Help with HP 11C keyboard

[osa...@gmail.com]

Hi,

Can anyone give me repair advice for an HP 11C keyboard? All was working great when the EEX, 4, 5, 6 and X keys stopped working. It was late so I turned it off and left if overnight. When I switched it back on they all worked again, until they didn't any more. Switched it off and left it overnight again but this morning they didn't work and have remained nonfunctional.

I have opened it up and don't know how to proceed. The keyboard is welded closed with plastic melts. If I pop them off I have no idea how to replace them and achieve any required pressure. The connector from the keyboard is very clean.

I'm guessing that one entire keyboard matrix connection has failed which has taken out half the row, but I've no idea how to proceed.

Can anyone advise me?

Thanks,
Owen

[Paulos, Richard G]

Many keyboards for computers and calculators use an array of rows and columns for the key signals. When you press a key it's not the N key, it's row X * Column Y. Part of the chip decodes the key location and uses that to assign the function/#. Since your 5 affected keys are in a row and adjacent to each other, the individual keys are surly okay. It would be the row wire/trace/connection. A weak or broken wire in the connector is most likely. Another place to look would be the trace between the % key and the EEX key. If the 11C keyboard array is 4 rows x 10 columns and run left to right, the break could be there. Such a crack in the trace would be nearly invisible. Or perhaps the keyboard is split into two arrays: 4x5 on the left side and 4x5 on the right side. Most HP's were vertical format keyboards and I could see HP deciding a split would be easier than redoing their decoding chip to accommodate the horizontal keyboard. Detailed schematics would show the layout if those were ever available.

I just repaired a circuit board where a cover tab was rubbing on a trace on the corner of the circuit board and wore right through the trace. Most commercial circuit boards are varnished. The abraded varnish looked like a small pile of white crud. This trace was so tiny, I soldered on a jumper to bypass the bad trace. I also cut off the offending bit of plastic cover. Works great now.

I just sold my HP11C last week. Retirement is approaching and I just didn't need it. IMO This line of HP Calculators are very reliable.

On my much older woodstock HP22 programmable, the stored programs are a list of the row X column of the key, not the numbers or functions. The program recreate the series of key presses.

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Subject: [External] [oldcalculatorforum] Help with HP 11C keyboard

Hi,

Can anyone advise me?

Thanks,
Owen

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[osa...@gmail.com]

Many thanks for the reply, some really useful pointers. You are correct, there appears to be two matrixes of connections. I’ve managed to conform that every key, when pressed, results in a good connection at the expected places on the matrix and an open connection when released.

Under a magnifier I can see a minute crack in the display / main board’s ribbon cable where it bends down before joining the clamp on the keyboard, and it’s on the line that connects the bad keys! I tested between the edge connector and the small roundel just before it goes it goes into the LCD panel and there’s no connection. If I move the tester probe to after the minute crack I get a good connection. So I think there’s a good chance that this is the issue.

So the next question, how do I repair that?! Soldering’s out. What about conductive paint?

[osa...@gmail.com]

It worked!! Tiny amounts of conductive paint applied with the end of a cocktail stick and now all the keys work. When I examined the cable strip I could see two splits, and a possible third. The only thing that made them work was when the cable was bent enough for the two sides of the break to touch. No wonder it became intermittent! 

I left addressing the lazy fourth digit in the display as I don’t want to stress the cable any more. The fourth digit lags behind all the others. As if if hadn’t noticed the display had changed and came in late. Sometimes you have to hold the calculator at a slight angle to see it clearly. It’s just the fourth digit. It all suggests that those particular 7 segments are not getting enough voltage, but I don’t know enough to be certain. It’s all sealed up in a plastic wrapper. I pinched lightly along the edges but it made no difference. Could be one of the rubberised edge connectors has failed slightly or some dirt has got in to it.

Are the displays in these calculators known to have issues like this?

Thanks again for all your help, I’m so glad I didn’t have to pop all the rivets on the keyboard!

[osa...@gmail.com]

In case it helps with anyone who has popped the keyboard rivets someone, on this forum or possibly a Facebook group, pointed me at Bob Smith Industries IC-2000 Black Rubber Toughened CA Glue. It's a supper glue, so sets almost instantly, with added rubber so it remains ever so slightly pliable.