This post reminds me of a 128K upgraded to 512k I got a while ago. After a good cleanup dusting and reflowing cold joints in analog board it got a similar screen. I actually always heard a very loud and clear bong with the screen flashing but vertical lines. Almost as if it was a flashing disk normally seen at boot up but with the vertical bars. I figured it was a memory problem so I replaced all the IC's with 4164 RAM and returned the mac to its original configuration without the 512k upgrade. Did not change a thing, still same screen. I spent all saturday replacing the ram,and I just gave up on it after that. I figured it was some other IC or perhaps a trace on the memory chips that was damaged but I had spent too much time and left it, never got around to tackling it again. Hopefully this thread will provide a more clear fix for my own lol so I can go back end perhaps get it up and running.
Things that were done,
Tested Analog board with a mac plus logic board and it booted fine so isolated issue to the 128K logic board.
Took ROM chips from PLUS and inserted them into 128K board but same artifacts on screen, so original ROMS are good.
Removed all memory ICs and replaced ( huge pain! ) but no improvement.
Clear BONG every time for me but vertical bars always present.
Some pics..
