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enter image description hereI am trying to repair a very expensive manlift joystick. I had great luck soldering in a new potentiometer on a different machine and getting it back to work, but this other machine has a potentiometer that I have not seen before.

It has 4 pins total, three like a normal rotary potentiometer and then a fourth across from the other three pins.

It is a Clarostat U-10K-OHM Type EJ pot. It has another number stamped "99148B 9507"

Clarostat has been purchased by Honeywell, so I am having a hard time finding any catalogs or datasheets for this guy. Can anyone point me in the right direction or advise what the fourth pin may be for? Would replacing with a standard three pin pot be a possibility?

Thanks!

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  • \$\begingroup\$ Well, that looks pretty much like the photo I posted- center tap 99.9% sure. Linear-taper extended-life 10K pot with a center tap (and whatever shaft and bushing length spec there is- I guess 6.35mm x whatever length). All that stuff was customizable if you bought a reasonable quantity. \$\endgroup\$ Commented Jul 25, 2016 at 18:54

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I second Spehro in what he thinks it is: an heavy-duty pot with additional center tap.

TT Electronics seems to sell a product similar to the part sported in your photo (datasheet).

enter image description here

Here is an excerpt from the datasheet with the technical drawing:

enter image description here

And here is the electrical circuit:

enter image description here

Even if it is not what you are looking for, it may be a starting point.

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  • \$\begingroup\$ Nice find. In stock at Digikey for < $15 USD. \$\endgroup\$ Commented Jul 25, 2016 at 20:03
  • \$\begingroup\$ That is perfect. Just ordered from Mouser 10.98ea for dust sealed. \$\endgroup\$
    – Trey D
    Commented Jul 25, 2016 at 20:17
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Not familiar with that series, but it appears to be an extended-life pot.

There are two possibilities for the 4th connection. The first is that it is a center-tap on the element. This might allow them to control the output at the neutral position of the joystick better. The second (and less likely one) is that it's a ground connection.

A quick test with an ohmmeter should settle the matter. Here is a picture of a center-tapped 10K pot. It would measure 5K from either end of the element to the opposite (center-tap) pin. The wiper would short to the center tap when in the center of rotation.

enter image description here

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    \$\begingroup\$ Since he is looking for a replacement, a quick check with a multimeter might not be a good option \$\endgroup\$
    – PlasmaHH
    Commented Jul 25, 2016 at 22:11
  • \$\begingroup\$ @PlasmaHH even if there is not a working pot on another axis it would serve to rule out the ground option(but I think that's redundant given the photo the op added) \$\endgroup\$ Commented Jul 26, 2016 at 0:02
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Four-terminal potentiometers were common at one stage on hi-fi volume controls to give automatic "loudness" control - see What's a potentiometer with four terminals in this schematic? - but these were logrithmic law and a joystick is more likely to be linear.

My guess is that it's a linear pot and that the centre-tap is used to provide a definite GND or reference to assist with a deadzone in the centre of the pot. This would be intended to prevent creep when the joystick is centred.

schematic

simulate this circuit – Schematic created using CircuitLab

I found a Clarostat brochure on Stephen Engineering and they seem to be handling the product now. They don't have any four terminal pots listed.

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It seems like there should be an equivalent circuit here (think Thevenin's theorem), but I can't immediately figure one out. I suspect you could make an equivalent out of a double-ganged 3-pin pot with appropriate connections. It would be something like:

A ---####C####--- B ---####D####--- E

where each gang is ACB and BDE, and where AE is the original resistance, and with B connecting to the center tap.

Then use two fixed resistors (represented by ####):

C ---####--- F ---#### ---D

and take point F as the original wiper connection.

I'm not sure that's exactly right, but it might help...

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