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Replacing the HP-9100 decimal select switch
Posted by David Smith on 6 Feb 2006, 7:07 p.m.
The HP-9100A and HP-9100B desktop CRT calculators have a thumbwheel switch on the keyboard for selecting the number of displayed digits to the right of the decimal point. This switch is a 10 position (36 degrees per step) continuous rotary switch (with no rotation stops). It is a custom switch that has produces a unique, custom four bit code for each position.
I had a 9100A that had this switch hopelessly damaged. I repaired the machine by replacing this switch with an off-the-shelf un-encoded rotary switch and a diode matrix to produce HP-9100 codes. It turned out to be surpisingly hard to find a suitible switch. It needs to be low profile with a short 1/4" diameter shaft (3/8" threaded mounting bushing).
The switch shaft needs to be rather short. I had to cut about 1/8" off of mine. The original switch had a round shaft, but the replacment had a "D" shaped shaft. This actually makes it easier to secure the tumbwheel knob. It is held in place by two setscrews. But, it does offset the thumbwheel markings from the switch markings by two positions (your offset may vary).
I used a Grayhill part number 05001-10N-9526 rotary switch that I purchased off of Ebay for $2.00. Digi-Key sells another Grayhill switch that looks like it would work, but it is around $40. I used standard 1N914 or 1N4148 signal diodes.
The switch has five wires connected to it. The common wire is BLACK. The other wires are colored (in my machine): A=BLACK/WHITE (with mayby YELLOW), B=RED/WHITE, C=GREEN/BLACK/WHITE, D=RED/BLACK/WHITE,
The original switch produces the following codes (thanks to Tony Duell for confirming my original tests): 0=ABD, 1=BD, 2=AC, 3=C, 4=A, 5=(no connect), 6=ABC, 7=BC, 8=AB, 9=B
The other six possible codes produce some interesting displays on the machine, including the display of the extra guard digits used in internal calculations. When you have the old switch removed play around and see what you can get. Also verify the color coding of your wires. Curious people with a good 9100 switch can set their switch to "5" and try jumpering the various codes into the switch wiring.
I made up 10 "single inline" diode packs consisting of four diodes each with a common anode connection. The common anode connection of each diode pack was soldered to a switch contact. The diode packs were aligned so that the diode cathode leads stood straight up from the back of the switch and the packs were radially aligned towards the center of the switch. This kept all the diodes within the footprint of the switch. There is no room for them to hang over the edge of the switch.
I then snipped out the unused diodes (see the next paragraph). Next, I wired the cathode leads of the diodes into four rings. The "A" ring was the outermost ring, the "B" ring next, then the "C" ring. The "D" ring was the innermost ring. This ordering evened out the density of the connections. I formed the leads by bending them over as close to the diodes as possible.
If I was going to do this over again I would have only wired in the proper number of diodes properly positioned onto each switch pin, but by starting with uniform four diode packs it was easier to get neat, evenly spaced rings of diode leads. It is very important to minimize the profile of the diodes on the back of the switch since you have little clearance to the side of the case. If your switch plus diodes were too thick to clear the case, you could wire up the diode matix on a small circuit board connected to the switch with 10 more wires.
After wiring all the diodes onto the switch, I attached the thumbwheel knob and then mounted the switch/knob onto the keyboard frame. It is held onto the keyboard frame by the nut on the switch bushing. Tightening the nut is a problem since there is very little clearance between the thumbwheel and keyboard frame. A very thin wrench would work, but I wound up holding the nut with needle nose pliers and turning the switch to tighten it down. FInally I soldered the common wire to the switch and the four code wires to their appropriate ring of diodes. Voila... 9100A good as new.
Edited: 6 Feb 2006, 7:17 p.m.
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