Let's look at -15V then.
First, examine the schematic to determine where -15V is used. This comes from A4 U60, which is an LM320T-15. It looks to be good for around 1.5A. I probably missed some:
Page 165, A4, U10, CC DAC
Page 165, A4, U9, CV DAC
Page 166, A3, R82, CC FULL SCALE
Page 166, A3, R81, CV FULL SCALE
Page 167, A3, R96, CC ZERO
Page 167, A3, U10, CC PROGRAMMING VOLTAGE
Page 167, A3, R155, Q7 follower
Page 167, A3, R90, REMOTE CURRENT METER ZERO
Page 167, A3, U11, CV DAC OUTPUT
Page 167, A3, R7?, CV ZERO
Page 167, A3, U7, CV PROGRAMMING VOLTAGE
Page 167, A3, U8, CV DAC OUTPUT
Page 167, A3, R38, U4 input bias
Page 167, A3, U4, OVP
Page 167, A3, U16, LOCAL OVP
So it looks like there are a bunch of op-amps with several control potentiometers and a few adjustment trim pots. Losing -15V makes sense that it would appear to be in OVP mode and would make the OVP indicator malfunction.
With the -15V J2 jumper disconnected, what do you get for resistance and capacitance measurements on the -15V line past J2?