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Another HP-21 repair
11-19-2019, 10:01 PM
Post: #1
Another HP-21 repair
I just got another HP-21 to play with (previous one here https://www.hpmuseum.org/forum/thread-13990.html)

This one was presumed dead but could revive it just by installing new batteries. Oddly enough it does not work when connected to the charger (with batteries installed) but it operates normally on just the batteries so all is well.

Some of the keys do not register at all when pressed so I opened it and will clean it tomorrow. If I understand correctly I should clean the contacts with vinegar, then rinse with water and clean again with isopropyl alcohol.

Here is a picture of the unit disassembled

   


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11-20-2019, 10:24 AM
Post: #2
RE: Another HP-21 repair
Hello!

(11-19-2019 10:01 PM)Moggul Wrote:  Oddly enough it does not work when connected to the charger (with batteries installed) but it operates normally on just the batteries so all is well.

Better forget about the charger altogether! When I get a new Woodstock series calculator, first thing is put a sticker with the calculator serial number on the charger and then put it in a box together with the others never to be used again.
The battery contacts are very prone to corrosion, even with new batteries, and even a well serviced calculator can be killed by it's charger a year later.

(11-19-2019 10:01 PM)Moggul Wrote:  Some of the keys do not register at all when pressed so I opened it and will clean it tomorrow. If I understand correctly I should clean the contacts with vinegar, then rinse with water and clean again with isopropyl alcohol.

If it is entire rows or columns of keys that are not working then the cause is usually one of the contacts which connect the two PCBs. Single bad keys are caused by the key contact or by corroded traces on the keyboard PCB. The easiest way to troubleshoot keyboard and display problems is to interchange these parts between different calculators. This can be done safely. From the picture I see that you have already takebn the keyboard apart which should be a last resort because it is very difficult to re-assemble in a way that will last over time.

For oxide removing I use both vinegar and baking powder (or baking soda as it is called elsewhere) because some of the oxides get better dissolved by acid and others by alcaline agents. I have never used isopropyl alcohol to clean a calculator but if, I would rather use it for the keyboard upper side (making sure that it will not dissolve the labels). I have isopropyl alcolhol in my toolbox but I use that for optical surfaces like camera lenses and my astronomical telescopes and eyepieces.

Regards
Max
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11-21-2019, 09:01 PM
Post: #3
RE: Another HP-21 repair
(11-20-2019 10:24 AM)Maximilian Hohmann Wrote:  If it is entire rows or columns of keys that are not working then the cause is usually one of the contacts which connect the two PCBs. Single bad keys are caused by the key contact or by corroded traces on the keyboard PCB. The easiest way to troubleshoot keyboard and display problems is to interchange these parts between different calculators. This can be done safely.

Thank you Max for the help!

I cleaned the keyboard PCB which had a lot of green oxide and reassembled the keyboard but the same keys still didn't work. Then I swapped the keyboard with another (non working) unit and the problem persisted so it must be the contacts. I will try again this weekend and clean everything.


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