Hello and asking for help with HP 67 power unit

[Tapti]

Hello alltogether,
I'm new to this forum. The reason to join you is that I'm a proud owner of one HP-32e and meanwhile two HP 67. One of the latter having a problem most probable with the power unit. I'm physicist by profession, so electronic circuit are not completely meaningless to me. But that's the issue with pysicists: They believe they know everything. Actually they know something of the most, but nothing very well ;-)
The problem with my 67 is the following: Overall it is in an exceptional good shape and it would be a pitty not to get him working. When the maschine is switched on (independently from the actual power supply) the reading is quite normal and it is possible to perform some calculation. After say 3 to 5 min the display starts showing nonsense (first only wrong numbers), then it starts decaying and in the end the only thing visible is the decimal point. After switching it off again and letting him alone for some time, the game starts from the very beginning. So my suspicion is that some part within the power unit (switching ps for creating the different needed voltages) is old or broken resp. Did somebody of you already observed a behaviour like that ? These are only some parts to exchange (2 transistors, some diodes etc), but I don't know the actual spare part numbers of these. Otherwise I would just rebuild the psu.
Thanks for your thoughts,
Stefan