HP 35s missing keystrokes

[MarkHaysHarris777]

(04-26-2014 10:29 PM)r. pienne Wrote:  After playing around with it a bit again, it seems that if the C key is pressed a bit harder it always works, so maybe it is a mechanical thing and I just need to get used to using a bit more force on that key. Ah well.

The problem you are describing is a classic HP keyboard problem that goes back to the early key switches of the Wang 700, and others back in the late 60s to early 70s and beyond. The mechanical keys on the HP are patented (maybe its expired now), but they have two distinct problems that are difficult to eliminate. The first is what you are experiencing, and the reason 99.9% of all calculators manufactured today us silicone bubble button technology... the little mechanical metal contacts oxidize over time, more under some extreme conditions, and stop registering. The good news here is that the key boards may be disassembled and cleaned (lots of patience, and lots of time). The second problem these keys have is contact bouncing. Sometimes they register more than once (or not at all) because of contact bounce (electrical noise created because the switch 'makes' more than once very quickly even though only pressed once. Contact debouncing circuits must be employed for these kinds of input switches (HP calcs included) but sometimes corners get cut!

The HP35s is assembled with screws, so the keyboard can be removed and cleaned if you are very careful. Sometimes you can forgo that if you simply press the key more directly (not violently) and firmly. My 35s is only two months old (by serial number) and it is already starting to exhibit this behavior. Sooner or later I will need to disassemble it and clean the contacts.

If your environment is very dusty (very fine particulates in a manufacturing area, or some other venues with fine dust) or if you or those around you smoke, you will see problems quicker which will affect more keys sooner. Cigarette smoke is notorious for ruining the contacts of all kinds of electronic switches, and the keys of your HP are no exception. Keep it clean, keep it covered, and don't smoke around it. Be thankful that (unlike the fx115 or the 36x pro) the keyboard is not plastic welded shut. The 35s uses about 28 little screws (its a pain, but its doable).

Cheers