Electronics > Repair
LCD display zapped by static discharge, repairable?
Gyro:
--- Quote ---... The HD44780 type LCD character displays have been around since at least as far back as the 80s and the vast majority of them are compatible and use the same pinout.
--- End quote ---
A classic (and rare) case of 'If it ain't broke, don't fix it'.
Buriedcode:
Given its age it could jsut be that - age. The contrast is often set with a trimpot because it can vary with temperature. Some displays that are designed to operate over a wider temperature range require a negative voltage on the contrast pin - close to 0V but sitll negative. Thats an STN display with what looks like 16 pins - pretty bog standard HD44780 as others have pointed out.
The contrast voltage - pin 3 - will be very close to the GND/0V rail, but not quite, so removing the resistor connected to that pin and adding wires to the pads for a pot could work. if theres no room for a small trimpot (or trimmer) then you can temporarily connect a trimpot, set it so the contrast is where you want it (at the viewing angel you want - these old displays were all designed to be viewed from a certain angle, 6 o clok, 12 o clock etc..) then measure the trimmers value and replace with fixed resistors.
Frankly I'd go with what the others have said and replace it. The only non-standard spec might be the dimensions of the PCB its on and the mounting holes, but all the 4 x 20 HD44780 displays I've seen over the years have been the same size.
https://www.mouser.com/c/optoelectronics/displays/lcd-displays/lcd-character-display-modules-accessories/?character%20count%20x%20line=20%20x%204&sort=pricing
Amazon, Ebay or Aliexpress will have plenty that are cheaper, but they don't always provide detailed dimension drawings.
amyk:
Post some pictures of the inside. I agree that the controller is likely to be HD44780-compatible, but the pinout may or may not be different from what you can find today.
Audiorepair:
I remember many years ago finding a solution to replacing a display where the contrast voltages were different.
I discovered that soldering a standard 1N4148 diode between contrast pin and ground gave around 0.6v contrast voltage, which happened to be perfect.
There must have been some kind of pullup resistor or circuit on the display that enabled this to happen.
Divarin:
--- Quote from: Buriedcode on April 05, 2022, 09:46:01 pm ---The contrast voltage - pin 3 - will be very close to the GND/0V rail, but not quite, so removing the resistor connected to that pin and adding wires to the pads for a pot could work. if theres no room for a small trimpot (or trimmer) then you can temporarily connect a trimpot, set it so the contrast is where you want it (at the viewing angel you want - these old displays were all designed to be viewed from a certain angle, 6 o clok, 12 o clock etc..) then measure the trimmers value and replace with fixed resistors.[/url]
Amazon, Ebay or Aliexpress will have plenty that are cheaper, but they don't always provide detailed dimension drawings.
--- End quote ---
So the specific model # for my display is p-s4a20ct
It looks like there is 5 volts going *into* a resistor just before pin 3 but only about 0.4 volts coming out. The surface mount resistor is labeled "103" which, looking that up, should mean it's a 10k ohm resistor.
There are no trim pots (or pots of any kind) throughout this entire device. So I'm thinking the purpose of this resistor is to set a constant contrast value and is not meant to be adjustable. Given the extremely low voltage coming out I'd say it's likely this resistor is what got zapped. I was just about to desolder it yesterday and was going to try exactly what you suggested: temporarily adding a pot, adjusting it, then measuring the resistance and replacing with an appropriate resistor. Unfortunately as soon as I powered on the soldering iron the power went out and stayed out until dinner time so I never got around to doing that.
If the issue is this resistor, or somewhere in the circuit before the resistor then replacing the screen won't help as the resistor in question isn't even on the display, it's on the main board just before a ribbon cable which goes to the display.
Here's some more pics:
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