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Thread: Need to find replacement resitor for MAC 512K

  1. #1
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    Default Need to find replacement resitor for MAC 512K

    So I have a bad 3 legged variable resistor (trim pot) for R56 of the original Macintosh analog board 1983-1984. Does anyone know the value of this resitor or where I can buy a replacement?

    here it is in piecespot.jpg

    I found these but I am not sure they are changeable from the underside which is how adjustment is meant to be on the macintosh: https://www.ebay.com/itm/10-x-500-OH...72.m2749.l2649

  2. #2

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    Many have a slot on the other side. The only issue is the one you have has a ceramic body and the one on ebay has a plastic body. Both are fractional wattage but the ceramic can most likely take 1/2W while the plastic body isn't much more than 1/8th watt.
    What is the ohms value you need? These sometimes have the value printed on the other side but some times it is just a manufacture number.
    Dwight

  3. #3
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    500 ohms and there seems to be a "U" printed right after the rating.

  4. #4

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    Looking at the schematic, it only has about 5 ma across it. It can't be any greater than 0.0125 watts so I wouldn't worry about it. It looks to be the voltage regulator resistor. When you power it up you should first adjust the wiper to the end closest to R35 and then adjust slowly to the R38 end, as you watch the 5v line. That should start the voltage low and bring it up slowly. Adjust by measuring the 5V line as it is more important than the + and - 12V to be properly adjusted.
    With the pot at the R35 end, the 5V line should measure about 4.1V. If it is off by more than 5% to 7%, there is something else wrong with the supply that needs to be fixed. The calculated range of the adjustment is 4.1V to 5.8V. It can be off by +- a few percent.
    Dwight

  5. #5
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    Thanks for all the info Dwight, I will try just that.

  6. #6
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    Default

    Well, the trimmer pots came in and made no difference. Im only getting about 1.3 VDC and the machine is chirping away at machine gun pace.

  7. #7
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    Unplug everything from the analog board besides the CRT if you haven't already and power it up. If it still doesn't register more than 1.3v, then something else is wrong with the supply, likely a shorted component that's dragging down the rail.

  8. #8

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    Quote Originally Posted by VERAULT View Post
    Well, the trimmer pots came in and made no difference. Im only getting about 1.3 VDC and the machine is chirping away at machine gun pace.
    Where are you getting 1.3V?
    Dwight

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