HP21 uses same RAM chip as HP25?

[jebem]

(05-21-2019 12:52 PM)calc-calcs Wrote:  OK Didier, that definitively clears it up. And the ACT having those internal registers is probably why my HP25 still mostly works without the RAM chip. Interesting.

Thanks
Alex

I would remove the suspect RAM ic and have it cleaned to make sure no electrolytes residues are left under the chip. Cleaning the pcb nay not be sufficient.

Probably you know how to remove the chip without killing it or damaging the pcb. Here it goes anyway:
You need a solder vacuum pump or even a manual vacuum pump to suck the melted solder in order to be able to remove the chip without destroying it.
Small flat pliers or even small cutters are usefull to shake the legs a bit to make them free while still in the pcb holes. This allows to remove the chip without destroying the pcb.
The soldering iron must have enough thermal energy to maintain the correct temperature, so a 30 watt with a small tip would be adequate.

Cheers.
Jose