Simple things first. Replace the batteries, followed by voltage tests after powering up.

Another culprit is the 40 pin connector. If original, the male and female should be replaced. Next would be to replace the Insulation displacement connectors. (Those are the ones where the wire is pushed into forks that cut the insulation and make contact with the wire)

Another thing is to check all your voltage levels when the machine is on, and then check again when it fails. Pay attention to the amount of AC on the DC power. It’s likely the capacitors on the power supply board are bad. Since they are over 40 years old, they should be replaced anyhow.

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