HP21 uses same RAM chip as HP25?

[calc-calcs]

I just posted this on another thread, so I hope its not a problem to post it here as well, as it seems relevant to this discussion.

Well the RAM chip is off the board, and yes there was a bit of remaining corrosion between 3 of the 4 traces that run under the chip. Extraction went quite straight forward although it was a bit tricky to clear all the vias because most IC pins are leaning on the inside wall of the via plating itself, so most of them needed to be centered in the hole first. It was a good experience in preparation for the ACT swap.

The RAM chip seemed clean underneath and between its pins, but I gave it a good cleaning with a soft toothbrush and 91% IPA, as a matter of course.

Without the RAM chip the calculator is still functioning exactly the same way as before. I am tempted to solder in a socket and insert the RAM chip to try again and see if anything changed. But I can't find any low profile sockets in my part bins. Guess I could always just unsolder the socket if needed. Also will only solder the few chip pins that are actually in use.

Oh and following the advise on a post I read these days, I lifted one leg of the charger input diode and put some heatshrink on it. Better safe than sorry.

I have some pictures of the restoration process since I first opened the calculator for the first time, but guess there nothing most of you have already seen or done yourselves.

Anyway, if I decide to reinsert the RAM chip with a socket I will report back any further findings.

Alex