HP-12C: Missing Segments

[Stefan Ramonat]

This question may well have been asked before, but I could not really find anything by searching.

Last month, a recent 12C that I had for about 2 years all of a sudden started to have missing segments in the display. It started in the first (far left) digit, with the segments at the top and the bottom right of the digit occasionally disappearing. It was on-and-off for a while, though at some point it became nearly constant, and the issue spread to other digits. Changing batteries had absolutely no effect, though, curiously, the segments would sometimes reappear if I took out one of the two batteries, only to disappear again if I put the battery back in.

Two weeks ago, I purchased a replacement 12C. This morning, the same 2 segments that originally gave me issues on the other 12C (the top and bottom right) have begun not showing on the replacement 12C. It appears to have started right after it dropped on to the floor, though the fall did not seem that bad (it was from desk height onto soft carpet).

Could the fall have caused something to become misaligned? Is there anything I can do to fix it?

Interestingly, I still have the older 12C and when I tried it this morning all segments were functioning normally, even though last time I tried it at least 2 in each digit were not displaying.

Thank you,
Stefan

[Randy]

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Could the fall have caused something to become misaligned?

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Yes, but not misaligned - detached.

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Is there anything I can do to fix it?

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Maybe.

The current production HP's use a thermally bonded flex cable to make the circuit board to LCD connection. This is very typical today for consumer electronics.

What has probably* happened with your 12C is that one or more of the connection points has lost contact. I would expect that is probably only one connection due to the segment multiplexing that is done in almost all LCD drivers. The original Voyagers used a 2:1 multiplex, the later ones a 4:1. I've not dug too deep into the current ones to know what the new arrangement is as they are for my purposes, unrepairable.

So, repair, while straight-forward on paper, gets a bit iffy in actual execution.

1. Remove the feet (they peel off) and battery cover
2. Remove batteries and all visible screws
3. Lift off the back cover (you might need to use a small plastic blade to pop the edges of the case back up and off, unsure on this model) and look for two small springs that connect the front metal plates of the calculator to the back metal plates. This is for ESD protection. Set the springs aside in a safe place, they are easily lost
4. Cut off the flattened over heat stakes that hold the circuit board in place
5. Remove board and place on a flat, static free surface
6. Power up with 3 volt supply
7. With a small insulated device (toothpick comes to mind) press on the connections of the ribbon cable and identify the bad connection point, it may be on the lcd or board end
8. Puncture the flex cable with the procedure below to restore the connection
9. Reassemble, securing the board to posts with small screws or glue
10. Screws, feet, batteries, battery cover back on and the process is complete

Here is a link to the bad connection repair method. Repair of a bad HP10Bii display

* - The "probable" asterisk is my way of saying a detached connection is the most likely cause. The problem may be a bad wire bond between the chip and circuit board or a bad chip. Chances are very small but they cannot be ignored.

[Stefan Ramonat]

Thanks Randy. I will give that a try on the weekend.

Stefan