Had an interesting case of a TOTAN that would yo-yo reset. Resetting stopped when the sound board was disconnected. After swapping the CPU board from another machine I knew the fault was on definitely on the CPU board. Suspecting the 3 state transceiver 74LS245 or the buffer 74LS244, I took it to the workshop for a close inspection. Levels measured on the outputs of U1 and U3 looked fine. Moving to the inputs (to see they matched the outputs) I saw on the Oscilloscope the logic level of the WDEN signal was not driving a nice 5 volts … only 2 volts on a logic high! A small amount of pressure on the ASIC socket had the signal return to a full 5 volts.

I removed the ASIC, using a proper puller. The socket showed an exceedingly small amount of corrosion, no corner cracks and no evidence of battery damage. Given how hard it is to desolder these sockets, it was time to use my trusty fibreglass scratch-pen. A quick polish of all the ASIC pins on the socket and the chip was called for. I have a small dental pick I use as well to prod the ASIC pins into a nice curved form. A thorough clean in alcohol and the chip was returned to the socket.

I replaced capacitor C31 while it was on the bench. Easy part to replace and I had a spate of bad ones lately.

The board was returned to its machine and I’m happy to say no further reset problems. I’m not so happy with my scores on the machine, but that is not a problem I can fix on the workbench.