HP-71B keyboard repair


Recently, I was given a dead HP-71B by a friend. This friend is a gardener, and he used this calc (or computer?) outdoors. The little beast had leaked batteries and had a good amount of soil in it. I cleaned the battery leakage, and let the calc rest in water for half an hour. After letting it dry over night, I tried to start it: All segment were lit, and it didn't react on keypresses. Some of the keys I pressed stayed pushed-down, because the dirt inside the keyboard blocked them. So I decided to repair the keyboard:

The heatstakes holding the keyboard PCB together could be removed with a pocket knife. You slide the knife under the mushroom head, and turn the knife. This breaks the plastic of the mushroom while leaving the heatstake as tall as before. With this method, I broke about 5 heatstakes, the rest were only poles without their mushroom head. Cleaning the now dismanteled, well-built keyboard (which was very dirty) was easy, and so was putting the thing together. I decided to test the HP-71B first, before melting the heatstakes of the keyboard. It came on, and reacted to keystrokes. However, without the pressure on the keyboard, some keys bounced, and sometimes pressing one key also registered the key above it (pressing 'A' displayed 'AQ').

Even without melting the heatstakes, keyboard-feeling was pretty good. This is due to the fact that there is very little room between the keyboard PCB and the boxes containing the 'Front ports'. So I decided to leave the heatstakes alone, and just put some elastic material (mine was 2mm foil for isolating walls) onto the front port boxes and the battery compartment. This did the trick.

Keyfeel is now as good as on my HP-11c, however the foil was a bit too thick, so the case is now slightly bent. But I was able to clean everything inside the keyboard (and there was a lot of dirt) and the front half of the case, so the keys and the display look like new.

The whole procedure (dismantling, cleaning an putting together took about 2 hours). Now I am seeking a Forth/Assembler module to do something useful with it!

I hope this story helps people who have a bad keyboard on the HP-71B.

Have a nice week! Klaus


Maybe you aren't as picky as some. I have tried repairing these and, in fact, remelting the heat stakes and the keyboard is never as good as original. Very hard to get the original fit. Keys are always a little loose, compared to the original, in my experience.

Sounds like you soaked the keyboard before you took it apart. Did you put the LCD under water as well?


Yes, I put the LCD under water. I used warm water to clean the dirt away, and later used distilled water to finish. While being in the hand-warm water, the LCD developed some white spots, they didn't look like condensed water (but I think it must be condensed water) and went away while drying.

About the keyboard: It's certainly an improvement over a dirt-covered keyboard, so disassembling was the only choice I had. My keys are not rattling, and it feels good. Let's put it like this: "If you have an unreliable keyboard, then disassemble. If you have a dirty keyboard, then think twice".


Soaking does little except get things wet and mayby solidifies dirt into cement. You need to use something like an ultrasonic cleaner to make headway on these things. I have found three cycles (shake out all water as best you can between each cycle) in distilled water works well.


Hi David,

there were many particles inside the keyboard (between the keys an the rubber shielding). Would ultrasonic cleaning and shaking the water out remedy this?


I have done it successfully on several machines that had been immersed in mud, coffee, and mystery goo with very good results. After cleaning the machine I let it dry in an oven on low overnight (be careful... some ovens don't go low enough) or in a hot car with the machine out of direct sun.


Also, one should only immerse or use ultrasonic cleaners on machines as a last ditch emergency measure. The key domes are held in place with a sheet of adhesive tape. Too much water could cause the tape to come loose. I have never had it happen, but it is a possibility.


Sounds like a ultrasonic cleaner does a good job.
On the HP-71B (at least on my machine) there was no adhesive tape. The metal domes are sitting on the heatstakes:

Keyboard PCB
Metal domes
Rubber foil
plastic keys
So I guess getting the thing wet would not cause any troubles.

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