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Electronics => Repair => Topic started by: pebcac on April 18, 2021, 01:29:22 am

Title: 18x2 LCD Display Failure Mode
Post by: pebcac on April 18, 2021, 01:29:22 am
Hello!

I picked up a Cyberpower UPS with a bad LCD display. I can make out what it's supposed to display (load percent, etc) but I was wondering if I can fix it myself.

[attach=1]

[attach=2]

I gave the ribbon cable a nudge, but that didn't seem to change anything.
If I change the viewing angle, I can make out more characters, as in image 1 (normal) to image 2 (from above).

1. Is this a bad LCD display?
2. Or is this a bad LCD driver?

If this is a bad LCD display, I can swap out the LCD assembly by desoldering the 16-pin connector.

Thanks!
Title: Re: 18x2 LCD Display Failure Mode
Post by: sleemanj on April 18, 2021, 02:14:54 am

I would disassemble and give the zebra strips a clean.
Title: Re: 18x2 LCD Display Failure Mode
Post by: coromonadalix on April 18, 2021, 02:22:20 am
there's no zebra strip,  it the flexible foil who make the contact on the lcd glass ...
Title: Re: 18x2 LCD Display Failure Mode
Post by: james_s on April 18, 2021, 03:00:31 am
there's no zebra strip,  it the flexible foil who make the contact on the lcd glass ...

The original Game Boy is built like that. I've fixed a couple of those by placing a piece of paper over where the cable is bonded to the glass and then rubbing it a few times with a hot soldering iron, the paper will prevent the iron from damaging the cable.
Title: Re: 18x2 LCD Display Failure Mode
Post by: tooki on April 19, 2021, 10:15:42 pm
18x2 LCD Display Failure Mode
*16x2. I’ve never seen or heard of 18x2. The next size up is 20x2.

Hopefully repairing the hot-bar ribbon works. It’s very likely that it’s trivial to find a replacement display that is electrically and software compatible, but that centered header is really unusual, so finding a mechanically compatible replacement might prove difficult.
Title: Re: 18x2 LCD Display Failure Mode
Post by: pebcac on April 20, 2021, 01:33:42 am
*16x2. I’ve never seen or heard of 18x2. The next size up is 20x2.
Yup, it's a 16x2 display.

Hopefully repairing the hot-bar ribbon works. It’s very likely that it’s trivial to find a replacement display that is electrically and software compatible, but that centered header is really unusual, so finding a mechanically compatible replacement might prove difficult.
You're exactly right. Not only is the header centered, the pin ordering is flipped (16-1 instead of 1-16). Let me know if anyone finds a suitable replacement. I did go over the ribbon cable with an iron with paper in between, but that did more harm than good.
Title: Re: 18x2 LCD Display Failure Mode
Post by: james_s on April 20, 2021, 02:28:07 am
I would connect an off the shelf display using individual wires and then find a way to mount it mechanically, a block of foam rubber or even a piece of wood ought to work. You might find that not all of the pins are even used.
Title: Re: 18x2 LCD Display Failure Mode
Post by: pebcac on April 21, 2021, 07:18:12 pm
I tried attaching a 16x2 LCD to the UPS control panel but no dice.

The backlight does turn on and off as expected. On when the unit is on, off after a timeout, and back on when a button is pressed. Otherwise, it's a blank screen.

The LCD does work when paired with an adapter, so that's not the issue.

The UPS control panel is driven by an STM8 and is fed data via SPI from the main UPS unit.

A few potential causes:

1. The backlight voltage from the UPS panel is 12V, which is much higher than the expected 5V. There are current limit resistors, but the backlight pin voltage (15, 16) does drop to 6V. Not sure if this is affecting the control panel itself.

2. The supply voltage from the UPS panel is 3.3V, whereas the display may be spec'd at 5V.

3. The LCD contrast pin (pin 3) may need tweaking, but there's no pot on the control panel board.
Title: Re: 18x2 LCD Display Failure Mode
Post by: james_s on April 21, 2021, 07:57:48 pm
A lot of those 44780 displays indeed will not work from 3.3V, although there are 3.3V varieties available. Just for testing you might try wiring the display up to 5V and use level shifters on the control lines. In practice resistors might be sufficient. In at least most of those displays the backlight LED is completely separate and not connected to the display circuitry at all.
Title: Re: 18x2 LCD Display Failure Mode
Post by: pebcac on April 23, 2021, 04:36:45 am
A lot of those 44780 displays indeed will not work from 3.3V, although there are 3.3V varieties available.

I added a 7660 negative voltage inverter to the unoccupied footprint of the LCD module and tweaked the resistors used to set the contrast.

Working now, thanks for everyone's help!