To start with, in diagnostic (protection cancel) mode, check the output of the various voltage regulators.
The manual gives a range of acceptable values for the PS line.
Generally, take 3.3 (volts) and divide by 255. Store this value in calculator memory.
Multiply this by the value shown in the diagnostics.
The resulting number, 1.8 volts for example, is the approximate voltage seen by the microprocessor for that Protect line.
If the PS line is 0 or 1, try doing a reset using the preset/reserve option toward the end of the diagnostic menu. I have seen several units with non-specific failures of the PS sensing appear to be fixed this way. Theoretically, the only way you could get a 0 or a 1 for the PS line is if ALL the voltages for that summing network have disappeared. This is unlikely but possible with a loose plug or maybe falsely triggered by a power surge or some such.
If there is some mid-range out of spec value, often you can guess which power supply line is low or missing. If the value is HIGH for example, a -15 volt regulator could be bad. If the PS value is LOW, you might be missing a +15 or +5 volt source, and so on.