HP-97 weird behaviour

[AndiGer]

Hi all,

I have a HP-97 here that does not calculate correctly, but I don't can get to it what's wrong.

Some keys work ok as X-square or square root but some don't.
Positive number + cos gives 9.999999999 99
Negative number + cos gives -9.999999999 99
Any number + tan results in .00 and a minus sign in the exponent.
Strange: 10 + 1/x gives 0.10, 100 + 1/x gives 0.01 (any other exponents of 10, also negative ones give correct result). But 50 or 500 or 5.000 + 1/x gives 5.00 (or more 0 zeroes as DSP is set to more digits).
657 + 1/x gives 6.57, after about 1/10 second the displays flashes shortly to all 0 including exponent (as I can see - looks like a program is running but only very short) and then 6,57 again.
Pressing ->R after power on results in 0.000000000 exponent 0d. Weird.

Some function key pressed set the internal program steps counter up by 1! The first step is incremented by 2.
E.g.: Calc off, Prgm on shows step 000. Prgm off, e to the x, Prgm on shows 002 51. Prgm off, e to the x, Prgm on shows 003 51. Incremented to 004 51 after same procedure.

In Prgm mode nearly all steps are registered as 51 (i.e. R/S), but some not.
tan sets the program counter back to 000 as does RTN.
sin, 1/x 1, 2, 7 or +-key result in Error in the display.
R/S pressed results in a shortly flashing display and step counter incremented by 2.
e to the x results in a shortly flashing display and also step counter incremented by 2.
Pressing x<>y followed by 7 results in a blank display.

I changed the complete keyboard including PCB with a known good one. Behaviour is the same.
I also checked connections from the chips to the golden fork connections and test points resp. All are ok, no interruptions there.

I have the service manual but does not help for that case I guess. Some advice from the experts here?

BTW: I can create program cards on my other HP-97 if necessary. The card reader of the confused machine is also repaired, so it can read programs. Don't know by now it runs the read programs correctly. Don't think so.

TIA
Andi