HP41C Display Problem

[GreyUser]

So the display in your original post was a full display of the character “8”? It would have helpful had you explained that, it was easy to assume it was a screen full of “5”’s and the problem was the dim segments.

Considering the information now, there are a couple possibilities. One, you’ve got a fractured solder connection between the main pc board and the display driver board. The fracture can be on either board, it’s more common on the display driver side. Two, a previous repair to one of those connections did not get cleaned properly, the flux residue must be completely removed. Moisture builds up and it causes very low current leakage that’s enough to cause display anomalies. Everything in a 41 has to be squeaky clean due to the extremely low current cmos logic signals.

Disassembly is quite simple. Remove any modules and port covers. Peel the feet off (with a very small flat blade screwdriver or pointed tweezers) and temporarily stick them on the key legend label on the case bottom. Remove the four Phillips #1 screws and lift off the case back.

Carefully examine the display interconnects and if everything is clean, lift up on each of the formed connection leads between the boards with a pair of pointed tweezers, a fractured connection is very easy to find that way. Clean any flux residue with IPA and a cotton swab. Do not drag it across the keyboard side connections, just roll it over them as it will just pull all the fibers out due to the turned up ends.

Do not under any circumstances spill liquid IPA into the display area, it will leave white spots. Work with a just dampened swab and be patient. The only way you clean the lcd and the window is to desolder all the display connections and remove the lcd/driver assembly to get to the window. That’s not something you should do unless you’re quite comfortable with the process.

BTW, once it’s open, you can get rid of all those bits of dirt inside the display area with a couple of blasts of canned air directed in at either end of the lcd. Just be sure the can is level so all you get is air, not liquid. Otherwise, you’ll get those white spots mentioned above. Yes, there’s more than one way to cause them!