25, or 4 to 6 [Not Chicago Content]

[PANAMATIK]

(01-29-2015 02:17 AM)Les Bell Wrote:  I have an HP-25 (non-C), for which I've rebuilt the battery pack and added a 6.3V zener across the battery contacts (not that I've even plugged in the charger, so far - in fact, it's still running on the initial charge in the new NiMH batteries).

It's got a keyboard quirk, though; sometimes pressing the '4' key places a '6' in the display. Repeated keypresses will give strings of 6's and sometimes 4's, and on occasion it will actually kill the display. A bit of jiggling sometimes resolves the problem, and right now it seems to be behaving itself, but the problem comes back after a few days of disuse.

A bit of Googling revealed a post in the old forum from someone with the same problem, but no fix was suggested.

Any suggestions? Without having stripped the calculator down and performed more comprehensive testing and cleaning, I tend to suspect dust under the key, but would appreciate opinions from those with more 25 experience.

Getting 6's instead of 4's is strange because these keys are in different keyboard columns, there might be some contact or smear, that has to be removed between the columns.

Each row, these are the horizontal five keys from left to right, is scanned every 308 microseconds. After 7 rows, that is after 2,1 ms, all keys are read. If a key is pressed, the voltage of the column goes from 6.4V either to 0V or to a midrange voltage about 3,5V , if the LED display segment is on, that is displayed with the same row as the keyboard. In my ACT chip I read the keyboard columns by its analog values, because they can have three different levels.

What do you mean with "kill the display"?

If you have an HP-21, HP-22 or another HP-25 or HP-29C, they all have they same keyboard, you can exchange them and check whether it is only your keyboard that fails.

I had a HP-21 keyboard with sticky 5 and it seemed to get better when moving the key from right to left several times, while still being pressed.

I have many (about twenty) HP-25/HP-21 etc. keyboards. I never had wrong key detection (4 to 6), always the key was sticky and needed more pressure.

Goeff is right to recommend cleaning!

Bernhard