HP41C problem...very disconcerted

[isanchez]

Hi,
I am addressing you because I am trying to repair an HP41C, the very early model, sn1943Axxxx. One of those with the metallic keyboard plate, not plastic sheet.
I have a lot of experience in repairing this model, but in this case I have to confess I am really disconcerted, this is why I am addressing you for help.

I proceed to describe how is the situation:

The calculator was in bad state, very dirty but only one important issue: the battery contacts were ruined.
I cleaned the calculator exhaustively: the internal foam of the case got degraded and it was full of dust. NO oxidation at all, all keys working fine, except the number "3"key . All keys made the typical "click", including number "3".
I replaced the battery contacts and mounted back the calculator. It worked all fine, except the "3". It cost a lot to make a good contact, so I decided to open it again, disassemble and clean the "3". TO do this I immersed the bottom first third of the keyboard in water with very soft soap for some minutes. The idea is that the water does not reach the display. The rest of the keyboard is basically mechanical and this has worked for me always,. Before this I tried with contact cleaner spray without success. After this I dried the keyboard with a hairdryer (just cool fan, no heating), when was fully dry, I assembled the calculator back and worked PERFECT: the number 3 was flawlessly and also the rest of the functions, keys, display etc.

This was yesterday.

Today I try to turn on the calculator and gets blocked, showing "*" and then "NULL", no response to on/off and also shows nonsense content (all display functions displayed, some numbers when keyed, but comes back to "*" "NULL".

I have tested/replaced the following:

-Changed/tested with at least five different battery contacts.
-Checked the display connection (soldering continuity) with the keyboard board.
-Replaced the Motherboard.
-replaced the zebra (original in both cases). In this calculator the motherboard is fixed with a couple of nuts.
-Cleaned again the keyboard with contact cleaner spray, with special interest in the "*"key...

When the display shows "*" "NULL" sound to me like if the "*" is somehow "pressed" or making continuos contact, hence the "NULL" message, and blocks the calculator....

The only issue I have not checked is the display driver...could this be he reason? I could replace it, but not sure if this has to do with it...

I have to confess it is the most difficult repairing process I have had to face...It is really a pity because the calculator looks really nice...one of the nicest I have owned.

UPDATED: solved!! The strange behaviour evolved and the bottom row of keys acted like the on/off button. The rest was fine. This made me understand that there was a remaining drop of water moving inside and causing abnormal contacts. Disassembled again, more hairdryer and work flawlessly again!!!!


Thanks in advance for you help.
May I wish all the best for you during this times of difficulties and pain. We shall overcome!

Ignacio

[SammysHP]

Have you measured the keyboard matrix with a multimeter? I had a similar issue with a 42s. Problem was a tiny spek of carbon from one of the plastic sheets that separated and made continuous contact. Poking with a thin shim between the membranes didn't help so I had to disassemble everything.

[isanchez]

(04-18-2020 09:56 AM)SammysHP Wrote:  Have you measured the keyboard matrix with a multimeter? I had a similar issue with a 42s. Problem was a tiny spek of carbon from one of the plastic sheets that separated and made continuous contact. Poking with a thin shim between the membranes didn't help so I had to disassemble everything.

Thanks! I shall check it!
Regards
Ignacio