HP-29C now has dots across the display...

[Maximilian Hohmann]

Hello!

(09-02-2024 01:50 PM)AnnoyedOne Wrote:  The engineer in me wants to experiment

Not with my HP-29C! That was one of the most difficult to find (and costly) calculators in my collection and I would not make experiments with it, especially not experiments with the supply voltage... Normally undervoltage should not cause damage to the circuitry, but who knows with the flimsy power supply design of the Woodstock series.

In my experience, in most instances of intermittent malfunctions, the silver rivets on the PCBs which connect to the battery are the culprits. Silver oxidises quicky, especially when traces of leaked battery chemicals are present, and siver oxide is a poor electrical conductor. Sometimes it does not help to clean the contact faces because the connection between the rivet and the PCB trace is oxidised. It is rather difficult to re-solder but worth a try.

As an alternative to regularly clean the contacts they can also be gold plated. For that a galvanising pen, gold electrolyte and a power supply are required. The cost (without the power supply) is in the order of 50$/Euros and probably a little high for treating a single calcaulator. And one has to be very careful when applying the voltage for galvanising the contacts. 4V is required which is already too much for the calculator. So make sure where to connect the pen.

My pragmatic solution is to bypass the awful contacts altogeher. On some woodstocks (more will follow...) I simply glued and soldered a little 2-pin JST connector to the calculator PCB and a little cable with the corresponding plug to the battery pack (something like this: https://www.ebay.com/itm/253329082894). No problems ever since.

Regards
Max