Graphic LCD : lines at the start and then off (or fully black)

[adrien62400]

Hello everyone,

I'm trying to repair a machine which has a graphic LCD LMG7520RPFC and I have an issue with it (with the display controller board actually I think).
At the start, the LCD shows random lines (either vertical or horizontal) and goes off (or fully black) after few seconds.

The display itself is good since I just changed it and it has been tested.

Do you know from where this kind of issue usually comes ?

The controller board is composed of a LC circuit to provide VEE and I had to change a transistor and a diod on it because they were dead. I didn't find any other components to change and I'm now stuck.

Thanks in advance for your help,
Adrien.

 

[kellys_eye]

What is it used in?

Is the display connected via zebra strip or 'hardwired'?

Do you have a schematic or pics of the display board and the parts you recently changed?

 

[adrien62400]

Hello,

First, thanks for your answer and sorry for the long time between yours and mine!
The display is connected via a flat flexible cable.

Here are two pictures of the top and the bottom of the display controller board :
http://hpics.li/b061e1e
http://hpics.li/87d417b
And one of the display's bottom where you can see the reference:
http://hpics.li/503f215

This is a graphic display which is used to control a wires preheater, so you have some buttons as well. I don't have any drawings, that's the hard point.

Here are two videos of my issue with the display:

It's impossible to tell if the display is on and full black, or just off. The light that you can see on the video comes from the backlight I think.

To explain you with more details, at the start, the backlight broke down. After a few weeks, the display broke down. I have repaired the backlight but the display was still off.
I did some checks and the VEE on the display controller board was at -1.20V instead of -22V (I used the display to determine what voltage I should have).
So I checked components and the transistor and the diod in red on the picture were dead. These are the only components I changed for now.

 

[kellys_eye]

The transistor circled in image BOARD2 - adjacent to L600 - looks to be blown. If you've recently replaced it I suggest you got the EBC lead arrangement wrong. Many of those TO92 devices have differing pinouts and need to be carefully checked before fitting.

 

[adrien62400]

The transistor is a s8550.

http://www.el-component.com/bipolar-transistors/s8550

It seems to be good.

 

[adrien62400]

I ran some tests this morning and I found something.
Now that I have changed the transistor and the diod, there is nothing on VEE : 0V.
I have 4.8V on the E of the TO92, nothing on the Base and on the Collector. My base is wired to an AIC1652.
I tried to follow the PCB to figure out from where VEE comes and it's actually coming from this AIC1652 just next to the LC circuit with which I had issues.

http://www.metatech.com.hk/datasheet/aic/dcdc_pdf/Aic1652.pdf

It's properly supplied (4.8V on Vin pin 1) but there is nothing on Vout (pin 6, 7) : 0.5V instead of -22V !
I think this component is dead as well... Do you think I could be right ?

I'll change the transistor as well if I change this AIC because you're right, there is a strange point on it, it must be blown.