From: Dick Waddingham
Email: dick@brimpton.demon.co.uk-nospam
I have an HP25 that had the same symptoms and the repair was very easy but took hours to find! The problem turned out to be plating on some of the pins of the external memory chip that had become detached from the pin itself, in particular the clock input pin. It was not easy to see this even when examined under a strong magnifier. The solder joints looked perfect but they were effectively open circuit because the plating was insulated from the pin! I stumbled across the answer when the sharp point of my scope probe broke through the plating on the clock pin, after having probed the same pin a number of times, and made the connection. The pin was resoldered, and the plating then came visibly loose. After cleaning up the pin and making the joint properly the calculator now works perfectly. On checking I found a couple of other pins were on their way to becoming open circuit too.
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