Faulty PRGM / RUN switch on 29C

[Maximilian Hohmann]

Hello!

As I am currently working on two HP-29C's I took the opportunity to take some photos for illustration:

This is how the PCB with the electronic components (including the LED display) is conneced and mechanically joined with the faceplate which contains the keyboard contacts. The photo shows the calculator facing downwards with the power connector rods sticking out to the top. The connection between the PCBs is through two such rows of goldized contacts, one between the topmost row of keys and the switches and the other below the lowest row of keys.



This is after pulling out the contacts (soldered to the component PCB) from their holes (which are on the keyboard PCB). If one or more of these contacts are corroded, rows or columns of keys or a switch will not work any more. The same happens of course if a trace on either PCB has been "eaten away" by corrosion.



That's what an intact keyboard PCB looks like. The black dots are the "heatstakes" which hold the PCB in place.



After mechanically removing the heads of these heatstakes (e.g. with a small knife or drill) the PCB can be separated from the calculator case:



These are heatstakes with their heads removed close-up:



And this is the part of the PCB where the switches are located. That's acttually the back side of the bit of PCB shown in the previous image. If a switch does not work, chances are high that there is some corrosion or other non-conducting stuff in that area.



Finally, the contacts of the switches. These look large in the photograph but are actually very tiny and can easily be lost upon disassembly of the calculator... Again, corrosion of the metal or a worn gold layer can cause them to stop working. There is absolutely no replacement for these contacts apart from other woodstock calculators.



Good luck with your repair!
Max