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How to Overhaul 500-C Pot

Discussion in 'Fisher Forum Stickies' started by Vinylcafe, May 21, 2012.

  1. Vinylcafe

    Vinylcafe Linvin' the Dream

    Messages:
    966
    Location:
    Vancouver, Canada
    How to Overhaul 500-C, 800-C, 400 Pot

    I just finished overhauling a 500C volume control/on-off switch.
    It's a few hours work, but I had some issues with my left channel and traced it down to that device.
    To start, you must physically remove the unit.
    I started by taking pictures and documenting every wire attached to each part of the pot, the center taps, on/off, etc.
    It's tricking removing and replacing the center tap connections, as they are underneath the pot.
    Once all leads are disconnected and labeled, undo the brass nut and pull the pot out.

    The pot comes apart in 3 stages.

    First unbend the 6 small tabs on the front plate attached to the threaded silver bushing. Once removed, you will lift off the right channel carbon path, which is connected to the phenolic base.
    Under that, you will also find a brass wiper attached to a keyed nylon bushing .
    You DO NOT need to remove the sprung ring or bushing/wiper assembly from the shft. I cleaned this part of the pot with rubbing alcohol, if you have isopropynol it's likley better.

    My pot was filthy black and gummed up with a ton of old Deoxit and dirt. It took about 10 minutes and a ton of Q-tips to remove all the residue.
    It's really gummy and smells like the fuel I used as a kid flying gas-powered model airplanes.

    To get at the left channel wiper and carbon path takes 2 steps.

    First undo the four metal tabs hidden under the first pot housing assembly.
    I just used a small jewlers screwdriver to carefully pry them up.
    You also have to desolder two solder points on either side of the pot.
    To get full access you now have to pull off the back of the pot which is the on/off switch mechanism.
    It's a keyed friction fit (mine had not solder points here), just cafefully remove it , and it will just pop off. My switch did not have the protective cover you see on some Fisher power switches.
    To further pull the pot apart from the back you must also carefully remove a small brass locking ring that sits on top of a brass washer.
    This holds the shaft in place from the rear (that's what she said).
    Once removed the pot will pull apart allowing access to the left channel wiper and carbon path.
    On my unit this was not in as good a shape as the right channel (no idea why), and also totally gummed up with old dirt and Deoxit.
    Many black Q-tips later, it was time for reassembly.

    I first put just a small bit of Fader Lube on each carbon path with a Q-Tip, I did not flood it with liquid.

    To reassmble you essentailly work backwards.

    The 2 metal housings for each phenolic plate holding the carbon paths are keyed in such a way that they only fit in one way.
    Same with the back nylon bushing holding the left channel wiper - it is keyed but can be put together wrong if you are not careful.
    Just be sure that both wipers are keyed in the same position and that one is not 180 degrees off-set. Use the wiper still attached to the shaft as the reference.
    Before you seal everything up, twist the shaft to each end of clockwise and counter-clockwise.
    Extreme clockwise has both main wiper points at the 11:00 o'clock positon. Going all the way counter-clockwise, you will end up with the main wiper points in the noon position. That is normal, until you snap the back switch mechanism back in place (again keyed), then your stopped position should be 1:00 o'clock.

    If you need a new switch mechanism "metalbone.net" has them (as of June 2012), and you can pop a new one on the back.
    The only caveat is that they are the older original switches that were more prone to failure due to current surge. To overcome this, he ships with a thermistor (CL 80 or 90). If you install this on the mains line, you avoid this issue.

    The last piece is putting on the plate with the bushing on it. A couple drops of fine oil inside the bushing well help things run smoother.
     

    Attached Files:

    Last edited: Jun 12, 2012
    totsuka likes this.
  2. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
    1,884
    Location:
    Montréal, Canada
    Wow! Bookmarked as reference...
     
  3. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
    19,111
    Location:
    Glen Burnie Md.
    NOTE: as this is the identical part number for the 400 and 800C, this procedure will work for all 3 units.

    Larry
     
  4. dsndblm

    dsndblm Well-Known Member

    Messages:
    943
    Location:
    Central Arizona
    Way cool! Great documentation and information. Thanks for taking the time to share.
     
  5. bhamham

    bhamham AK Subscriber Subscriber

    Messages:
    2,034
    Location:
    France
    Way to go, Vinylcafe :thmbsp:

    There's probably a few hundred that have been tossed and replaced with Alps, etc that could have been restored.
     
  6. keithpgdrb

    keithpgdrb Super Member

    Messages:
    2,295
    Location:
    Bolingbrook, IL
  7. ascernman

    ascernman New Member

    Messages:
    4
    Excellent job Vinylcafe! I'm going to try my hand at your rebuild procedure on an old 500C volume pot tonight. I'll post the results good or bad when I am finished.
     
  8. JBL GUY

    JBL GUY Addicted Member

    Messages:
    7,023
    Very informative!

    Thanks for the post and the pictures.
     
  9. Vinylcafe

    Vinylcafe Linvin' the Dream

    Messages:
    966
    Location:
    Vancouver, Canada


    Slow and steady wins the race. Good luck. If I can help in any way let me know.

    While mine's together and smooth as silk, I've not yet had time to reinstall. Here's hoping it dealt with the left channel issue ;)

    Interesting side note ... put a multi-meter to both the right and left channel pots, resistances are slightly different for each channel at the same setting.

