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Replacement part for unusual 5-pin potentiometer

Discussion in 'Electronics & Electrics' started by boka, Apr 11, 2020.

  1. boka

    boka Member

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    I'm having trouble finding a replacement part for a potentiometer that controls a small 6V 20W halogen bulb inside a microscope. It's labelled PIHER 25K 109M and is wired up like so:
    20200407_165047.jpg
    I understand how a basic potentiometer works but wouldn't say I'm electronically minded so don't know what's going on with this 5-pin configuration or whether I can get away with replacing it with a more simple one. It's connected back to a mainboard that then connects to a microscope.

    I'm not a frequent visitor but didn't know where else to ask :) Thanks for any info
     
  2. MUTMAN

    MUTMAN Member

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    images.png

    try belling out with this in mind
     
  3. MUTMAN

    MUTMAN Member

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    is a led replacement lamp available ? and change the setup over to a sweet pwm driver
     
  4. merlin13

    merlin13 Member

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    Looks to be 'way too light to directly control a halogen bulb - max power value for that bulb indicates it can eat over 3 amps, and there's no way in Hell I'd ever consider pushing 3 amps through a plastic switch like that...

    I'd suspect that's actually a 25K trimpot on the three inline terminals with a switch on the pair at the back, controlling a bit of circuitry to handle the bulb power.

    Piher website thus, although they're not very friendly if you're looking for that particular series of pot. Would be "reasonably" easy to find a substitute, but not at Jaycar etc.

    Shove Comes To Push, if it's actually a trimpot and switch setup then as a temporary measure you could Frankenstein a trimpot and separate On/Off switch combo, to keep your 'scope going until you find the right replacement.

    So what's actually wrong with that one - sticky operation, doesn't adjust/turn on any more, it's let the Angry Blue Genie out?...

    Depending what's wrong with it, try washing it out with Isopro or contact cleaner then a squirt of WD40 or ideally a light silicon electrical grease.

    Else ask around for someone with the right skillset that could pull that one to bits and see if it can be resurrected. Or find a suitable drop-in replacement, but ideally would need to see how much room there is in the 'scope where that one lives...

    Oh, and yup, I'd also suggest looking at an LED replacement - cooler to run, better light spectrum (purer white) than an incandescent...
     
    Last edited: Apr 11, 2020
  5. OP
    OP
    boka

    boka Member

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    Thanks for the detailed info, there's a motherboard of sorts that sits between the pot and the microscope, so not directly connected to the bulb.

    Yes there is an LED version of the bulb available for the microscope and it does technically perform and render images better but it's not popular for the user as it's very bright and harsh even on the lowest setting and sometimes the light needs to be dimmer. Similar sort of deal as looking at oncoming LED headlights vs traditional headlights.

    What happened was that the control knob sits facing up on top of the table beside the microscope - I accidentally sheared it off when moving a piece of heavy equipment across the table. At this point, the only damage was that the "stem" for the knob is snapped in two and I just have to join the two parts of the stem together but could not for the life of me get the piece of stem out from the pot. I tried heating a flat piece of steel so that it would melt in to the two pieces and hold them together but that was too tricky. In the end I tried to drive a small screw in and use the screw head for leverage to pull it out with pliers but no luck (the screw actually worked ok as a control knob itself, I guess I just wasn't happy enough with the result. I then tried very carefully epoxying the two parts together without taking the stem out of the pot but that didn't work either and at this point I'm thinking I'll see if I can just replace it.

    The reason I wanted to try myself first is from previous poor experience with the equipment supplier, which is another story for another time. But yeah, wondering if there's a readily available equivalent part out there.
     
  6. merlin13

    merlin13 Member

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    You won't get the stem out of the pot unless you seriously disassemble it, and even then it'd be bloody unlikely that you could without some serious risk to totally bork it all up.

    More than likely you'd be pulling the remaining stem with the pot wiper and probably the switch bits still attached out of the top of the housing, after you drill the rivet(s) out and pull the entire thing to bits. Feeling brave?

