TFT Repair

I rescued a 17" Sony SDM-S74 TFT from a WEEE bin. It powered up ok and displayed an image, but the screen went black after about 2 seconds. So I dismantled it and had a look around.

 

 I wanted to know whether it was just a problem with the backlight, or something more complicated involving the display driver.

There are two boards inside - the one on the left provides four HV supplies for the CCFL tubes and logic rails for the other board. The other board takes in VGA and DVI and drives the display panel.

The HV supplies turned off with the display after a couple of seconds, so it was impossible to tell whether it was the problem. or just a side-effect. Shining a torch on it didn't reveal anything. I happened to have a spare CCFL driver left over from a scanner I dismantled a while back, so I hooked it up to one of the tubes. Sure enough the display lit up dimly to reveal a perfect image - proving it to be a fault on the power board.  The CCFL driver can be seen in the centre of the photo:

I looked around the power board. There was an interesting chip labelled BA9741F. I looked it up and found a datasheet. This is fairly unusual - for some reason you usually get one of those annoying sites that just list chip names or, even worse, one that tries to charge you for datasheets. I had a poke around with the scope and found that the voltage on pin 15 of the chip ramped up until the backlight turned off. The datasheet is pretty appalling, but this seems to be to do with some kind of short circuit protection that turns the supply off once its output has been shorted for some period of time.

I  checked the outputs and they were not shorted, so I figured something must be wrong with this short circuit detection. The pin was connected to an external capacitor which it charged up. So I went for a really nasty hack... and replaced the capacitor with a 2.7k resistor!

The display works!

I've been using it for a good few weeks now and it hasn't caught fire, so I think I made the right call. Still feel a bit dirty, though, should probably have got a replacement chip, or otherwise probed deeper into the matter. 

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ctx pv720 BA9741F / display goes blank after 2sec.

Submitted by Anonymous (not verified) on Sun, 06/01/2008 - 19:48.

ctx pv720 Hi-Voltage PCB(BA9741F) / display goes blank after 2sec. aka (17" Sony SDM-S74 TFT same problem)

I checked out the data sheet. I placed a 3.3k-ohm resistor inplace of the cap. off of pin 15.

It fixed my 17" LCD ctx pv720 too.

Thanks for the report.
An old bench tek in Atlanta, GA.

obscurity

Submitted by jeff on Mon, 06/02/2008 - 02:29.

I can't believe this article was actually relevant to someone! Glad it was helpful. The internet is a beautiful thing.

Thanks to you both fixed a

Submitted by Anonymous (not verified) on Wed, 07/16/2008 - 11:05.

Thanks to you both fixed a CTX PV720. I suppose the article is relevant to enough people. Now you have helped fix another monitor, make someone happy and save the earth from more waste.

PV720 Circuit diagram

Submitted by Josh (not verified) on Sat, 10/04/2008 - 15:07.

Does any of you have the circuit diagram for this monitor? I have one here that is displaying only a complete blue screen with vaguely the image it should display. The monitor's menu displays (RGB)correctly and all options are available demonstrating it's not a color circuit malfunction.

Thanks

ctx pv720 BA9741F / display goes blank after 2sec.

Submitted by Anonymous (not verified) on Tue, 12/23/2008 - 01:53.

Follow-up from report on 06/06/2008. The display is still working great...

Good fix
Thank you again for the posting.
From:
"An old bench tek in Atlanta, GA."

Fixed my Philips 170B2T

Submitted by Markus (not verified) on Sun, 10/12/2008 - 19:01.

Hi,

Thanks for sharing this information.
I just managed to fix a Philips 170B2 with the same problem which has the same BA9741F IC on the inverter board.
I did a quick check of the voltage on the capacitor connected between pin 15 and 16 and it was indeed rising until the panel backlight went off.
I replaced the capacitor with a 2,7k resistor and the backlight stayed on.
The screen was in operation for 40 minutes now and is still working.
I wonder how long the hacked monitor will work though.

Cheers,
Markus

15" AOC LM525

Submitted by jc (not verified) on Sat, 11/15/2008 - 21:00.

Got this given cos it turned off after blinking a few times - anyone know what could cause this? Have pulled it apart and nothing unusual - all diodes look good, caps and res's look ok....

I've got a Philips 170B2T

Submitted by sam (not verified) on Thu, 06/04/2009 - 16:28.

I've got a Philips 170B2T which is doing the same thing (back light goes off after a few seconds). Anyone diagnosed the problem further, or have any suggestions on how to debug it? Has anyone had any problems with this fix? It seems pretty dirty and I was hoping to diagnose it some more.. :) Surely it's shutting down for a reason (something else is overheating maybe?)

Sam

Wow! This fixed my 17 inch

Submitted by Anonymous (not verified) on Sat, 01/10/2009 - 12:23.

Wow! This fixed my 17 inch LCD.

Thanks

Another one working

Submitted by Angel (not verified) on Thu, 02/19/2009 - 12:54.

Another CTX PV720 fixed. Now working for an hour with 2K2 resistor.
Thx a lot, now I can reuse my old 17" after some months useless.

Greetings from Spain.

Glad your monitor is

Submitted by jeff on Fri, 02/20/2009 - 00:31.

Glad your monitor is working; thanks for the post!

This helped fix my monitor!

Submitted by Dan (not verified) on Wed, 09/09/2009 - 23:40.

My lenovo 6135 had the same problem! Its a nice 1280x1024 manufactured in july 06, saw fairly little use in its first life as the monitor for a security system comp. I was sad when it crapped out on me a month ago (got it free), and I couldn't find any issues with the backlight itself.

After reading this article, I cracked it open and found a similar IC, the TL1451A. After looking up the datasheet, I found out how to disable the short circuit protection. (same thing, resistor across the lytic near the chip) At first a 3k resistor didn't work, but a 500ohm did. Totally shorting the cap also works.

As far as how long it'll hold out, I don't see any reason this wont work forever. I can remove the resistor or short once the monitor has powered up and is steady state, and it stays on fine. So basically overload protection is only when the tube is first firing up, probably due to slight aging and higher current draw. As a result, the IC is only being overloaded as it trys to power up, and trips.

I have a 3M microtouch monitor with this problem, I bet I can apply this solution there as well!