Data East The Real Ghostbusters arcade pcb repair
Board booted to garbage on screen. Logic probe quickly showed the main CPU was not running (no activity on data or address bus). First thing to check is a clock signal is reaching the CPU and this seemed fine. Next up is to check the CPU isn’t stuck in reset – and this wasn’t fine. The reset pin was stuck low – this means the CPU wasn’t even trying to run as it was being held in reset state (the reset on most CPUs works by pulling the line low for a little while, then releasing it to high).
There are no schematics available for this PCB so tracing where the lines go was the next step. In fact it turns out the reset line goes all over the place on this board – the main CPU reset is shared with the sound CPU, the Yamaha synth chip and several other components. The common source is the LS10 chip at E11. The actual reset is generated on the lower board by a PST518 as is common with this era and this too traced to the input pin 1 on the LS10. The logic probe on the LS10 appeared to show valid inputs, but no outputs – so problem found? Well, the LS10 tested fine off the board, but curiously with the chip removed the output lines became logic high. That means an input somewhere on the board had to be driving that line high. So what was actually happening was the LS10 was trying to drive the line low, something else was trying to drive it high, and it ended up as a non-signal in the middle somewhere.
So, more tracing and the problem was found at the i8751 protection microcontroller – it’s reset input appeared to be putting out +5V. When removed there was an obvious problem – a pin was bent and not making contact – in fact it’s the ground pin for the MCU, so this was a very bizarre case where because the MCU was not grounded properly all sorts of lines were putting +5V onto the board. With the pin reconnected the LS10 worked properly, the CPU could reset properly and there was activity on the data & address lines.
But – still garbage on screen. There was one suspicious looking IC on the board – a Fujitsu LS00 at 13C - lots of corrosion on the legs even though all the IC’s around looked perfect. A lot of these Fujitsu’s have a high failure rate – I assume some kind inferior alloy was used for the legs and corrosion has travelled inside the chip package where the lines are even thinner. A logic probe confirmed the chip was getting inputs but the output lines were dead. Swapped this chip out and game working 100%