HP30b - reluctant key

[Chasfield]

Emboldened by my recent success at fixing the the reluctant numeric keys on my Texas TI59, I decided I would attend to the similar problem affecting my HP30b, which has had a duff "2" key out of the box. It never fired on the click when minimal pressure was applied - all other keys would do so. It required a follow through, with extra pressure, before it would register. It also had a tendency to double fire when so treated.

I decided that I would dive in and try to fix it. The risk of wrecking the calc. in the process was not a problem as it was heading for the trash anyhow. B.T.W., this thing had the usual Rolls-Royce UK pricing for US imports when I bought it.

Apart from five small screws (two hidden under the rubber bumper of the rear case), this is a pop-apart device and brave use of the small blade on my Swiss army knife was required to pop the front silver case upwards out of the rear case/bezel (best not to start the popping near to the fragile LCD panel). I used a Stanley knife to split the heat stake ends that hold the motherboard down. This made it easy to rive off the melted over bits. The motherboard, complete with stuck down key dome retaining membrane, could then be lifted off the front case, leaving a rubber membrane and the key tops in place. The last part of the tear down was to peel back the white plastic dome retaining membrane far enough to expose the gubbins of the "2" key.

Here lay the surprise. I was dead sure that I would find some visible foreign body that was interfering with the click dome's action. However it was surgically clean in there. Nevertheless, I polished the contact surfaces with a WD40-soaked Q tip and put everything back together.

I didn't try to make good the ravaged heat stakes. Instead, I applied some blobs of translucent silicone such that they would retain the motherboard against keying pressure by filling the gap between it and the rear case. I won't need to dismantle the calc. again - if the keyboard acts up one more time, out it goes.

Anyway the fix worked and the "2" key now responds perfectly.

[jebem]

Thank you for sharing your experiment on this machine.

While I did similar repairs on other models and brands, I didn't try to repair this specific model (not yet).

This model is well known for presenting this defect, even on "like as new" specimens.
Therefore this issue doesn't look like to be caused by the usual metal wear and oxidation resulting from prolonged usage.
I would say that early key contacts isolation is something related to poor metal resistance to oxidation combined with manufacturing contamination.

Did you by any chance took some pictures of your work that you could share here as well?

[Chasfield]

Sorry, I didn't take any pictures of this tear down. The process was reasonably straightforward - the heat stakes are relatively few and easy to deal with. The casing clips are robust so that they don't come to harm when you split the calculator.

Note: just edited my original post to mention the 5 screws that hold the calculator case halves. together.

[Dave Frederickson]

(11-13-2017 05:51 PM)Chasfield Wrote:  I polished the contact surfaces with a WD40-soaked Q tip and put everything back together.

The WD-40 website doesn't mention use on electrical contacts. DeoxIT is preferred. Where did you hear of using WD-40?

Dave

[Chasfield]

No recommendation led me to WD40. I know, however, that the carrier solvent in it is potent stuff and thought it would be a good bet for breaking down whatever it was that was preventing good switch dome contact. In my judgment, the very slight residue of lubricant would probably be helpful insurance against future oxidation and would not hinder electrical contact. Time will tell.

The product is not suitable as an electrical contact maintenance spray - it is flammable for one thing and can break down some plastics, so I wouldn't go squirting it into electrical devices.

I have also found WD40 to be a good softening treatment for battery compartment verdigris, prior to polishing the contacts with a mini rotary brush. Once again, application should be minimal via a Q-tip. never spray it around.