9100 Keyboard Rebuild (with pics!)


When you pull your 9100 keyboard and flip it over, you'll see that Cherry and Alps probably weren't making standard key switches in the 60s:

Each key switch is an apparently custom-made set of contacts with integrated springs. Since they're just hanging out in the air they can get dirty/dusty, which can make for marginal connections. Fortunately they're easy (if tedious) to clean individually. HP abandoned simple mechanical contacts in their subsequent machines (9810/9820) for fantastically complex contact-less switches that were extremely reliable but hell to fix when they malfunction due to the complex electronics involved.

BTW, see those two black cylindrical things at the bottom of the image? Those are high precision (fraction of a % tolerance) resistors. If they're bad your keyboard won't work, and you can't get 'em at Radio Shack. You can, however, get them at Mouser.

While you're waiting for the resistors to come in, you might as well pull each key, remove its crumbling black rubber O-ring, and replace it with a nice orange silicone O-ring that will last pretty much forever.

At this point both 9100s are fully functional: they can record, read, and execute the test program from the 9100 manual; and the digit spacing problem on the first unit (where the floating point display was not grouping digits by threes, the fault of a bum transistor in the horizontal deflection board) has been repaired. This had the beneficial side effect of brightening the display somewhat so that now it's only slightly dimmer than the display on the other machine.

(Actually only one keyboard works, since we're still waiting for the replacement resistor for the other; the working keyboard was plugged into the other 9100 to verify its operation.)

At this point it's mostly cleanup: once the parts arrive and the units are burned in for a few days, they'll be disassembled, thoroughly cleaned, and the upper and lower cases REPAINTED. Fortunately the plastic fascias of the keyboard and display/card reader area look great after a simple cleaning. The result should be two new-looking and new-operating calculators.


A most interesting keyboard (electrical) design. I did not realize that it used this analog sensing of keypresses (hence the 0.1% tolerance resistors). This is the technology used in automobile steering wheel (infotainment system) switches and home (Sony) CD (mega)changers.

Is there a documented schematic of the 9100 keyboard available?



I would hope that Tony Duell's hand-drawn 182-page schematic for the 9100B would contain the keyboard schematics...but I haven't looked.


0.1% tolerance isn't exactly precision in the resistor world when it comes to analog circuits. The resistors are in the key strobe circuit and form a voltage divider so it is the ratio between the two values that is more important than the absolute value. Low TCR wire-wound or film resistors will work fine.

It is odd that a wire-wound device of that age would simply fail. Just out of curiosity, what is wrong with it/them?

BTW, yes, the schematics are in Tony's set.


I don't know why it failed; but it's a 2K Ohm resistor and measures over 3K.

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