Jittery Display on 512k

[bedroom]

Hey guys,
I recently was gifted a Macintosh 512k for Christmas. I have been using it kind of nonstop, and I noticed that after having it running for a little over ten minutes or so, the display begins to freak out a little bit. The top and bottom edges start to bounce around a little bit and faint lines show up on the bottom and the top of the screen, seemingly mirrored. I've thumped the side a few times when this happens. Sometimes it helps, sometimes it doesn't. The display has never gone out entirely, nor have I ever gotten a thin white line across the screen, but I'm worried that if I continue to use the Mac in this state I will end up screwing something up.
My best guess is that the solder joints on the logic board are cracked, and when the computer is on for a period long enough for the components to start to generate noticeable heat, the flowing heatwaves irritate the already straining connections. Then again, I am relatively new to the world of compact Macs. I know for a fact that the analog board has been recapped, and upon inspecting the logic board, it doesn't seem like any of the capacitors are bulging.
I'll appreciate any and all help I can get. Thank you!

In terms of specs, this is a stock 512k, not an upgraded original. No add-ons have been installed. It has only a 400k floppy drive, no hard drive. I have only been inside it once to grease the disk drive up a little bit. The screen problem predates this.

 

[Byrd]

Welcome bedroom,

It'll likely be one or two dry/cracked/loose solder joints on the analog board of your 512K - look at the connector where the CRT yoke harness plugs into the analog board (both the connector itself and the underboard of the board), one of the pins will likely be scorched, blackened, in need of solder removal and to reflow with fresh leaded solder. Take note also of the traces going elsewhere on the underside of the board and check if they too are not looking worse for wear. Sometimes a yoke pin may require replacement if too damaged, or a trace repaired on the underside of the board.

Of course, going down the rabbit hole is that while working now, if never properly serviced you will also need to eventually recap the analog board for long term reliability.

 

[cheesestraws]

Agree with @Byrd — I think your diagnosis of a dodgy solder joint is likely correct here.

Those analogue boards are a bit notorious for the chunky solder joints on the connectors cracking. I'd add to Byrd's point that if you're comfortable doing so, resoldering all the connectors is probably worthwhile preventative maintenance here. Sounds daunting put like that but there's only three of them and they only have a handful of pins each .

 

[AndiS]

You should also check the live voltages at the 12 and 5 V lines. This can be done frome the Floppy port without opening the case. I had wobbly screen problems with a Mac Classic that were caused by a low 5V supply. If you find this to be the case, you still have to dectect the source of the problem, but it might give you a hint.

 

[bedroom]

Thank you very much for all the replies! I have never checked live voltages before so I will save that one for last. I'm almost 95% sure that the connections are the issues, as Byrd and Cheesestraws said; sometimes when the lines appear I give it a gentle thump on the left side and they go away pretty quick. I will crack it open this week. Thanks again!

 

[alectrona2988]

Those early Macintosh systems had no active cooling, so it's guaranteed you would run into issues down the road. That's why those "system savers" existed which went right onto the "handle" of the macintosh case. Due to all that excess heat, these weren't exactly reliable computers. They were damn well built like a tank, though.