Dave Magram
From:
San Jose, California, USA
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Posted 22 Nov 2023 6:31 pm |
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WHY IS AN EXTERNAL “TRAVEL-STOP“ FOR YOUR POTENTIOMETER USEFUL?
Because according to what I have read on the SGF, most potentiometers do not have robust internal travel stops, so every time you hit the end of the potentiometer’s travel with your foot on the volume pedal, it can wear away on the internal potentiometer mechanism—causing it to wear out sooner or break (usually just as you kick off the first song of a three-hour gig). 
Adding an external travel-stop allows the player to set a firm, adjustable stopping point, to avoid damaging the vulnerable internal stopping point in the potentiometer.
The external travel-stop shown below is for an Emmons volume pedal; the design could probably be adapted for most other volume pedals. The cost to make one is less than 50 cents—and it protects your $25+ potentiometer from being ruined.
COMPONENTS
SHEET-METAL PLATE: The “travel-stop plate” is made from a small piece of 18 gauge sheet metal. 18 gauge sheet metal is thin enough to bend easily to fabricate it and fit easily between the locking nut and the potentiometer, but is strong enough not to bend from repeatedly stopping the set-screw on the string spool.
- To make the travel-stop plate, I used a Simpson “Strong-Tie Angle A21”, available in most hardware stores in the USA for less than 50 cents. Two travel-stops can be made from one A21 angle plate.
LONGER SET-SCREW: I also bought a longer socket set screw (8-32 x 3/8”) to replace the short original socket set-screw that holds the string spool onto the potentiometer shaft (about 15 cents each).
- This longer set-screw protrudes about 1/4” above the string-spool so that it stops the potentiometer travel when the set-screw bumps into Tab “B” of the travel-stop plate. (A set-screw longer than 3/8” will not work; it will bump into the top of the volume pedal and not allow full rotation of the potentiometer.)
HOW IT WORKS:
The “travel-stop plate” is held in place on the volume pedal frame with the locking nut that holds the potentiometer in place.
- Tab A of the travel-stop plate braces the travel stop in a fixed position against the volume pedal “arm” so that the travel-stop plate cannot rotate.
- Tab B creates a firm stopping point for the new longer set-screw in the string spool. By adjusting the string spool on the pot shaft, it is very easy to create a potentiometer-travel stopping-point a few degrees before the end of the audio-taper, protecting the potentiometer from breaking or wearing out prematurely.
FABRICATING A “TRAVEL-STOP PLATE”:
I made this “travel-stop plate” with just a hacksaw, a vise, a file, and a drill press (a handheld drill would work too). It will probably take about 30 minutes to make.
- The gap between the bent-over Tab “B” and the string-spool needs to be about 1/16“, just enough to clear the string spool as it rotates, but close enough so that it hits the new longer set-screw squarely.
- WARNING: Be sure to file off all burrs and round off all sharp edges after you’ve made the travel-stop plate so that you don’t cut yourself when installing or adjusting it. Don’t ask me how I learned this tip. 
ADJUSTING THE TRAVEL-STOP:
Before reinstalling the potentiometer, I put a small piece of white tape on the body of the potentiometer, and marked a heavy line on the tape where the internal travel stop was.
- I then drew an arrow up to that heavy line showing the direction of rotation, and made a second mark on the tape about 1/4" BEFORE the heavy line.
- After installing the potentiometer and travel-stop plate, I rotated the string spool so that the new longer set-screw would hit Tab “B” at the second mark, and tightened the set-screw onto the string spool.
1) Volume pedal not yet fully depressed for max volume; the protruding set-screw still has about 10 degrees of travel before it hits Tab “B” of the travel-stop plate.
The maximum internal travel of the potentiometer is shown by the heavy line (with arrow pointing to it) on the tape affixed to potentiometer body.
2) Volume pedal fully depressed for max volume; set-screw is stopped by the travel-stop plate Tab “B”, a few degrees before the internal potentiometer stopping point is hit.
- As can be seen by heavy line on tape (and the flat section of the pot shaft), the travel-stop has been adjusted to stop the travel of the potentiometer a few degrees (about 1/4") before reaching the max internal travel of potentiometer--to protect the pot from damage.
- Although a bit difficult to see in this photo, the second mark (thin line) on the tape lines up exactly between Tab A & B, because that is where the shaft rotation is stopped by the set-screw hitting the leading edge of Tab B on the travel-stop plate.
- Dave
Last edited by Dave Magram on 27 Nov 2023 11:48 am; edited 4 times in total |
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Mitch Ellis
From:
Collins, Mississippi USA
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Posted 22 Nov 2023 7:30 pm
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Dave,
Your travel-stop plate is a great idea, and your design is some really good thinking. You will probably save a lot of steel players, as well as six-stringers, quite a few dollars. Thank you for sharing your idea with us.
Mitch |
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