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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