HP25 repair - need advice (TESTING POINTS PICTURED)

[albertofenini]

Update : success, but ...
We checked all the traces and discovered that the Ground terminal from the battery had a bad
connection on the upper layer of the main PCB.
We fixed it by soldering a thin wire and rechecked all the traces.
At this point we mounted the Toroid, one transistor (0550), the 2,2nF capacitor and the resistor and
we checked that the basic oscillator was working, and it was !
We mounted all the components for the PSU and we checked the Vdisp, Vss and Vgg where good.
The PSU was working.
We installed then the ACT and the Rom and the HP25 finally turned on !
However, when we added the RAM, we weren't able to use memory registers nor programming
All the programming steps look like in the picture.
This make us think that the RAM chip 1820-1564 is defective.
Any way we can check this ?
We browsed this link http://www.brouhaha.com/~eric/hpcalc/chips/ but it looks like the only possible donors could be an HP22 or an HP91
Does anyone has found a different compatible chip ?
Thanks for all the support !