Repair Question - math question....need some help | The Canadian Guitar Forum

Repair Question math question....need some help

Discussion in 'Amp Building/Technical/Repair' started by GTmaker, Dec 5, 2019.

  1. GTmaker

    GTmaker Gold Member

    Joined:
    Apr 24, 2006
    Location:
    Guelph, Ontario
    Hi Folks

    I have a master volume pot on a Polytone amp I have that's getting wonky so I want to replace it.

    There doesn't seem to be any markings on the pot but I have to admit I didn't take it out of the chassis yet.
    But I did measure the ohms readings between a outside pole and middle pole with the pot on 10 ,
    and this is what I got.

    METER set to read 200K and the actual reading I got was 75.0.
    So the question is what value pot do I need to get to replace the wonky one.?

    any questions please ask.
    G.
     
  2. laristotle

    laristotle Gold Member

    Joined:
    Aug 29, 2019
    Location:
    acton
    If you can read schematics, scroll down to Polytone.
    Schematics
     
  3. dtsaudio

    dtsaudio

    Joined:
    Apr 15, 2009
    Location:
    Hamilton, ON
    That's showing a 75k pot, which is unlikely. You're getting a false reading from something.
    Everything I've looked at from polytone suggests it is a 10k pot, so best to check a schematic. What's the model of amp.
    If you're reading is reasonably correct, most likely a 100k log pot.
    FYI - When you check a pot, measure the two outside pins, and it usually doesn't matter where the knob is set.
     
    greco likes this.
  4. greco

    greco Gold Member

    Joined:
    Jul 15, 2007
    Location:
    Kitchener, Ontario
    You have two Polytone amps.

    Which model is this one with the wonky pot?
     
  5. greco

    greco Gold Member

    Joined:
    Jul 15, 2007
    Location:
    Kitchener, Ontario
    Deleted: Information refers to wrong Polytone model.
     
    Last edited: Dec 6, 2019
  6. Hammerhands

    Hammerhands

    Joined:
    Dec 19, 2016
    Location:
    Winnipeg
    You could measure from the outside pole to the other outside pole, that should give you the total resistance.
     
  7. GTmaker

    GTmaker Gold Member

    Joined:
    Apr 24, 2006
    Location:
    Guelph, Ontario
    The amp in question is a Polytone Mini Brain head...
    The pot in question is a Master volume pot and a reverb volume at the same time...it has a double set of flippers.
    Here is a pic of the head.
    G.
    P Mini Brain Head.jpg
     
  8. dtsaudio

    dtsaudio

    Joined:
    Apr 15, 2009
    Location:
    Hamilton, ON
    Looks like it is 10k from the schematic.
    How does that work. If you change the master you change the reverb? Odd.
    That would be a dual pot.
    I haven't found a mini brain schematic, but all the others I looked at had those two separate, and they were both 10k. I'd say log taper as well.
     
    greco likes this.
  9. greco

    greco Gold Member

    Joined:
    Jul 15, 2007
    Location:
    Kitchener, Ontario
    Does the amp have two volume controls? (from your pic)


    Screen Shot 2019-12-06 at 9.17.43 AM.png Screen Shot 2019-12-06 at 9.18.02 AM.png
     
  10. GTmaker

    GTmaker Gold Member

    Joined:
    Apr 24, 2006
    Location:
    Guelph, Ontario
    Actually the amp has 4 volume controls...
    Front panel has a volume control for the main circuit and a volume control for the sonic circuit.
    And a volume control for the reverb.

    On the side panel there is a single master volume control for volume and reverb.
    G
    DSC06804.JPG
    DSC08344s.jpg
     
    greco likes this.
  11. reckless toboggan

    reckless toboggan

    Joined:
    Mar 9, 2019
    Location:
    CA
    Solve for x?
     
  12. greco

    greco Gold Member

    Joined:
    Jul 15, 2007
    Location:
    Kitchener, Ontario
    It certainly would be nice to find a schematic for this amp. I have been searching and have had no luck...not even any promising leads.
     
  13. GTmaker

    GTmaker Gold Member

    Joined:
    Apr 24, 2006
    Location:
    Guelph, Ontario
    Problem Solved .....( I think).

    OK....so I went with a hunch that the suggestion of a 100K pot was needed.
    Dropped in to my favorite electronics store (Neutron Electronics) and asked for a 100K pot with double flippers ( sorry for the technical terms).
    NO such thing at the store...BUT....they do have single flipper 100K pots.....so I bought 2...

    I replaced the old pot and placed one of the wires on there....
    For the second pot, I removed the "main Output" jack and taped it safely in the inside of the chassis.
    Installed the second pot with the reverb wire on it and placed it in the now empty "main output" hole.

    Thats it folks....works as it should only now I have separate Master and reverb controls...
    Unless someone is going to tell me that those 100K pots are going to blow up my house....I'll keep it as is.

    first pic is the wonkey pot. ( what was happening is that the amp volume was very low until I found a narrow sweet spot at about 8 or 9 and then all was normal).
    thanks for all your help...much appreciated.
    G.

    DSC08351s.jpg
    DSC08349.JPG
    DSC08347.JPG
    DSC08353.JPG
     
    greco and laristotle like this.
  14. Hammerhands

    Hammerhands

    Joined:
    Dec 19, 2016
    Location:
    Winnipeg
    This seems to be a dual-ganged potentiometer, like a stereo pot. Often each circuit has it's own housing.

    If it had two controls it would be a concentric pot, like you find on some guitars.

    You can measure each ganged potentiometer, they are usually the same value, but sometimes you get one that is 10K and one that is 100K.
     
  15. greco

    greco Gold Member

    Joined:
    Jul 15, 2007
    Location:
    Kitchener, Ontario
    Well done!

    I admire you for working on an SS amp like that. Especially when you didn't have a new replacement for the original pot.

    The wire in this pic looks like it might be shielded (coax) style wire.
    Did you purchase it at Neutron?
    I'm assuming the centre conductor is stranded....correct?
    Is there a label marked on the outside of the cable?
    What is the approximate outside diameter of the cable?
    I'm asking as I dislike working with vintage braided shield wire and I am always looking for a good substitute.

    Thanks for the info.


    Screen Shot 2019-12-07 at 9.51.39 AM.png
     
    Last edited: Dec 7, 2019

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