Update: work on the 2nd 7603 I have is progressing well.
Refurbishing the power supply and regulator board is complete. The rectifier board received a new batch of electrolytic caps (on their nice adapter boards). The rectifier for +/-15v rail was faulty (CR811) and replaced. Also CR820/821 was replaced with modern equivalents (1N5625GP). Most of the carbon comp resistors were a little high. They are there to discharge the electrolytic caps, so I left them as is.
The main interface was removed and given an overhaul of electrolytic caps, plus a cleaning of sockets and pin connectors.
Finally the regulator board was carefully inspected. 6 out of tolerance resistors were replaced, and R952 was upgraded to a 1W metal film resistor (in line with changes to later production units). the original 0.5W resistor dissipated a little heat, and was beginning to discolor the PCB.
Tested on the bench, the -50v rail measured -49.9 prior to adjustment. Not bad for a 38 year old piece of gear. I will assess ripple performance later when work on this mainframe is complete. (Note that the ICs are removed from the logic and trigger/vertical boards - I did not want a faulty regulator to damage them!)
Initial results look good
Pile of rejects.