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Macintosh Plus reboot in a loop or vertical bars

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I have a Macintosh Plus that have 2 problem:

  1. The Analog board only provide 9.4V measured on the connector on the Analog board side (Same on the J8 from the Logic board). All the connector have been soldered (with new solder past). The board had bad corrosion traces but I did measured all connection with an ohmmeter and they all seems to be fine -> I will probably need to change the 12V 5W Zener diode to solve the first issue. (I guess).

    I know that the Analog board has an issue because I tested it with a known good logic board and I had the same issue (9.4V instead of the 12V)

  2. The logic board was cleaned and the ROM and RAM where re-seated. When testing the logic board with a known working Analog board, the Macintosh Plus will reboot rapidly in a loop. The screen is not even able to show a good icon, only try to show a partial image. To be safe I removed the Programmer and Logic switch to check it wasn't due to a reset being inserted by the bad contact on these switch.

    I check on the Dead Mac Scroll but I wasn't able to seen a similar issue. Power line on the logic board aren’t shorted...  however on a good logic board I was able to measure ~150 ohm between +5V and GND, while on the "bad" logic board I only had ~70ohm


Anybody had an idea on how I should proceed? I do have a working unit to compare and test parts...


Thanks in advance






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  • 2 weeks later...
7 hours ago, Gsbtom said:

Might be the picture, but a lot of the traces on that analog board don’t look good, like there’s a lot of corrosion. Judging by the patch wire, looks like repairs have been attempted before.

Hi Thank you for your reply,

Yes, you are right, several traces are corroded. I did the patch wire while I was measuring with an ohmmeter connections linked by "corroded" traces. The Analog board need to be checked. I received the Zener doiod yesterday so I will see if I can save the Analog board.

For the logic board, swapping the analog board with a known good one shows that the Logic board has still this reboot-in-a-loop behaviors. I can see a something quickly draw on the screen before it reboots. What is draw is something like an Sad Mac but it's not clearly drawn...  and hard to tell since it only last for tens of seconds.

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19 hours ago, Nathanplus said:

Check the voltage pot i had that issue multiple times I would do a full recap after ajusting it to see if its your problem. You would want to turn it to the positive side a little it looks like your putting a little too much power into the MB

For the Analog board there is clearly a problem with the 12V (measured at 7V). But I don't really understand your suggestion regarding the MB.

I have a good MB that works fine on a known good Analog board. So I suppose that this good Analog board provide the correct voltage to the working MB.

Now using the MB that I want to fix on the good Analog board, the MB/Logic board keeps rebooting.

Do you suggest to:

  1. Look at the voltage provided by the Logic board since it may be too high for this MB but fine for the other MB?

  2. Check the voltage on the bad MB and change the Capacitors on the MB...  (I assumed capacitor on MB didn't have to be replaced)

  3. Else... 

Thank you for your help

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  • 3 weeks later...
  • 2 months later...
On 5/7/2019 at 4:21 AM, CharlieFrown said:

Yesterday  I replaced the original ROM chips (giving me the identical bars) with two  27c512 containing Plus ROM images.

Works perfectly, chimes, no bars, boots to desktop. 


I can confirm that the vertical bars are usually caused by faulty memory or ROM chips. 

I did have them once but it not always the case. Its only the roms when there is a short to ground. The first time I tried to put a scsi connector in my plus I fried the roms but other than that I haven't seen a computer with normal config do that unless there is corrosion. 

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