Mike, your remote prognosis was correct! I tried the piggyback method with a RAM chip from the working board, and sure enough UE4 was bad. When I piggybacked the chip Jupiter Lander started working much better - the black lines on the screen were mostly filled in with red. So I socketed the board and put the donor chip in, and now jupiter lander works 100%.Mike wrote: ↑Mon Jan 03, 2022 7:55 am I wanted to check that BD7 (bit 7 of the VIC side of the data bus) reliably appears at VIC when sent from the 6502 over the '245.
At this stage, I'd take the entire mainboard trace of BD7 under extreme scrutiny - continuity, shorts to other traces, any cold solder joints, checking all socket pins. If available, I'd check the signal on BD7 with a scope at the '245, VIC, VIAs, RAMs.
However, I still get some weird behavior when reverse characters appear on the screen - for example directory listing.
This directory listing still looks the same: https://imgur.com/pzlJSDx
I went back to original VA10 hypothesis and put a new chip in UE8 - no change. I've tried the VIC 6560 and character ROM in the other board.
Possibly another bad RAM chip? Can I replace a 300ns RAM chip with 200ns? The 200ns 2114s much much cheaper and easier to find.