OK, I've had some time to play with this today. I am getting more and more convinced that the voltage references are out... more specifically, the section in the attachment.
Would someone be willing to probe a couple things for me in case you have one of these PSU's in working condition? It seems many of these HP/Agilent PSU's are similar, perhaps some other models in the same family could do the trick as well. The places to probe are listed down below.
That would be much appreciated!

So here is my thinking: the +15V power supply is OK I guess, no ripple, a bit low perhaps at 14.1V, but probably OK. Then the zener is dropping 4.97V, leaving 9.13V for the LT1021 reference input. According to the datasheet, the reference output should be between 4.995 and 5.050V - I am reading 4.45V... I thought that maybe the source has to be drawing a minimum current or something for it to work, but reading the datasheet it seems not to be the case, to the reference is simply out it seems.
I ignored that for a moment though and read the output of the U31 opamp that is supposed to be at +10V - it reads 9.56V - that makes sense actually - that circuit basically doubles the reference voltage - it is slightly low but well, perhaps within specs still.
Then I read the -10V reference again, that reads -14.7V. I thought it could be because the opamp output was not loaded, so I pulled it down with a 15k resistor just in case, but still same reading.
Therefore, just to check if these readings are indeed abnormal, I would appreciate if someone could probe the following voltages in a similar PSU:
- +15V rail
- Voltage drop accross CR32 zener diode
- Voltage output of the LT1021 +5V reference U40 (pin 6)
- Voltage output of the +10V reference (pin 1 of the U31 opamp)
- Voltage output of the -10V reference (pin 7 of the U31 opamp
The attached picture shows the region where the components are on the board. They're not hard to probe.