Taito Frontline arcade pcb repair
Game did not boot and just displayed a solid color screen. The Z80 cpu, located on the lowest board of the 4 board set didn’t show any activity with a logic probe. It hadn’t failed though – instead the WAIT line was being held which prevented the CPU from doing anything. There are actually 4 different potential sources for WAIT on these boards, so any of them could have failed as well as the OR gate that combines them. Testing with a logic probe revealed a bad LS74 which was replaced and the game could now boot.
Quite obvious though that some graphics were wrong. Frontline and other Taito SJ games are a little bit unusual – instead of the graphics hardware directly accessing the graphics ROMs, the graphics hardware draws out of RAM. So the CPU has a banked port into the graphics ROMs which it copies to video RAM as needed.
So in this case the problem wasn’t actually on the video board, but in how the CPU was reading the ROMs. The schematics show a bunch of 40193 chips that control the address lines for the graphics ROMs. One of these had failed – a logic probe showed dead outputs. A replacement was fitted and the game was 100%.