Thank you all for helpful comments and hints. I hope finding the time over the next weeks go systematically through the boards.
I have the mentioned power supply document with schematics - in principle it looks rather simple, compared e.g. to a 9845 PSU.
For testing I clamped the 7805 to the sheet metal case as a heat sink so this should be o.k. for test operation. In the long term I will replace it with a proper LM 309K, just for the looks. Voltage in front of the regulator is about 18 V (unloaded) so this looks fine to me.
I will also make some measurements on the PSU of the second working terminal for comparison (that was why I wanted to get two of these).