I have a Macintosh Plus, which as soon as I turn it on shows the floppy icon and then resets itself immediately afterwards. Note that it does so as soon as it needs to power up the floppy drive. For example, if I unplug the floppy drive connector, the Macintosh Plus will not reset. The floppy drive works correctly because it has been tested on another macintosh plus, and the logic board also works correctly, because this is also tested on another plus. The only thing I have done on the analog board is to go over the solders (especially in the junction connectors), and change the two capacitors C1 and C2. I did not recap the other capacitors. Note that even if I turn it on without floppy and try to vary the screen size horizontally, as soon as I enlarge it a little more, it resets.

I have replaced all the capacitors of the video card, I have also gone over ALL the contact tracks of all the components of the video card but without openable results. I am sure that the defect lies in the video card because (having two macintosh plus) I tried to put this card in the working one and it gives me the same problem. Unfortunately I do not have an oscilloscope available and I was wondering: it is possible to connect only the video card, without connecting it to the logic card, and from the first check the output voltages from the cable that goes to the logic card, with a normal tester and possibly check voltage drop by manipulating the trimmer of the screen adjustment? Or risk something?

1 Answer 1


My guess is transient undervoltages on the +12V rail. I'm basing this on both the floppy drive (schematic top-right, G4) and the flyback circuitry (figure 2) drawing power from that rail. If you have an oscilloscope it'd be useful to check whether the voltage on that line droops when the floppy is accessed or the horizontal screen size is changed. Based on figures 2 and 7 the main capacitors on the +12V rail are C25 and C31 (both rated 1000µF 16V) and C2 (4700µF 16V). The document linked below makes some additional suggestions on how to remedy this (p. 9):

All three outputs are filtered with textbook LC “pi” networks. Adjust the voltage trimpot so that the +12V output (as measured at the external floppy connector) lies between 11.9V and 12.75V, and the 5V output lies between 4.85 and 5.15V. Inability to get both the 12V and 5V outputs within spec can be caused by a failure or drift in components, but most often is caused by simple deterioration of connector contacts. Simply cleaning power harness contacts, or resoldering where they connect to the circuit boards, fixes 90+% of such problems.

Figure 2 (partial): Figure 7:
Figure 2 (partial) Figure 7

The Mac+ has lethal voltages present inside the case especially on the analog board (left of the CRT, IIRC) and on the CRT itself. DO NOT probe the circuitry or attempt any repairs unless you are experienced with working with high voltage circuitry.


  • Very probably. Seems definitly like a power supply problem.
    – tofro
    Mar 11, 2021 at 11:55
  • I had to edit the post as I did not have enough space to fit the editing of subsequent works Mar 13, 2021 at 7:39

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