HP-25 - what could be wrong, simple question

[albertofenini]

Hi everyone,
we posted few days ago this Restoring HP25 S/N 1702S13138 - Very strange take a look
The aim was however to restore another HP25 RE: HP25 repair - need advice (TESTING POINTS PICTURED)
The symptom was that the unit was working, performing simple operation but we were not able to store data in the registers or store a program.
We thought that was a bad RAM chip 1820-1564
So we recovered this chip from this last unit, note we don't know if it was working or not, but once transplanted it didn't solve the problem.
Are we just out of luck and also this RAM chip was not working ?
The question here is, just to be sure what should be looking for, can we assume that a working unit
where the only not working part are the registers have a defective RAM or it could be a different cause and we should be looking also for something else ?
Thanks anyone for help