    Else assuming the rest of the pot still works, grab Mr Soldering Iron with a sacrificial tip fitted and carefully weld the broken-off part back onto the remaining stub. Addition of some prayer would help.

    To reinforce that beforehand though, if you're reeeeeally careful and super-overly confident I'd also ponder drilling a small hole down the middle of the broken-off piece, suitable for a thin piece of stiff wire, poking the wire down through the broken bit and leaving less than 5 mm poking out.

    Then use Mr Soldering Iron to heat the wire and carefully (!!) push it down into the stem. After that then weld the two bits together with the iron.

    Also ponder a very long and very fine calibre screw, and drill down into both bits. Feeling really brave?

    Or keep looking around for either a genuine or OEM replacement. As I'd suspect that pot clips into place rather than screws in (there's no thread for a mounting nut on your one there) pretty sure you'd need to be pretty precise on the dimensions of any OEM one, or find one with a long enough mounting thread for any mounting nut to sit on the outside of the case.
     
  7. OP
    OP
    boka

    boka Member

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    I took up your suggestion to drill out the rivets and took a look inside for any other way to retrieve the stem but it looks like it's not meant to ever come out. Taking a look at the bottom part, it just looks like a circuit track that just goes in, around, and back out, not connecting to/interacting with anything:
    20200414_141220.jpg 20200414_141209.jpg

    If that's the case, I think I'll just try find a 25K pot and see how it all goes

    Edit: I was cleaning up and found a ball bearing among the rivet filings on the floor, I think the bottom part is involved in providing that on/off tactile click
     
    Last edited: Apr 14, 2020
  8. mtma

    mtma Member

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    So the two extra pins are a switch.

    There should be (have been) a cam on the back of the shaft that pushed the copper bridge away, to disconnect the circuit.

    It looks like it was commoned up with the pot itself though, so it might function to ensure that it either goes up to 11 or down to -1.
     
  9. merlin13

    merlin13 Member

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    Yup, surprising to see an equipment manufacturer do halogen lighting control in a sane manner.

    As in that setup is such so when the operator turns the halogen lamp off it drops the power/current to it down to zero.

    Then when they turn it back on it the current ramps back up from zero, rather than stampede in at full draw into a cold bulb.

    Which is when most bulbs/electronics etc decides to blow and/or let The Angry Blue Genie out... ask any incandescent bulb (he says, after replacing both rangehood bulbs over the weekend that decided to blow within 24 hours of each other... don't make me go there).

    So now the OP needs a 25 k ohm pot (or equivalent) and an On/Off switch set up to do the same as that dead one - that'll involve either sorting out a genuine replacement or asking the appropriate OEM bits supplier for a suitable equivalent.

    Or even going all ghetto and having the pot 'n switch as separate entities, even as a temporary jobby, just remembering to dim the bulb all the way down to off before playing with the switch. And unless they've got a decent supply of halogen bulbs within arm's reach I'd suggest that'd be Do Not Want as a long-term fix.

    OP - if you've been having hassles with the local equipment supplier, either give them another go, or bite the bullet and go straight to the manufacturer of the 'scope and tell them their local rep isn't very helpful.

    Oh, and as an aside I'd also hazard that one day you might even find a teeeeeeny tiny spring somewhere, that was used in conjunction with that ball bearing... ;)
     
  10. _zak

    _zak Member

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    I went down a bit of a rabbit hole on this one, but from the description I think the potentiometer is a Piher PC16 series. Obviously the name matches up, but the configuration seems right as they're available with a switch in them. Unfortunately a quick look around at the usual distributors doesn't seem to show the variant with switch in stock anywhere, but you might have luck approaching them directly?
     
  11. OP
    OP
    boka

    boka Member

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    Thanks all for the help! I tried my luck and submitted a request for samples from Piher but of course they told me they don't do parts/replacements. I then emailed the equipment supplier but no response for the past week, I think I'll end up calling them. Perhaps they are trying to get in touch with Piher themselves at the moment. Will have a google for the PC16 part in the mean time, good thing it's not urgent while I'm staying at home at the moment
     

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