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Thread: TRS-80 Model III potentiometer replacement

  1. #1
    Join Date
    Feb 2015
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    Default TRS-80 Model III potentiometer replacement

    Brightness and contrast pots are frozen: Tried DeOxIt, WD-40, electronics cleaner and pliers without success; time for replacements. I can find references in the schematics to resistance values (500 Ohm contrast, 500K Ohm brightness) but nowhere a wattage. What is a reasonable value? 1/4W, 1/2W, 1W?

    Thanks,

    -CH-

  2. #2

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    I hope someone comes along with the answer for you/us. I will say I took mine out of the bracket and pried the tabs back to open it up. Once it was open I was able to scotchbrite the wipers and used a little dielectric grease on them before putting it back together and re-bending the tabs. It’s worked so far.

  3. #3
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    I was tempted to try more aggressive methods but my old pal Murphy suggested I wait until I located a replacement.

    I tried looking up the part by the number stamped on back but didn't get any hits. I don't think it will matter if I opt for higher rating as long as the resistance and form-factors match. The 1/4-inch D-profile shaft is perhaps going to be tricky to find. I may have to modify a rounder / longer one.

    But I'll report back when I find something that works.

    Thanks for your reply.

    -CH-

  4. #4

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    Go to archive.org and search for "sams model iii" - the Sams Computerfacts manual for the Model III should list the pot values.

    g.
    Proud owner of 80-0007
    http://www.f15sim.com - The only one of its kind.

  5. #5
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    Quote Originally Posted by geneb View Post
    Go to archive.org and search for "sams model iii" - the Sams Computerfacts manual for the Model III should list the pot values.

    g.
    Just wanted to say thanks for this, not needing to fix anything myself right now, but I didn't know this book existed and it looks great. I see there's a Model 4 GA version as well which is awesome (plus a tandy 1000sx which I can use too).
    -- Brian

    Systems: Amstad PCW 8256, Apple IIe/II+/GS/Mac+/Mac 512k, Atari 800/520STFM, Commodore 64/128/Amiga 3000/PET 4032/SX-64, IBM PS/1 2121-B82, Kaypro II, Osborne 1, Tandy 1000 SX, TI-99/4A, Timex Sinclair 1000, TRS-80 Color Computer 3/Model 4 GA

  6. #6
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    Quote Originally Posted by geneb View Post
    Go to archive.org and search for "sams model iii" - the Sams Computerfacts manual for the Model III should list the pot values.

    g.
    Thanks. Here it is (1/2 Watt):


    PotSpecs.jpg



    -CH-

  7. Default

    Quote Originally Posted by TXNathan View Post
    I hope someone comes along with the answer for you/us. I will say I took mine out of the bracket and pried the tabs back to open it up. Once it was open I was able to scotchbrite the wipers and used a little dielectric grease on them before putting it back together and re-bending the tabs. It’s worked so far.
    +1 to this, you've nothing to lose.

    The benefit is you don't have to do any rewiring, nor wait for new parts.

    I recovered my model 3 this way.

  8. #8
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  9. #9

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    Let us know if these work. I have two model IIIs that might need them some day. One benefit of working in Mansfield TX is their will call.

  10. #10

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    Seems to me unless the image is wrong, that the shaft will be way short for this application,
    I normally take long shafts and cut them to size, but so let us know they work out.
    I buy mine from a local Electronic Store (yeah an old fashion one) so I can compare length of shaft to make sure.
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