Macintosh Plus voltage adjustment

[larsove]

I have a mac Plus that have has both ab and lb recapped. It’s stable and works well with a blue scsi. I just started cleaning the floppy drive and as I was testing it today I noticed that the screen starts to flicker some, and the drive makes a low ticking sound. When I Insert a floppy the machine goes into a sort of restart loop. I suspect low voltages. So I measured with the floppy both connected and not connected. It reads 4.8-4.795 on both occasions. And 11.7 and -12.01.
Adjusting the potentiometer does almost zero difference. So there is some component failure somewhere.
Both CR15 and R26 looks damaged. But I’m not sure if these have anything to do with this problem.

 

[bibilit]

If the issue only is present when using the floppy drive, the drive can be faulty... hence the restart loop.

If you are not able to adjust the potentiometer, maybe the Optoisolator (U3) can be faulty

 

[larsove]

If the issue only is present when using the floppy drive, the drive can be faulty... hence the restart loop.

If you are not able to adjust the potentiometer, maybe the Optoisolator (U3) can be faulty

I will replace U3 with a 4N35, so I’ll have to order one.

As for the drive I was thinking that the drive is causing a drop in voltage when trying to start the motor/reading a disk causing the reset loop. But it may also be faulty. I have several others I can clean and test.

 

[larsove]

So, I finally got to work on this machine today. I changed U3, and also the zener diode at CR15 and CR17.
I went over an checked for cracked solder joints and reflowed many of them just to be sure.
when I now turn the Plus on I only get a repeating semi high pitched noise (maybe chirping?).
I have checked if I gave shortened anything while soldering but I can’t find anything.

Another thing is that the CRT does not discharge itself . I usually never get a spark from discharging the CRT, but this time I got a pretty decent spark.

I have one suspect and that is the potentiometer, it’s been so long since I’ve worked on this machine that I suspect that the potentiometer was left in one of the extreme positions (fully Clockwise or counterclockwise)…