HP21 uses same RAM chip as HP25?

[jebem]

(05-22-2019 08:30 PM)calc-calcs Wrote:  Stop the press!

Remembered ordering two 10-pin low profile socket pin strips some time ago, took me a while to find them, but eventually in they went and the RAM chip as well.

Upon first power up, everything is now working, all 8 registers, statistical E+, programming... all is good. Ok maybe the lingering heat from the pull and the resoldering... So removed the external power and let it sit for a while.

30 minutes later, still everything is working.

2 hours later, still going.

Just put the calculator back together, and installed the battery: still working.

Maybe the problem really was the hidden residual corrosion after all...
Or maybe the added heat during desoldering...

I'll try again much later and update. So far so good

Alex

Excellent news!

The socket can also be a source of bad contact due to oxidation/dry electrolysis after some years.
In the 80's the use of dip sockets was common and the first thing we used to do when repairing these computer boards was to press the chips against the socket to hear a good satisfying crack noise even if that couldn't fix the failure most of the times.