Quadra 700 fuzzy video

[VMSZealot]

I've just acquired a Quadra 700 - but it has an unusual problem, and APD thinks that all checks out with the box!

When the Quadra is doing nothing, the video (on the Macintosh Colour Display more usually seen connected to my LCIII) is stable and crisp.  When it's processing, the video gets a slightly stippled pattern on the right side of any item on the screen (window, mouse pointer, text etc) and when it's accessing the floppy or the hard disk the stippled pattern goes berserk and extends many pixels (over 60 by the looks of it) from whatever the item it's stippling from it.  This is regardless of the OS booted.  It's most noticeable at 1 bit depth.  In colour, it's there - but not quite so noticeable.

My guess is that it's power related - and a repair to the PSU would fix this issue.  But has anyone else seen this problem, and what do you recommend?  Does anyone sell capacitor kits for servicing Mac power supplies?

 

[cruff]

Sounds like a bad capacitors issue. Maybe start with the power supply then move to the motherboard.

 

[jessenator]

Hrmmm. The Quadra 700 had tantalum capacitors from the factory. Not saying there might not be a failure there, but they aren't the electrolytic kind, which leak over time. From what I've gathered, when tantalum caps go they GO and take a lot with them, similar to, but not quite like, a Maxell clock battery bomb. I would wonder though if it is a power issue or something else on the board—

@VMSZealot have you tried removing the VRAM simms to see if the artefact persists? That might be a good, simple place to start. It has 512K soldered, so you should be able to get a display signal still, with the SIMMs removed.

The only other shimmering experience I've had is on a IIsi (which incidentally uses the soldered RAM for VRAM), but that was somewhat cap-goo related. The other instance is the logic and analog boards on my SE/30 causing the screen to have issues. Since the arch of those two are very different from the Quadra 700, I would try the dedicated VRAM first, and then perhaps give your board a good visual inspection (any battery, or other misc spillage/goo) and perhaps give your board a good clean and rinse with 91%+ isopropanol before thouroughly drying.

A power supply re-cap or replacement (ATX mod or mAcTX adapter) is definitely good due diligence with something nearly 30 years old.

The board should be labeled, but here's a diagram of what RAM is what: