(HP-41) strange behaviour, needs repair

[Hiwi]

Hi,
i recently bought a very nice looking HP-41C serial 2130A.. from ebay. The seller said, that it is fully working.
The calculator looks like new, never been serviced, but does not work correctly:

You can do normal calculations, SIN, COS ... I think that is what the seller has tested. But I
can't enter any programs.

Switching to PGM mode shows a NONEEXISTENT message. SIZE 000 ,0049 0063 is working and i can access and store data in the registers. But no program.

When USER mode is on, i can make calculations but not SIN,COS,TAN in this case ALPHA turns on, and i can see the KEYCODES! Switching OFF returns to nomal display.

GTO... does not work either, i see forever 'ACKING _ ' instead of PACKING. SAME with function XEQ PACK.

Is the CPU faulty or ROM, can I do some other checks, PSU?

RAM should be ok, since I can store data in different SIZE options and make KEY assignments.

In case it is not repairable, has anyone a spare mainboard for sale?

Ralf