I purchased this unit on Ebay knowing it was failing the self-test and had the following errors: 612, 613, 615, 617, 618, 619, and 621. A quick search on the internet led me to this @EEVblog forum thread: https://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/
After changing out some electrolytic capacitors I swapped out chip U201 - not an easy task without SMD tooling. I found this video from @ToltecMerc very helpful: https://youtu.be/dlhpkA7d60M?t=931
The chip did the trick and the unit now passes all self-tests.
Timestamps:
00:00 Introduction, self-test errors
01:55 eevblog forum thread
02:40 Schematic analysis
04:00 Capacitor ESR results
04:48 18V rails fix themselves
05:30 Chip replacement strategy
06:09 A new strategy
09:07 Self-test passes, resistance checks, super stoked
