LCD screen went crazy on me


I have an old plywood Flashforge 3D printer.
I tried to add some anti-static features to it - surround key user elements with aluminium tape and connect them to the ground via 10MOhm resistor.

After this tuneup the screen on my printer went crazy.

The abracadabra patterns on the screen change when I push the arrow buttons that usually allow to manipulate the user menu of the printer.

I took off all the grounding strips, the screen worked fine.
Then it started to go crazy on me again.

Screens on the old FlashForge 3D printer are 20 by 4 line LCD text screens.

The original screen is labeled J204A.
The replacement that I bought on Aliexpress is 2004A.
There is a difference between the old crazy screen and the new screen: there are two SMD capacitors that are missing on the new screen.

The new screen doesn’t work at all when I connect it in place of the old one.
The old one shows crazy abracadabra.

I dam a little lost.
I hope that swapping out parts would not cost me more than an new 3D printer.
Thank you.

The random patterns on these displays is unfortunately a common phenomenon.
But anyway, if you have swapped the displays and nothing works on the new display, it could simply be the screen contrast setting is not appropriate for the new display. There is usually a potentiometer connected to pin 3 (V0) to control this voltage.
What, incidentally is the voltage between pins 1 and 2 ( Vss adn Vdd) 5 volts ? and is that what the new screen was advertised as supporting ? Some support only 3.3 volts.

From looking at the photos, it isn't a contrast control issue, you can see the pixels just fine.
The issue is that characters on the display are incorrect.
This usually happens when the LCD looses 4 bit nibble sync with the host.

Loss of 4 bit nibble sync can happen for a variety of reasons.
But basically what happens is that the LCD is not seeing all signals cleanly/correctly so it looses a nibble from a byte and then from that point on it sees the 8 bit values composed of one nibble from one byte and the other nibble from the next byte.
It may be seeing phantom E signal bumps which is what causes a loss of nibble sync or it may be incorrectly seeing the 4 data lines and mis interpreting the nibbles.
Or it could be that host LCD code driving the LCD signals is violating certain timing parameters of the hd44780 interface.

Given your description of things changing when you are altering grounds, I'd suspect something like ground bounce.
Ground bounce is when the ground signal floats up between two different areas of the circuit.

This can easily happen with poor wiring or lack of proper decoupling in the circuit or from a noisy power supply.

Can you provide some photos of your actual h/w so we can take a look at.

--- bill