Clarostat CM43464 pot.

[ColinTheAmpMan1]

I have a Tektronix P6201 FET probe which I think has a faulty DC offset pot. I say this because the knob rotates continuously with no end-stops and I don't think this can be right. The pot is a Clarostat CM43464 and is 5k but has to be physically quite small to be in the housing. Would anyone have one of these to sell, or does anyone know if its behaviour is correct?
TIA, Colin.

[ronbryan]

Colin

The DC offset pot on my P6201 rotates 3.5 turns before a light end stop can be felt. The pot knob can be turned past the end stop and then rotates ad infinitum. This happens for both CW and CCW rotation. Maybe the pot has some kind of slipping clutch to protect the mechanism.

Ron

[Radio Wrangler]

It's probably a normal pot with a built in epicyclic friction reducer gear..... think of those Jackson Bros slow motion drives. Some of the American pots with ball reducers had the balls exposed at the back, ready for fluff and dust to get in.

David

[factory]

Originally Posted by Radio Wrangler

Some of the American pots with ball reducers had the balls exposed at the back, ready for fluff and dust to get in.

David

Picture of the "pot with balls" from the hp 1824A, think it might be a bit bigger than the one in the Tek probe.


David

[ColinTheAmpMan1]

Originally Posted by ronbryan

Colin

The DC offset pot on my P6201 rotates 3.5 turns before a light end stop can be felt. The pot knob can be turned past the end stop and then rotates ad infinitum. This happens for both CW and CCW rotation. Maybe the pot has some kind of slipping clutch to protect the mechanism.

Ron

Thanks for this info. I have just checked mine out and it shows the same behaviour. I hadn't noticed the light end-stops before, but they are definitely there. I checked the resistance when rotating the knob and there are definitely two limits - around 1R and around 2.5K (which is the measured resistance across the pot, not 5k). There is no mention of this behaviour in any of the documents I have, nor is it mentioned in TekWiki, where photos of this pot can be seen. I wonder why Tektronix decided to do this but not document it?

Colin.

Tektronix decided to do this but not document it?

No need, you just twiddle it, overthinking is the problem. In normal operation you would never go outside the limits so it doesn't matter, it just works.

[ColinTheAmpMan1]

Originally Posted by merlinmaxwell

No need, you just twiddle it, overthinking is the problem. In normal operation you would never go outside the limits so it doesn't matter, it just works.

I can see your point, but:

(1) Pots usually have end-stops - if they rotate continuously they appear to be broken.
(2) It would surely have been cheaper to use a standard pot.

Or am I underthinking (if there is such a word).

Colin.

[knobtwiddler]

If you really do need to find another pot, there is a firm in NY USA called 'State Electronics'. They bought all the Claro parts up (as well as Allen Bradley), and can make pots to order. However, make sure you're sitting down when you see the price...