I've started my self-education in electronics by troubleshooting/repairing bench power supplies. The ones I've worked on all have service manuals, with a principle of operation, block diagram, and schematic. I haven't worked on enough of them to have a good sense of all the different variants, but, it seems that there will always be output elements, often old-school transistors, perhaps sometimes derated MOSFETS. There will be a control stage that compares actual output voltage to the set voltage. There may be a driver stage which is basically another little PSU, between the control and pass elements. Given the amount of current this thing can deliver, I'd assume it has a driver stage. There may be another, smaller, regulated power supply for the control and driver stage, and an unregulated DC supply for the output. Other bits are something to drive and calibrate the meters, something to control the current, overvoltage protection (crowbar?).
So, if this were mine, I'd start trying to figure out what does what, probably by starting to work out a schematic, by working inward from inputs, outputs, meters and major power elements (transformers, beefy heat-sinked semiconductors).
My SWAG, based on the symptoms you have described, is that the meter is driven off a signal in the control or driver stage, but that there is something wrong with the driver stage, or the unregulated DC to the output stage. It goes to full-scale because there is no feedback from the output.
Misc observations/questions from the photos:
- It looks like this uses classic transistors for the output elements. I see 8 on the heatsinks. Are there more on the other side? It seems like there should be more than 8 for 40+ amps of output. Or maybe there are I bunch of power resistors too? I think I spotted one nestled on the backside of a heatsink
- Some of the transistors may be part of the driver stage. Try and figure out if the collector/output of some transistors connect to bases of the others.
- It looks like there are thermostatic switches mounted on the heatsinks too (near the top). Do these control the fans? Do they somehow cut the output if the temps get too high?
- I see a beefy internal heatsink near the back, what's mounted on that? Driver transistor? Rectifier?
- Forward of the beefy internal heatsink, what's that I see? Is that a fan? Or something else?
- Looks like the beefy switch on the right of the front panel switches the output. The nearby button is perhaps a way to reset an output crowbar without power-cycling.
- There is a beefy transformer, this surely steps down the AC that gets rectified to unregulated DC to feed the output stage. It might also have accessory taps for other purposes. How many taps come out of it?
- I also see a smaller transformer that probably provides stepped-down AC that powers the control and maybe driver stages.
I don't know how helpful that will be, I'm interested to see what other people come up with.