12 Jan 2020

Super Monaco GP - Sega (X board) 1989

Got that board in a deal to be able to test my PCM reproduction on X board and Y board hardware.
I was told the board worked with graphic issues. Let's see!

First both main and sub CPUs were missing so as three ROMS: one main CPU, one for sprites and one for sound. I populated the board with fresh parts and I plugged the game. Nothing, black screen, no sound, no activity on busses whatsoever. What a surprise...

I probed the main CPU RAMs and found activity for only a fraction of second before CPU crashed... Data signals were unhealthy and since two RAM chips used were shiny ones (on the 4 tied to main CPU) they were the most probable cause to me. Of course tested out of circuit they failed miserably. I also noticed they were many of those RAMs peppered on the board so decided to probe them all and sure enough two were also faulty on the sub CPU. 4 bad RAMs to start with! The good news is game then started, with sound, but with graphic glitches as well.

Typical mask ROMs failure here on sprites. I dumped them all (16 of them) and two turned out to be bad. The second good news is X boards have DIP32 sockets even though original mask ROMs used are 1Mb DIP28 chips, and non JEDEC DIP32 EPROMs can be used in place. Replacing them got me a fully working game:

For 5min before screen got badly corrupted.

Probing here and there I found half of the address lines on the tile ROMs were floating. Tracing them back led me to a LS374 with all outputs floating. Inputs were toggling so as control signals. Piggybacing a good known chip cleared the issue so it was definitely proven bad.

Since then I've played the game for few hours without any new appearing issue. Let's hope it lasts!

Game fixed.

No comments:

Post a comment