HP-41C LCD Communication

[haiduc32]

Hi,

I got a HP-41C with LCD misbehaving. I took the logic board and put it into a 41CX and worked fine. But putting the 41CX logic board into the 41C still leads to random LCD characters.
Most of the time the LCD will just show @@@@@@@@@@, I managed to get to show some numbers very faintly with its original zebra connector by squeezing the logic board against the keyboard.. if that makes any sense. I replaced the zebra connector with a brand new one. Could not make it show numbers anymore, just @@@@@@@@@@.

Connecting it to a oscilloscope shows the digital signals on DATA/ISA/SYNC, but in between also some attempted pull-down signals, that get stuck somewhere in the middle. So I am assuming something is wrong with the LCD driver, which I believe is not the original one, as the person soldering it did a messy job.

Any ideas to why it would show only @@@@@@@@@ and why the digital signals can be broken by the LCD?

Also, anyone got any info on the LCD communication? I would attempt to write an Arduino sketch to test the LCD by itself but can't find any info on the subject.