How to fix a bad key on HP 9100A

[Don Gray]

I was lucky enough to recently acquire an HP 9100A 'calculator'. It seems to be in pretty good shape except for the '7' key which does not work (although I dont yet have a means to test the card reader). The '7' key, feels like any of the other keys when pressed (similar resistance and I can feel te click of the switch engaging) but I get no output to the screen. I haven't tried to remove the keycap yet or to remove the keyboard; I thought I would see if anyone in this forum had any suggestions...

Thanks in advance, Don

[Tony Duell]

I know the 9100B pretty well, and I think the 9100A is very similar. The keyboards on these machines encode the keypresses to 6 bit keycodes (these are the codes listed on the pull-out card at the front, with STEP being the obvious missing one), and if one key only
is faulty, it can only really be the keyswitch itself. Unplug the mains cable and open the case (2 (or 4 on very early machines) screws on the underside. Swing the cover open so it latches open. Then undo the 4 obvious screws that hold the keyboard in place, pull it forwards slightly and unplug the cable from the edge connector on the processor chassis. Unplug the power switch cable from the 3 pin connector at the front of the processor chassis.
Now take the keyboard apart. Undo the 3 screws on the bottom plate, lift off the plate and free the grommet from the slot. You can now see the keyswitches. First try cleaning the '7' key. Soak a piece of paper in contact cleaner, put it between the contacts, press the key (so the contacts come together) and pull the paper out. Repeat a few times.
If that doesn't work, I seem to remember you can unhook the spring and remove the moving contact. It's probably best to get the PCB out of the housing first (undo about 14 screws, note which holes don't have screws in them -- they take the screws that hold the baseplate on).
Do not attempt to pull the keycaps off a 9100. If you want to remove the keys for any reason, take out the keyboard PCB (as above), take off the keyboard bezel (3 screws at the top between the toggle swtiches, and use a screwdriver from inside the keyboard housing to free the locking barb on the keycap. Then slide it out.
Each keycap has a metal rod that presses the switch. Originally, there were rubber O-rings on them to improve the keyboard feel. They've all long since cracked and fallen off. On my pair of 9100Bs, I took all the keys off and put new O-rings (1/8" ID, 1/4" OD) on the pins.
The card reader will almost certainly need a new roller. I've done this on mine, but it's a big job to do it. You need to take off the old roller (which really needs a special puller tool), make a new roller with a groove for the O-ring, and press it back on. You need a reasonable metalwork workshop to do this.

[Don Gray]

Tony,
The wetted paper did the trick. Also, lucky for me, the roller for the card reader appears intact. Now I will try to make a fake card to test it out (I found dimensions for the card in a past thread).
Thanks a bunch for your help, Don

[Tony Duell]

So it was just dirty contacts. I thought it probably was...
On one of my HP9100s, I had to take apart just about every keyswitch and clean up the parts to get them to work properly. It took an afternoon, but it was worth it. That 9100B had all sorts of other faults as well (blown EHT fuse, blown transistor on the microcode branch board, card reader roller, etc). I don't
know what had happened to it.