How to fix vintage 12C enter key?

[Peter Matthews]

I have just purchased a 12C off eBay and the ENTER key is not working well. It doesn't press down very much. Everything else is fine but the ENTER. It almost feels as if it does not move (but it does work if you mush it down).

Is this fixable if I open it up?

If not, where might I get a "part" to fix it?

I recall seeing a photo with a bubble-like keyboard layer. Maybe that's the culprit?

Please advise, oh HP experts!

Thanks

[Randy]

Try pushing on TOP of the key, directly between the Clx and 1 keys... chances are good you are pressing in the middle or at the bottom. Since the key hinges at the bottom, the normal travel can only be felt by pushing the top of the key.

[Peter Matthews]

Hi - thanks but I think the key needs to be mechanically fixed. I have a 11C and the travel is totally fine.

I tried opening up the 12C but there seems to be many layers of plastic wrapping and some kind of black paper wrapping. I didn't want to cut through the paper without knowing ahead of time what's going on.

[Jeff O.]

The following is from memory from disassembling an 11C a few years ago.

You should be able to unwrap or unfold the plastic wrapping to expose the back side of the keyboard printed circuit board, i.e., no cutting should be necessary. It may be necessary to peel off an adhesive edge on the plastic, I do not really recall. With the back off and plastic out of the way, the rest of the calculator (keyboard pcb, key contacts and top half of the calculator shell) is held together by about 40 plastic pins, known as heat stakes. The top of each of these pins is mushroomed over the keyboard pcb to hold the “sandwich” together. So further disassembly requires these heat stakes to be removed. Do not just slice the head off of each stake, instead carefully trim just the mushroomed portion off of each stake, leaving the center of the post sticking through the pcb. Once they are all trimmed, the keyboard pcb should come off. The key contacts consist of a sheet of metal domes, my guess is that the dome for the enter key is collapsed. I do not know if it will be possible to repair a bad dome, you might need a new sheet of contacts from a donor unit. If you succeed in repairing/replacing the contact, then you get to the hard part: reassembling the keyboard “sandwich.” You must hold the “sandwich” tightly together while either melting over the remaining portion of each heat stake or gluing each stake with your choice of adhesive. I believe someone here once reported building some sort of jig to hold the sandwich together while gluing or melting the stakes, but I do not recall who or if they reported total success.

I am not really a repair expert and like I said, the above is from memory. So, while I believe the information to be accurate, follow at your own (or your 12C’s) risk.

...

[Peter Matthews]

Wow thanks Jeff! That really puts things in perspective and is great info before I even attempt such a thing.

[David]

If you do pull apart your calculator, you have some luck on your side:

The buttons each have a 'snap disk', which is both a simple electrical switch, and also what makes them feel right. The Enter key has two, one which is used (at the top) and one which is not (at the bottom); this because the Enter key takes two positions on the 4x10 grid.

You could conceivably take the lower disk and put it in the upper position, restoring snappiness.

Fair warning: this will be a lot of work with risk of not getting the disk position correctly. The disks are joined into strips of 5 disks, and you would be moving a disk from one strip to another. Plus the adhesive film will likely be destroyed and will need to be replaced. Not a big deal as it is pretty much the same a box sealing tape, though you may find it challenging to find some tape wide enough.

Electrically you should be fine making this change, because the PCB wires the disks together redundantly with respect to their continuity being formed from strips.

The other disk is utterly unused and is there surely for the manufacturing economies.