Macintosh 128k Video Issue

Discussion in 'Apple Collectors' started by rikkitikki, Jun 9, 2019.

  1. rikkitikki macrumors newbie

    Joined:
    Jun 9, 2019
    #1
    I am restoring a Macintosh 128k and have an issue where the vertical position of the image is lowered to almost halfway on the crt. The image is clear and crisp and the desktop is complete, just compressed to halfway. I have researched and performed all of the basic checking I.E. voltages, capacitors, position rings on the yoke. Any ideas? I also have Larry Pinas book on repairing the Macintosh and have gone through his video problems sections.

    Any help is appreciated. I have attached two photos showing the problem.

    thanks,
    Rick IMG_1737.png
     

    Attached Files:

  2. MacTech68 macrumors 68020

    MacTech68

    Joined:
    Mar 16, 2008
    Location:
    Australia, Perth
    #2
    Try checking page 56 of Larry Pina's 'The Dead Mac Scrolls".
     
  3. rikkitikki thread starter macrumors newbie

    Joined:
    Jun 9, 2019
    #3
    I did all of those adjustments... no luck
     
  4. rikkitikki thread starter macrumors newbie

    Joined:
    Jun 9, 2019
    #4
    I also tried adjusting the purity rings on the yoke and they moved the image up a little bit but nowhere near it needs to go. I am starting to think this is more of a mechanical issue rather than an electrical one. I don't have enough knowledge of tv circuits to understand how the upper left hand position is determined. Is it an RC circuit constant or something else that may be bad in the vertical circuit...
     
  5. MacTech68 macrumors 68020

    MacTech68

    Joined:
    Mar 16, 2008
    Location:
    Australia, Perth
    #5
    Page 56 mentions 2 components that may have failed.

    R2 Q2 No top half vert deflection annot.jpg
     
  6. rikkitikki thread starter macrumors newbie

    Joined:
    Jun 9, 2019
    #6
    I already replaced those... and also replace R3 and Q1, no luck.

    thanks
     
  7. chabig macrumors 603

    Joined:
    Sep 6, 2002
    #7
    I don't know much about analog video signals. But you're getting a clear picture, just not the right size. That makes me think it's not frequency related, but that the amplitude of the vertical sweep is reduced. The most likely cause for this is a failing capacitor, think. Resistors and inductors don't fail as often. All you have to do is find the right one. I'd guess that it's one of the larger ones, maybe C1 or C2. Is there any evidence that either might be bad or have bad solder connections?
     
  8. rikkitikki thread starter macrumors newbie

    Joined:
    Jun 9, 2019
    #8
    yes I thought the same, the trick is figuring out which one is bad without just replacing all of them.

    thanks.
     

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7 June 9, 2019