Faulty PRGM / RUN switch on 29C

[David Hayden]

I finally got some time to tackle this repair and thought I'd report back.

Many thanks for the instructions on disassembling the 29C. The only thing I'd add is that the calc and case are also held in place with a small clip at the bottom of the calc, just behind the "Hewlett-Packard 29C" badge. After removing the screws and pushing the down on the charger contacts to separate the calc from the case, you can use a finger nail at the bottom to push the calc up slightly towards the display to disengage the clip.

Cleaning the PRGM/RUN switch would require removing the heat stakes so I tried cleaning the switch by putting a little DeOxit on a piece of paper and gently sliding it back and forth under the keyboard PCB. Cleaning didn't work.

Next I got my multimeter and realized that the switch shorts out two easily accessible traces. Low and behold, the switch was working fine!

So I cleaned the little tuning-fork shaped connectors that Maximilian's photos show. Still no joy.

At this point I notices something odd. When I put it back together, if I pushed on the bottom of the calc, various segments of the display would come on and off. Last night, in my Turkey-induced Thanksgiving food coma, I realized that I was pushing right over one of the connectors. It also occurred to me that while I cleaned the front and back of the connectors, it was actually the skinny sides that made contact with the holes they slid into when the calculator was assembled. So I planned to clean the little connectors again, this time using deoxit on a string or thread so I could get the edges.

This morning I disassembled it to clean again. But as I looked closely, I noticed something else: the tines of the little tuning forks of the bottom connector were closer together than the tines of the top one. If they were too close together, they wouldn't contact the holes then slid into and that might produce exactly the symptoms I was seeing. Look at Maxamilian's second photo and imagine the smaller tine bent in a little towards the larger one.

To fix this, I used a very small flat-head screwdriver. I slid it between the tines of each connector and gently turned the screwdriver to widen the gap a little. That did the trick and the calculator is now working again. Yay!