One thought...
Do you have a reliable bench PSU?
If so, you could temporally use it for the -12.6V rail, and see if the rest of the instrument power's up, with the internal regulator disabled and disconnected from the rest of the instrument.
Pull F4 (the 12.6V rail fuse) and connect to the downstream side of that from the external PSU -ve, and it's +ve to chassis ground.
You'd have to make sure that both PSU's are switched on (and off) at the same time, and that the bench PSU? has a working current limit/foldback, set for at most 4A.? (Disconnect the flood gun filaments too perhaps, "just in case"!...)
The only other "dependency" on the -12.6V rail, on the PSU schematic I can see, is a resistor to the base of Q12 in the Calibrator circuit.? I don't think that's going to worry about anything...
If nothing smokes, you could also vary the voltage slightly +- of the set-point to check that the "Sensor Amplifier" (Q11) in the 141T's PSU circuit is behaving correctly.
If the external PSU is stuck at it's current limit, or folds back, then the root cause of all the trouble is deeper in the instrument.
As above.? Just an idea..
Cheers.
Dave B.
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