Phase Inverter pot failure in Fisher 500c

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

  1. HVHiFI

    HVHiFI Hudson Valley HiFI Subscriber

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    Hi guys, I just finished a restoration on a beautiful 500c and have run into an issue I have not run across in over two dozen of these that I have restored. After the work was done I ran the unit for a few hours and it sounded wonderful. This unit had a big mixed bag of tubes so we replaced. New matched quad of EH7591 biased using a 10k pot installed on the rear of the chassis. New 12AX7EH phase inverters, and 12AX7LPS Sovteks in the preamp and phono positions.

    After about 2 hours of use I started to notice a crackling in the left speaker. I reduced the volume and there was still a crackle with the volume all the way down. The sound became worse and I started to notice a little stream of smoke from in front of the OPT! Shut down! Put the unit on the bench and powered it up slow on the variac. Little stream of smoke seems to be coming from the PI potentiometer! I marked the position and attempted to turn the pot and it is very rough and crusty feeling. Also, the pot does not seem to turn to the extremes the other does. This unit did have a considerable amount of corrosion on the chassis so it was obviously stored in a damp location. I cleaned the pot as best as we could but still had a poor "feel" to it. Ran the unit again, played perfectly. After half hour or so crackle came back while playing but not when volume down. Seems like it is crackling when there is a demand for power. I have heard of the pots causing noise but the smoking is odd.

    So...never had to replace the inverter pots. Manual shows them as 25k. I have a few parts units I can simply replace but wondering if a new part would be better. Something like this : https://www.digikey.com/product-detail/en/cts-electrocomponents/026TB32R253B1A1/CT2160-ND/203781

    Or am I barking up the wrong tree??

    For reference here is the underside : http://hudsonvalleyhifi.com/wp-content/uploads/2017/05/fisher500c.jpg
     

     

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  2. gadget73

    gadget73 junk junkie Subscriber

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    Should just be a 1/2 watt linear taper pot. No real magic to it. As long as the replacement mounts up properly I'd think it will work fine. Maybe slot the end of the new pot if its not already so you can adjust it easier.
     
  3. DOBIEGUY

    DOBIEGUY AK Subscriber Subscriber

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    Mike
    Read my post from this past Wednesday. After repeated Deoxing of the PI pots the annoying noise is gone.
    I plan to replace both with NOS Allen Bradley pots. Should be good forever.
    Good luck
    Jimmy
     
  4. HVHiFI

    HVHiFI Hudson Valley HiFI Subscriber

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    Thanks both, Jimmy do you have a part number of something that would fit well?

    If the pot is dirty and the wiper does not make contact would that explain the smoking? I'm figuring full resistance or and open resistance so it's burning up?
     
  5. DOBIEGUY

    DOBIEGUY AK Subscriber Subscriber

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    Mike
    Mine are linear 500K. Found a seller (networkw) on the big auction site. Has many different pots in his store.
    NOS vs CCC for our Fishers is the way to go!

    I'm seeing a 25K 2 watt in his listings
     
    Last edited: May 26, 2017
  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Mike; I replaced the pots in my 400 last year. Mine had 500k 1/2 watt linears. Got 4 from AES. Cut slot in shaft with Dremel and Epoxied them to the inside chassis next to where old ones went.

    https://www.tubesandmore.com/products/potentiometer-alpha-linear-38-bushing $1.60 ea.
    As the 500c have them coming thru the chassis, Just pull the old ones and mount new ones. No Fuss, no Muss.
    Larry
     

     

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  7. HVHiFI

    HVHiFI Hudson Valley HiFI Subscriber

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    The service manual says 25k for the pots. Maybe it's 25k for the 500c and 500k for the 400. I'll pull these and check.

    Thanks for the answers fellas! It's so funny with these Fisher receivers, when I get one in and rebuild it, I forget how good they sound. I have a prestine 400 in my personal collection that I need to refurb and enjoy at home more often.
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Mike -- Unlike the 400 receiver the phase inverter controls in the 500C connect directly to the B+ source powering the phase inverter tube. If the pot is smoking, then invariably, the pot has trash inside it that is causing the tract to conduct to the metal case of the control, cause the carbon track to heat up and smoke. You're either disassembling the control and really cleaning it up, or better, simply replace it. Either way, the control itself or the connections to it is almost surely the problem.

    Dave
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The 500C uses a 25K linear pot.
     
  10. HVHiFI

    HVHiFI Hudson Valley HiFI Subscriber

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    Makes total sense Dave, thanks.

    Guess I'll order up a bunch of the pots and add it on to the list of 500c restoration items. If these are becoming an issue then might as well nip it when doing the overhaul.
     
  11. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Mike; Don't let it get out the pot is smoking. Every :beatnik:hop-head:beatnik: for 50 miles will be headed your way. Now where did I put that road-map of NY STATE?!?!:naughty:

    Larry
     

     

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  12. monkboughtlunch

    monkboughtlunch Super Member

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    How common is it for these pots to fail? Should they be replaced as a preventative measure?
     
  13. HVHiFI

    HVHiFI Hudson Valley HiFI Subscriber

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    Lol!
     
  14. HVHiFI

    HVHiFI Hudson Valley HiFI Subscriber

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    That's what I'm thinking. If we usually see issues rearing their ugly head in several units we add it to the restoration. This may be a good idea on future restores. On the 500 and 800 not to difficult or pricy either.
     
  15. thornev

    thornev Active Member

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    If I may steal this thread as it has been inactive since May 2017... I broke one of my PI pots trying to get solder out of it (Don't ask.) I cannot find any of these Fisher 25K pots anywhere online. Does anyone know where one can find them? I can find 25K pots, but I don't know the shaft length/diameter and they have the slot in them so one can use a screwdriver to adjust. I guess a split shaft pot would work if the other dimensions are correct. Or I can use a dremel to cut a slot. Thanks. Thorne

    PS - I found the pots in the attached image. Are any of them OK as PI pot replacements?
     

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    Last edited: Apr 14, 2018
  16. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Use the 1st one in the pics. Alpha, linear, Solid Shaft. Mount it, and dremel a slot.
     
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  17. HVHiFI

    HVHiFI Hudson Valley HiFI Subscriber

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  18. thornev

    thornev Active Member

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    Thanks HV and Larry.

    HV - Are you sure the 500-C takes the 3/8" (.37") bushing as opposed to the 8mm (.31") bushing? My serial# is 76944 if that helps. That's a later model around 1967-8 I'm guessing.

    HV - I'm in POK. Not too far from Chester, eh?
     
  19. HVHiFI

    HVHiFI Hudson Valley HiFI Subscriber

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    They fit, drop in replacements. I stock them if you need and want to take a drive.
     
  20. thornev

    thornev Active Member

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    Thanks, HV. Wish I knew you had some. I already ordered 4 of the 8mm pots. Should be here Tuesday. But, are you around this morning or early afternoon today? Maybe I'll get a few from you so I can fix tonight. PM me please?
     

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