Midway Terminator 2 arcade pcb repair
Game was initially dead with video or sound output. The TMS34010 CPU is the fore-runner of the CPU used in my earlier Battletoads repair and shares most traits with it including control pins that can halt the CPU for external memory access. The logic probe showed a very similar problem to Battletoads initially – the CPU was resetting properly, getting a clock signal, but had no activity on the data/address bus. Something was holding the LRDY (local ready) pin low so the CPU appeared to be in a wait state for something external. That something was traced to the Intel security chip which is not replaceable if it has failed. Luckily it hadn’t failed – the problem was that chip was not getting a clock signal so it was also stuck causing a deadlock. I checked with the schematics and found the clock signal for the Intel chip came from… the TMS34010 CPU. This CPU actually provides clock outputs as well as having a clock input and it seemed this part of the CPU die had failed – a replacement CPU taken from a Mortal Kombat 2 board got the board to boot to the test menu, albeit unstable and crashing after a few seconds. (The MK2 pcb failed to boot with the T2 CPU proving the CPU itself was bad).
Finding the instability took much longer… there was a previous repair to the board with 4 RAM chips replaced. These all share a common data and address bus but a continuity check showed 2 chips had a couple of unconnected data lines – the soldered pins were not making good contact with the traces/thru-holes (specifically traces on the top side of the board rather than solder side). With the solder reflowed the board was 100% again.