Macintosh Plus horizontal jitter/wiggle + bomb :(

[twirlip]

Hi everyone, I'm working on my Macintosh Plus revival project! Now that it's back from the dead (CR21 had shorted), I could use some help with some odd display behavior I'm seeing. I've captured it in this video. Essentially, I have two and a half problems:

1. The display jitters on the horizontal axis. Left side is fairly stable, right side will wiggle and jitter randomly. The center is also affected a bit.
2. Occasionally, I see faint thin horizontal lines appearing and disappearing across the display. Almost like horizontal static. Sometimes I boot the computer and they're not visible, other times they're very noticeable.
3. The computer only boots occasionally. 75% of attempts, it fails with the bomb and a "coprocessor not installed" error. When it does boot, and I open and close MacWrite, I get another bomb and a "stack collision with heap" error. I'm hoping this is a combination of bad RAM and bad floppy (I'm upgrading the RAM and picking up some scsi2sd solution soon).

The left side of the display is fairly stable, but the right side will wiggle and shift randomly. The horizontal dimensions will also change by ~1cm randomly, back and forth. The jittery behavior is unrelated to any read access from the floppy drive, and happens even if the computer is completely idle without a boot disk inserted. This doesn't seem to stabilize with time, but I need to test that more. Tapping the AB doesn't seem to affect the jitters one way or another.

This behavior does appear somewhat similar to what I see in the Dead Mac Scrolls. I'm going to attempt to replace transistor Q3 this weekend. Tbh I'm also just worried this could be the flyback. The epoxy seal is very brown, although there aren't any cracks, visible arcing, or ozone smell.

The last thing I'll mention is that connector J4 was burned when I opened up the mac, and I did reflow the solder, although I'll give that another shot this weekend along with some cleaning in the yoke connector. I did reflow any other connector or large component on the AB, although I may try that again with more flux this weekend.

I'd appreciate any thoughts or suggestions!!

 

[Fred1212]

Have you changed the caps on the AB?

 

[Kouzui]

Seconding the AB caps, and will add the 4-pin yoke connector - if it looks burnt, it needs to be replaced as well. You can search for it on the forums here and find the part number (can't recall what it is ATM), and remember that one of the big electrolytics, and the one that usually fails, is nonpolar. Replace it with another nonpolar cap or you'll get an unpleasant surprise on power on.