Potentiometers

Potentiometers are a common source of problems in old scopes. Often, just working the potentiometer back and forth over its range is sufficient to rejuvenate it. Other times, it is necessary to spray some cleaner into the pot.

Many of the potentiometers in Tektronix instruments are parts Tek bought. Some of the potentiometers were made by Tek.

1960s stop-less potentiometers

In 1962, Tek-made potentiometers started being used in many instruments. These are typically identifiable by their gray delrin cover (and their Tek part number).

pristine 311-259 pot in Type O

They are "stopless" pots, which means that they have a detent but can be rotated through the detent. This feature was intended to reduce shaft and pot problems that result from excessive torque on traditional pots.

close-up of cracked plastic part, partial fix using tightened wire

These types tend to develop cracks in the cross-shaped plastic part that holds the setscrew clamping the shaft. Tightening that screw then causes the crack to widen, and the shaft is not clamped strongly enough anymore, causing it to slip.

Trying to fix the problem using epoxy (alone) is usually not effective. Solutions require to compress the broken plastic part in a circular fashion, either by wrapping wire or slipping a machined ring or suitable steel clamp over it.

Jim Adney says:

The most common problem with the Tek-made VAR pots seems to be noise or intermittent connections. These can usually be overcome by simply running the pot back and forth across the bad section and thru its full range.

OTOH, several of them that I've come across have suffered from broken plastic that keeps the set screw from holding tightly onto the shaft. The plastic at the center of the cruciform section is a bit too thin at the center, so it cracks there, allowing the plastic and set screw to back away from the shaft.

I solved this problem by making some small aluminum rings with #4-40 tapped holes. The rings fit snugly around the cruciform section and and hold those parts together. A longer stainless set screw replaces the OE set screw and passes thru the OE threaded part of the plastic and can then be tightened against the shaft. The OD of the rings I made is about 3/4". The ID and width are made to fit the pot. For my use, I drilled and tapped two different #4-40 holes, at different offsets, so I could pick the one that was the best fit to the pot.

The set screw shown in my repaired 3A8 may look like it could be shorter, but it causes no problem in this application and ensures full thread engagement in the aluminum ring. I will use a shorter set screw in the future. I think the photos show the idea pretty clearly.

Some types that may be affected are:

Type	Resistance	Used in
311-068 (311-0068-00)	500 kΩ	VAR GAIN in Type Z (R7620)
311-234 (311-0234-00)	1.5 kΩ	VAR GAIN in 3A72 (R445, R545)
311-259 (311-0259-00)	710 Ω	VAR GAIN in Type O (R6530)
311-116 (311-0116-00)	770 Ω	VAR GAIN in Type CA (R3338, R4338)
311-279 (311-0279-00)	770 Ω	VAR GAIN in Type CA (R3338, R4338)
311-289 (311-0289-00)	1.3 kΩ
311-290 (311-0290-00)	355 Ω	VAR GAIN in 75 / 3A75 (R437)
311-298 (311-0298-00)	680 Ω	VAR GAIN in 53/54L / Type L (R6352)
311-304 (311-0304-00) 311-309 (311-0309-00)	185 Ω	VAR GAIN in 3A74 (R426)
311-334 (311-0334-00)	375 Ω	VAR GAIN in 3A2 (R139)
311-0565-00	215 Ω	VAR GAIN in 3A8 (R437)
311-0603-00	185 Ω	VAR GAIN in 1A4 (R75 ×4)
311-0659-00	10 kΩ	VAR MV/DIV in 3S1 (R293, R593)
311-0693-00	10 kΩ	VAR MV/DIV in 3S2 (R293, R593)
311-0701-00	5 kΩ	VAR UNITS/DIV in 3S5, 3S6 (R401, R421)

Pictures

311-259

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  311-259 in Type O, pristine condition

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311-279

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  311-116 in Type CA

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  close-up of cracked plastic part, crude fix using tightened wire

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311-304

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  311-0304-00 pot, VAR GAIN in a 3A74

Repairs

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  Pot with Jim's repair rings

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  Repaired Pot in 3A8