lots of carbon left on it. Even a good cleaning with Caig's De-Ox-It would not save this pot. The new Dunlop pot has different terminals and a slightly smaller case. The terminal wiring is clearly shown below where the Dunlop pot is shown soldered with the wire colors clearly visible. Since the pot case is used as a stop at full volume, the pedal does travel a little farther in the down position to reach the smaller diameter Dunlop pot case. I found this to be no problem whatsoever and I was able to achieve a pot resistance of 464K ohms down to 160 ohms with the pedal in both extreme positions. This was achieved without having tht pot reach its end stop before the pedal reached its end stop. This is important as you do not want the pot to take any abuse by being used as a pedal stop.
The Goodrich 120 pot bracket had a slot that the tab of the Allen Bradley pot case fit into to prevent rotation. I found that by properly tightening the Dunlop pot it would not rotate. Time will tell if if stays put! In my case it was not necessary to shorten (cut off) the Dunlop pot shaft. Also the pics are detailed enough to see the routing of the string in the full open and full closed positions. I routed and attached the string with the pedal in the full on position as this left the shaft pulley set screw facing up in an accessible position.
I included pics of the setup I used to measure the taper of the pot and the results are tabled below also. It's definitely a logarithmic audio taper. I plotted the pot rotational positions on paper, attached a chicken head knob from my Fender Hot Rod Deluxe, affixed the pot to the paper via its solder tabs, and rotated it to eight different positions noting the resistance at each position.
So far the Dunlop pot works great! Same tone, same taper, full off, full on and no drag just like the old high quality Allen Bradley pots. Just as good as buying a brand new pedal. Check back here once in a while and I'll update how well this pot performs over time.
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|Incorrect String Routing:|
|Position||Resistance||% Travel||% Resistance||Delta Rotation Deg.||Delta % Resistance|