this section gives +15v, -12v and an unknown voltage at X17 pins , maybe guess 24vdc at least
and an control line going to the v35 diode.
I see there is mention of an powerfail sense line too
If you say you have an 20 ohms impedance at the psu main output, it is definitely an problem, maybe you have some diode protection shorted out
Takes many photos of your boards and show us ... we could find something ?