Phase Inverter pot failure in Fisher 500c

Discussion in 'Fisher' started by HVHiFI, May 26, 2017.

  1. gadget73

    gadget73 junk junkie Subscriber

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    unbolt it and use a drill bit as a gauge pin. 8mm is 5/16.
     

     

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  2. thornev

    thornev Active Member

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    Why would I need to drill if I put in the pot with the smaller bushings?
     
  3. s-petersen

    s-petersen Scott Subscriber

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    He was just saying to measure the hole in the chassis using a drill bit.
     
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  4. gadget73

    gadget73 junk junkie Subscriber

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    right, remove the pot, use the shank of the drill bit to figure out the size of the chassis hole.

    If you go with the smaller pot, you may want to bridge the hole with washers so the nut has something to sit against. Better solution would be a bushing so it centers the pot nicely.
     
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  5. thornev

    thornev Active Member

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    Thanks gadget. I've ordered both sizes so I'm covered.
     
  6. thornev

    thornev Active Member

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    Looks like the 3/8" is the correct bushing size. I got those today and measured them.

    What does one do when the tab that goes into the chassis slot to ensure that when turning the pot adjustment that the pot doesn't spin and the tab is on the wrong side of the pot?! Turning the pot upside down is not an ideal solution because the components' leads won't reach. I guess I could bend back or cut the tab and ensure the pot is real tight on the chassis. Too bad the manufacturers don't offer a left or right tab positions. I guess I could cut a slot in the chassis. That seems like the best solution but fraught with dangers such as little pieces of metal getting stuck on other parts.
     
    Last edited: Apr 17, 2018

     

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  7. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Just bend it back and tighten the nut down. It won't move unless you turn the knob to the stop and horse it around like a 400# Gorilla!
     
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  8. thornev

    thornev Active Member

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    I've been playing around with all sorts of different options and it is feasible that I can transfer the OEM pot's retaining device to the new pot... except for the tab... which I could bend back against the pot's body. I enjoy researching all these options. And in the meantime, while I have the chassis apart, hmm, what else can I do? (he said rhetorically).
     
  9. s-petersen

    s-petersen Scott Subscriber

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    You could use a lock washer above and below the chassis, it works well.
     
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  10. thornev

    thornev Active Member

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    HV - I hope you don't mind if I hijack your thread. If so I'll start my own.

    I received a replacement pot today - a 25K ohm 3/8" bushing pot. I had it all hooked up for a mock test to see, before soldering, if I could adjust the phase inverter for V12 to be equal to the static value of V12 pin 3. Well, it looks like the pot has large dead spots. Is that possible on a new pot from AES? I tried connecting DMM leads to various combinations of posts on the pot and I get a lot zero values, and when I get non-zero values, they never settle on a single value. The values keep jumping around. I tested the resistors and their values are as documented so they aren't the culprits. Anyone have any thoughts?

    How does one test a pot to verify it is not defective? I tested the pot for continuity and I get continuity when the pot is adjusted to full clockwise (leads on middle and left) and counter-clockwise (with the leads on middle and right) which seems like correct behavior.

    Thanks, Thorne

    UPDATE: It was indeed a bad pot. I ordered 2 of the same pots and the other one works fine. Does anyone have any experience with AES and their honoring defective units with replacements?
     
    Last edited: Apr 18, 2018
  11. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    They're pretty good about defective parts or damaged by USPS.
     

     

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  12. thornev

    thornev Active Member

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    Finally... Noose removal is DONE ! And my 500-C sounds as good as ever ! Happy now. Thanks for letting me rob the thread. Over and out.
     
  13. gadget73

    gadget73 junk junkie Subscriber

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    usually I test pots with the analog ohm meter. Connect the leads to the center wiper and to one end. Adjust it and watch for a smooth sweep of the needle. If it drops out or twitches around oddly as you adjust it, the pot is not working correctly. The only thing that "new" means is that nobody has ever installed it. It doesn't mean that it ever worked.
     

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