HP-50G trying to fix failed keys

[HPing]

I recently purchased an HP-50G from eBay and some keys don't work. Kindly looking for tips from gurus to (try to) fix the keys.

Back story- The eBay seller kindly reversed all the charges. I would like to fix the HP and pay him back, if possible. It would be a fun project with no financial downside.

Errors - Three keys don't work (0 . G).

- There seemed to be the slightest bit of crud around these keys which I could see in daylight. At one point maybe there was more crud but some was cleaned and some fell inside the calculator.

- I tried a bunch of things to the calculator to get the keys to work (remove batteries, self-test, set beeper with keystrokes, press the keys multiple times...).

This post gets me pretty far in the disassembly. But once the case is split, I'm not so clear on further disassembly and typical key fixes. Could someone provide some tips? Thank you.
https://www.hpmuseum.org/cgi-sys/cgiwrap...ead=171784

[HPing]

Today, we disassembled the 50G and determined why three of the keys (0 . G) do not register.

1. Main problem: There are 3 holes punched in the soft black rubber mat between the keys & dome switches. So when the key is depressed, the "key pin" just goes through the rubber mat but no pressure is put on the dome switch (picture 1).

2. Secondary issue: A broken hinge on two of the keys (0 .); note each of the keys has 2 hinges (picture 2). In the photo, the broken keys are on the bottom row, from right hand corner the second & third keys.

3. OK: The white dome switch panel looks ok, with some wear on the white cover for 2 keys (0 .). All 3 of the keys register when I use a finger to press the white dome switch panel (maybe the 0 is slightly less tactile).

Potential causes:

1. Black rubber mat wore from use. Also, the "key pins" are very small diameter. Bigger diameter might not have punched through the rubber mat.

2. Key hinges are feeble. Beefier hinges might not have failed.

Potential fixes.

1. New black rubber mat or some type of (very) thin plastic over the mat

2. New keys. I have seen some people epoxy the keys on the hinge (glue back side only to not interfere with top case)

Any recommendations?

BTW - We found an easy way to separate the 50G's top & bottom cases. Method from an obscure old video that didn't describe steps well. Will document that later. Along with the helpful links and videos that helped with the disassembly.

[HPing]

The orange key disappeared. I'll try looking around the house again. Everything that could go wrong with this calculator has.

Project is on hold until I find that key.

[franz.b]

(10-17-2024 09:43 PM)HPing Wrote:  Today, we disassembled the 50G and determined why three of the keys (0 . G) do not register.

1. Main problem: There are 3 holes punched in the soft black rubber mat between the keys & dome switches. So when the key is depressed, the "key pin" just goes through the rubber mat but no pressure is put on the dome switch (picture 1).

2. Secondary issue: A broken hinge on two of the keys (0 .); note each of the keys has 2 hinges (picture 2). In the photo, the broken keys are on the bottom row, from right hand corner the second & third keys.

3. OK: The white dome switch panel looks ok, with some wear on the white cover for 2 keys (0 .). All 3 of the keys register when I use a finger to press the white dome switch panel (maybe the 0 is slightly less tactile).

Potential causes:

1. Black rubber mat wore from use. Also, the "key pins" are very small diameter. Bigger diameter might not have punched through the rubber mat.

2. Key hinges are feeble. Beefier hinges might not have failed.

Potential fixes.

1. New black rubber mat or some type of (very) thin plastic over the mat

2. New keys. I have seen some people epoxy the keys on the hinge (glue back side only to not interfere with top case)

Any recommendations?

BTW - We found an easy way to separate the 50G's top & bottom cases. Method from an obscure old video that didn't describe steps well. Will document that later. Along with the helpful links and videos that helped with the disassembly.

I eagerly await the full repair report. If you can link us the hidden video in the meantime...
Also I ask you if you can highlight in the photos the points you mention in the description, I'm not sure I've identified the details of the problems.
Thanks