We restored three power supplies to operation, replacing bad capacitors and other failed parts. The fourth supply is the big one, delivering +5V for most of the logic, and it was blowing its 12A fuse immediately on powerup.
We discovered that all the electrolytic capacitors were bad and replaced them - fairly expensive parts to match the mounting method and available space. We then found two of the rectifier diodes were blown and replaced them. Now the fuse does not blow, but no power is produced yet.
On the board, the logic for the power supply is powered by an 18V regulator chip for operational amplifiers and in turn a 5V zener diode, that feeds off the 18V line, for TTL logic. The output of the LM7818 was zero, which can either be due to a failed regulator or a short circuit downstream. We ran out of time to work further on this unit, having spent the day restoring the other three and doing the capacitor and diode replacements.
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