Macintosh 128K Voltage Setting

Discussion in 'Apple Collectors' started by sunapple, Mar 13, 2017.

  1. sunapple macrumors 6502a

    sunapple

    Joined:
    Jul 16, 2013
    Location:
    The Netherlands
    #1
    Last week I opened up my Macintosh 128K to solve the issue of the screen going black after booting, leaving only the mouse pointer visible.

    I found that the two dials for screen brightness and height (or width, the one next to it) had some corrosion in them and turning them would solve the issue. The solution didn't last long though, so I sprayed some contact spray in there to make sure it makes good contact.

    Then I saw two other dials, decided to spray them as well. I figured I could just rotate them and calibrate the screen afterwards.

    This is when I found out about the voltage setting. I made a mistake turning that knob without remembering how to set it back.

    I found out how you can measure the 12V and 5V voltages from the disk drive port on the back from a PDF file on the internet.

    It just leaves me to this: how do you set the dial before measuring the first time? I don't have a starting reference. Could the minimum setting (to the right from the cardboard side) be save and then go from there, or should I put it in the middle?

    Hoping not to make a bigger mess than I already have :D

    sunapple

    (@MacTech68 ;))
     
  2. MacTech68 macrumors 68000

    MacTech68

    Joined:
    Mar 16, 2008
    Location:
    Australia, Perth
    #2
    Turning the pot fully counter-clockwise (viewed from the solder side) should get the voltage as low as possible.

    You can then SLOWLY turn clockwise whilst observing the 5 volt rail. 5.00 is good but you need to compare the 12v rail too (see the pdf you got for further info).

    Use an insulated screwdriver. Some models of adjustment pots may allow the center adjust head to be shorted to the left or right pins of the component with a large screwdriver. Other models of adjustment pot are insulated - it's pot-luck (pardon the pun) as to which you have.

    Good luck! :)
     
  3. sunapple thread starter macrumors 6502a

    sunapple

    Joined:
    Jul 16, 2013
    Location:
    The Netherlands
    #3
    Thanks for your respons. I'll put it on the lowest setting and work from there. I understand that the voltages might slightly alter when the system warms up, but I'll just wait and see what happens. It's going to be interesting, that's for sure.

    The project will have to wait a few days before it starts, but I'll post the results here for future reference. Thanks again!
     
  4. sunapple thread starter macrumors 6502a

    sunapple

    Joined:
    Jul 16, 2013
    Location:
    The Netherlands
    #4
    It worked perfectly.

    Put it on the lowest setting (equivalent to about 10V on the 7th/8th pin) and work from there. The first time the computer didn't boot, but after setting the 12V right and rebooting it comes right up. I checked the 5V setting afterwards and it seemed to be alright as well. There's a pretty big margin so putting it in the middle of it works fine.

    Then I set the display brightness, focus and height back to the right settings and now everything works like it should. Easy as pie. Now on to fixing my Apple ][ PSU...

    Supporting documents:
    1. http://tim.id.au/laptops/apple/legacy/macintosh_128k.512k.pdf
    2. https://68kmla.org/files/classicmac2.pdf
     
  5. MacTech68 macrumors 68000

    MacTech68

    Joined:
    Mar 16, 2008
    Location:
    Australia, Perth
    #5
    Excellent news - good to see another historical artifact resurrected.

    :)
     

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