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Unusual HP-21 keyboard repair
06-06-2021, 12:18 PM (This post was last modified: 06-06-2021 12:24 PM by PANAMATIK.)
Post: #1
Unusual HP-21 keyboard repair
One of my Woodstock HP-21 had a very bad keyboard. Many keys didn't respond at all. When I opened it in february this year by cutting off all heatstakes, I was unpleasantly surprised: all contacts were corroded beyond repair. But this explained everything.

[Image: uc?export=view&id=17RFpY2HTijSd9un0SWAf7ATPowe06SZP]

Beyond? No, I thought that I could manufacture a new flex keyboard for the HP-21, replacing the corroded copper with new perfect gold plated contacts. Would it work?

[Image: uc?export=view&id=15ZFLMqysKu_2TmET78bhHw3S2BR_iWgs] [Image: uc?export=view&id=16l4hUtvMxo2fSAGc84nHXEUGlzJbWAYb] [Image: uc?export=view&id=1Rj205tVhJEP0BWbqSEL44oMFjMFw9H_o]

Last week I received the flex boards from DirtyPCBs. I had made a mistake by designing the 37 holes for the heatstakes too small. This caused the flex board not to be flat. Therefore I bought a special tool to enlarge the holes, which is normally used to punch holes into leather belts. Drilling was not possible, because the holes were not flat after the procedure. Also the location of the 7 contacts for the keyboard rows were not precise and I had no hope to get proper contact without soldering.

[Image: uc?export=view&id=1Cr_SA3mgkbnsy2RSy4BocelMVEOB6oef] [Image: uc?export=view&id=1gWD5OMmAU8yalyq_3P_HsyBh0Vs2cUX-] [Image: uc?export=view&id=175rXCWhxCwd9AQqgAsHsyruBBFpzw9dG]
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Aligning - Soldering - Measuring - Melting the heatstakes - It Works!

It is very important to press the keyboard PCB firmly against the buttons before melting the heatstakes. Otherwise the keys will not have the original touch and feel. It is possible to melt the parts together with the original heatstakes, but only one time, later there will be not enough material left, after you have opened it again. Perhaps there is a material for 3D printing, which can be used, but I don't know.

I got 12 flex PCBs, two are destroyed by necessary experiments, two are used for my repairs, 8 are left. But I'm not sure, whether I should sell them, because the repair procedure is not easy. Is there a demand?


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06-06-2021, 01:48 PM
Post: #2
RE: Unusual HP-21 keyboard repair
The originals look to be in pretty good shape except for surface corrosion. You could try burnishing them to remove the oxide then gold plating them.

Tom L
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