Potentiometers: Difference between revisions

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In 1962, Tek-made potentiometers started being used in many instruments.
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).
These are typically identifiable by their gray delrin cover (and their Tek part number).
 
[[File:311-259 1.jpg|thumb|400px|right|311-259 pot in [[O|Type O]]]]
They are "stopless" pots, which means that they have a detent but can be rotated through the detent.
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.
This feature was intended to reduce shaft and pot problems that result from excessive torque on traditional pots.


[[File:Tek 311-0304-00 pot.jpg|thimb|350px|right|Tek-made pot, part number 311-0304-00, VAR GAIN in a 3A74]]
[[File:311-304 cracked 1.jpg|thumb|400px|right|close-up of cracked plastic part, partial fix using tightened wire]]
These types tend to develop cracks in the cross-shaped plastic part that clamps the shaft using a setscrew. Tightening that screw then causes the crack to widen, and the shaft is not clamped strongly enough anymore, causing it to slip.
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 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.


Some types affected are:
Some types affected are:
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| [[311-259]]              || 710 Ω  || VAR GAIN in [[O|Type O]] (R6530)
| [[311-259]]              || 710 Ω  || VAR GAIN in [[O|Type O]] (R6530)
|-
|-
| [[311-279]]              || 770 Ω  || VAR GAIN in [[CA|Type Ca]] (R3338, R4338)
| [[311-279]]              || 770 Ω  || VAR GAIN in [[CA|Type CA]] (R3338, R4338)
|-
|-
| [[311-304]], [[311-309]] || 185 Ω  || VAR GAIN in [[3A74]] (R426)
| [[311-304]], [[311-309]] || 185 Ω  || VAR GAIN in [[3A74]] (R426)
|}
|}
==Pictures==
'''311-259'''
<gallery>
311-259 1.jpg          |
311-259 2.jpg          |
311-259 3.jpg          |
</gallery>
'''311-279'''
<gallery>
311-304 cracked 1.jpg  | close-up of cracked plastic part, partial fix using tightened wire
311-304 cracked 2.jpg
311-304 cracked 3.jpg
</gallery>
'''311-304'''
<gallery>
Tek 311-0304-00 pot.jpg | 311-0304-00 pot, VAR GAIN in a 3A74
</gallery>


[[Category:Repair issues]]
[[Category:Repair issues]]
[[Category:Electromechanical components]]
[[Category:Electromechanical components]]

Revision as of 13:30, 28 June 2024

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).

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.

File:311-304 cracked 1.jpg
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 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.

Some types affected are:

Type Resistance Used in
311-068 500 kΩ VAR GAIN in Type Z (R7620)
311-259 710 Ω VAR GAIN in Type O (R6530)
311-279 770 Ω VAR GAIN in Type CA (R3338, R4338)
311-304, 311-309 185 Ω VAR GAIN in 3A74 (R426)

Pictures

311-259

311-279

311-304