LCD replacement (again)

[Liteace]

Hello to you all, its been a long time, a long time ago I asked for help, here:

https://www.eevblog.com/forum/repair/lcd-replacement/msg2881066/#msg2881066

That machine has gone to the junk, Ive now got another machine, its basically the same just different branding and Ive broken the screen again on this one, this screen is the same size but has normal size chips /  IC's on the rear of the PCB, so question is can I use the same LCD I bought for the other machine on this one or is this totally different

[wraper]

As long as displays are rated for the same voltage, should work.

[tooki]

Looks the same. The only thing I’d double-check is the pin numbering relative to the backlight pins (15 and 16).
 They’re usually:
1-2-3-…-13-14-15-16
But sometimes you see:
14-13-…-3-2-1-15-16

(Basically, 1-14 either left to right or right to left — originally they didn’t have backlights — with 15 and 16 tacked on for the anode and cathode of the backlight.)

[coromonadalix]

4x20   lcd  ...

normally check for the 5vdc  and ground trace  ... they will tell you the right pin arrangement


rarely had problems with contrast adjustment, but i had one who needed negative voltage on the contrast pin,  instead  of the regular models  trimpot between 5vdc and ground ...

and the other surprise could be  the lcd backlight   led based or  ac voltage ??

[tooki]

rarely had problems with contrast adjustment, but i had one who needed negative voltage on the contrast pin,  instead  of the regular models  trimpot between 5vdc and ground ...

Yep. Extra-large ones and ones with extended temperature ranges often require a negative voltage.

Similarly, a regular one may need a negative voltage when operating at 3.3V instead of 5V. (“3.3V” models are actually just 5V models with an ICL7660 charge pump on the back to generate the negative contrast voltage. The same board is used on 5V models, just with the 7660 unpopulated, and with different jumpers installed.)

and the other surprise could be  the lcd backlight   led based or  ac voltage ??

Have you ever seen one in the wild using an AC backlight? (What was it? Electroluminescent? CCFL?) Those are common on bigger LCDs in the past, but the character LCDs seemed to always be either no backlight or LED. Originally they’d just have the yellow-green LEDs, then in recent years white became common. (And later ones using reverse contrast, i.e. light characters on black background, used any color LED that existed at the time.)

[Ian.M]

I still have a 40x2 HD44780 LCD out of an '80s  Laser printer that originally had an EL backlight.  EL panels degrade with age so a couple of decades later there was no hope of running it, let alone the hassles of generating the few hundreds of volts AC it required, without the printer PSU, so I cut its pins and slid it out and slid in a card with aluminum foil tape on it as a reflector so it could be used as a non-backlit LCD.

[tooki]

Yeah, I’ve been meaning to replace the EL backlight in my Keithley 197A with something else. (And retrofit a backlight into my 197, too.)

Since I’ve never seen one myself: do the character LCDs with EL backlights have the backlight inverter onboard, or do they just expose the bare terminals of the EL panel?

[Ian.M]

Mine had pads on the end of the LCD PCB that the EL pins anchored to.  I assume the backlight connections weren't brought over next to the 14 LCD power and signal pins, due to creepage and clearance considerations.

[Liteace]

Thanks for the info Guys, why has the original (broken) one got full ICs | chips on the rear and the replacement not + anyone got a link to a quality display, Id rather spend a bit more if its going to be better quality

[wraper]

Thanks for the info Guys, why has the original (broken) one got full ICs | chips on the rear and the replacement not + anyone got a link to a quality display, Id rather spend a bit more if its going to be better quality

Because displays with packaged ICs rather than COP are long in the past, primarily due to cost.