    Hopefully not enough to hear any difference in volume between left and right channel.

    *** pot is now back in unit, sounds really good and there are no longer any issues with the left channel. Thanks for your comments!
     
    Last edited: May 24, 2012
  10. nate86

    nate86 AK Subscriber Subscriber

    Messages:
    549
    So, anyone know if this procedure is similar/identical for a 500-b?
     
  11. rswojo

    rswojo AK Subscriber

    Messages:
    935
    I am totally impressed with your patience and skill.:thmbsp:

    With pictures this would have been a sticky.
     
  12. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
    19,111
    Location:
    Glen Burnie Md.
    Same part #. It'll work for the 500B and 800B also.
     
  13. Fisherdude

    Fisherdude Regular Dude - Super Mod Super Mod Subscriber

    Messages:
    27,606
    Location:
    Out West.
    Great job! Stickied!!:thmbsp:
     
  14. bhamham

    bhamham AK Subscriber Subscriber

    Messages:
    2,034
    Location:
    France
    On my last Fisher 400 rebuild (Rodney) I had trouble with my volume pot - the front one - so I pulled it, cleaned it, and retensioned it. When I got it back in it was better but about 1:00 the volume would cut out, so I guess I didn't retension enough. Anyway, today I decided to do a proper job and strip it down per Vinycafe's expert tutelage here and clean both pots and retension again.

    I first have to say this procedure isn't for the meek and timid. Frankly, getting the switch housing away from the rear pot housing was a real bitch! Those Fisher factory folks loved to sling solder. Solder had seeped down between the two housings and it took me a long time to finally suck/mop it all up so they came undone.

    Anyway, success at last. The volume pot tracks beautifully now. Thanks again, Vinylcafe :thmbsp:
     
  15. bhamham

    bhamham AK Subscriber Subscriber

    Messages:
    2,034
    Location:
    France
    Since having done one and it was somewhat fresh on my mind, I thought I'd pull the volume control from my Fisher 400 spare parts unit and do it. I've got another 400 where the original pot was replaced with a non-original. The volume range is shorter than the original plus it doesn't have a threaded shaft so I can't screw down the faceplate on that side. So, I'm going to replace it with my spare.

    You would think that having done one that the second time around would be a lot smoother. Well, it was in most ways but it sure was a PITA to get that switch housing off - probably a bit harder than the first one.

    Here's my enemy... the 'solder BLOBs'

    [​IMG]

    And, the other side:

    [​IMG]



    The problem I have is when I suck/mop them up some of the melted solder seeps between the two housings further bonding the two. Basically, I just kept prying with a jeweler's screwdriver on both sides until I broke the bond. Took quite a bit of prying and pulling. Here's a couple of shots showing some of the 'sticking' residue after I finally got them apart.

    [​IMG]


    [​IMG]


    I'm anxious to hear from others how they tackled the blobs. Vinlycafe wrote me that his didn't have blobs.


    Can't have too many pics of this thing so here's one with all the parts laid out.


    [​IMG]
     

    Attached Files:

  16. Vinylcafe

    Vinylcafe Linvin' the Dream

    Messages:
    966
    Location:
    Vancouver, Canada
    Bruce,


    These photos are fabulous, and really take show the contruction of this pot well.
    As we discussed, I think you're right, my pot had the on/off switch replaced, as there were no solder blobs anywhere and there was no large end cap either, just the switch - which literally popped off.
    When re-building, I never put any solder back in that area, things seems to be holding together fine
     
  17. bhamham

    bhamham AK Subscriber Subscriber

    Messages:
    2,034
    Location:
    France
    Pulled the non-original pot and found that it must be a 'short-shaft' version of the original. Made by Centralab USA, same as the original but numbers were different. Funny, it had a shorter track. In other words, would start about '2' and finish around '8' on the Fisher faceplate numbering scale. I could live with that but not being able to fasten the faceplate was my biggest complaint. Anyway, would do as a replacement if that's all you can find but you'd need an extension. Here's a couple of pics...


    [​IMG]


    [​IMG]
     

    Attached Files:

  18. Sonytubes

    Sonytubes Active Member

    Messages:
    184
    Location:
    San Francisco Calif.
    Excellent thread. Thanks to all contributors.

    I replaced my 800C volume pot years ago with an operational one from another unit.
    I kept the old one, just in case.
     
  19. PShuff

    PShuff Active Member

    Messages:
    298

    So, what were you fixing with your overhaul?

    Did it solve the problem?

    We have some issues with ours and what you guys are doing looks like a very cool solution but a whole bunch of work! :yes:

    We have found someone that can sell use a brand new volume pot with a new on-off switch for $60 bucks

    We only really need the volume part and that's only $40 bucks but what the heck, a new on-off for just $20 more

    You think we should try this first? We can change how our channels sound and their volume playing around with the pot

    Phyllis
     
  20. bhamham

    bhamham AK Subscriber Subscriber

    Messages:
    2,034
    Location:
    France
    It is a lot of work and not really a job I'd recommend to a beginner - not saying you're one. I'd buy a new pot for $60 in a heartbeat and save you the trouble of remounting the old switch which often goes bad. I'd make doubly sure it's an exact replacement - has the loudness taps and the shaft is the same dimensions as the old one.

    Can you provide the seller's details so others might buy one?

     
    Last edited: Oct 3, 2012

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