HP41C Display Problem

[Ignatz]

I'm having this problem with the display of my 41C. It was working perfectly, it gets very little use and then one day I turn it on and...



I've put new rechargeable batteries in it: no joy. As you can see, the contacts are as clean as anything... Is there anything I could try?

[grsbanks]

The LCD is soldered onto the keyboard PCB so it's not a bad contact there. However, the CPU board (yours is a fullnut) is maintained in contact by pressure exerted by screws that screw into posts that are part of the front.

These posts are notoriously fragile in 40-year-old calculators and my bet would be on the posts in your 41C having broken over time.

[GreyUser]

Repair a broken post only with solvent based plastic model cement, the stuff is like water, not thick at all. Do not repair with cyanoacrylates (super glue), that will not work.

A good reference: https://www.hpmuseum.org/cgi-sys/cgiwrap...ead=197569

[Steve Simpkin]

Try squeezing the top and bottom case of the calculator together. If it solves the display issue, broken posts are probably the cause.

[Ignatz]

Thanks, all... squeezing was my first go-to... no joy, there, I'm afraid... Like I said, The calculator has simply sat on my desk since I've had it: the problem appeared one morning at switch-on. I'll try opening it to have a look.

I forgot to mention that it showed exactly this problem when I first received it, but I think that was due to its having been sat outside in my delivery space in freezing temperatures. As it warmed up, the LCDs came back to life... any chance the two things are connected?

[cdmackay]

I don't know about the 41C LCD in particular, but LCDs in general can be very temperature sensitive, and even damaged by slightly too-high temps.

Also, you may be experienced at 41 repair, but note I've seen people say that the best way to get broken posts is to open one up.

[Ignatz]

Aaaaand... just like that:



I switched it on this morning and the display is back... not 100%, but good enough. The calculator hasn't been moved, opened, nothing... no idea what can be causing this...

[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!

[Ignatz]

Thanks, GreyUser... a bit beyond my abilities, I'm afraid... the problem, however, was the dim segments, which kept getting dimmer with time until they almost weren't there at all. If I look at the display now, it looks as though the whole cycle is beginning to repeat itself...

[jebem]

(06-15-2020 09:54 AM)Ignatz Wrote:  Thanks, GreyUser... a bit beyond my abilities, I'm afraid... the problem, however, was the dim segments, which kept getting dimmer with time until they almost weren't there at all. If I look at the display now, it looks as though the whole cycle is beginning to repeat itself...

That symptom looks like a bad supply voltage on the LCD display.
Just to eliminate possible causes, you could borrow an external 6 volt power supply to feed the calculator. Alternatively, use a battery holder for 4 X 1.5V penlight batteries.
Use a pair of cables with clips/alligators to apply the voltage to the calculator battery terminals.
Use good fresh 1.5 volt batteries (AA or AAA).
See the calculator behaviour by using it for a few days in these conditions.

If a known good power supply do not fix the issue, you may like to apply warm air into the LCD for a few minutes to see if it improves, by using an air dryer.
It is a long shot but it may improve the LCD display if the issue is caused by component degradation due to humidity.