Got a Rabio Lepus (japanese release of Rabbit Punch, a cute shoot ’em up released by V-System Co. in 1987) in lot of faulty PCBs.Hardware is made of a CPU and a VIDEO board:
Board had a graphical issue, sprites were splitted and misplaced:
Usually this kind of problem indicates an addressing fault so wrong address are generated hence wrong data are read/written.Sprites data are stored in six devices (four 27C256 OTP ROMs and two 28 pin 1Mbit MASK ROMs)
Reading them gave good dumps but the logic probe revealed that address line A7 was stuck high in all of them (address bus is shared)
Tracing this address line back lead me to an output (pin 19) of a 74LS273 @8A, the corresponding input (pin 18) was floating.All the other outputs were active (generating the other address lines for sprite ROMs) and inputs were toggling too.Each input is connected to a data line of a near 2k x 8-bit static RAM @10A so following this logical arrangement I figured out that pin 9 (data line D0) of the RAM should have tied to pin 18 of the 74LS273 but it was not so :
I restored the connection with a jumper wire and this fixed the issue:
As I expected, doing a visual inspection on solderside revealed a severed trace:
End of job.