HP-67 1805S02334 [RA and RB noisy, doesn't read but writes]

[albertofenini]

Did the swap ...

When the 02334's card reader has been put in a working unit it has proven
to be working, although reworked I would exclude the card reader for the fault

When I have put into the working unit the 02334's CPU board
I have had the same faulty behavior than before

Finally, I have put the working CPU board and the 02334's card reader
in the 02334's chassis so that I have used the 02334's reworked keyboard pcb
and I have been able to read and write

Possibly the 02334's CPU board is the responsible for the noisy RA and RB,
would it be the 1820-1751 CRC chip ?
How can I check it ?

[EDIT] I have put the 02334's CPU board back in its original 02334's chassis,
with a 0.00 on the display and the card reader disconnected, RA and RB measured at the CRC chip
are clean, but once the card reader is connected they immediately get noisy
This, does not happen if I use a different CPU board

any help is appreciated !
take care !