Faulty PRGM / RUN switch on 29C

[Maximilian Hohmann]

Hello!

(03-19-2019 06:43 PM)David Hayden Wrote:  Any advice?

Disassembling the calculator is straightforward: Carefully remove the two rubber feet under the display (e.g. by wedging them out with a tiny flat screwdriver or a needle) and unscrew the two screws (small phillips screws) underneath them. The calculator then comes apart with very little force which is best applied by pushing against the contacts of the power plug.
The calculator itself consists of two main pieces, the PCB and the keyboard, which are plugged together with two rows of connectors. Just pull them apart to separate them.

As a first test I would clean the connectors and reassemble the calculator. Often the fault lies not in the keys or switches, but in a bad connection between the two parts. If this does not help, I would try applying some contact cleaner spray under the keys and dry it with a hairdryer or compressed air. If that doesn't help, the keyboard itself need to be taken apart, which unfortunately can not be done in a non-destructive manner due to it's "heatstaked" construction. The chances of getting it working again are 50/50 and in any case it will never be as good as before once it has been taken apart... For a real good descrption of how this is best done we all will have to wait for Geoffs magic book :-)

Regards
Max

NB: I am right now in the process of making one good HP-29C from two broken ones. One has faulty chips and the other one has a keyboard which was totally destroyed by the previous owner in an attempt to repair it. From the outside it looks like new but not one key is working any more. So don't take the keyboard apart unless there is no other option!