Hi all,
I'm really stuck with this repair, and I could use some advice.
I recently picked up a wacom cintiq which wouldn't power on the display. All of the LEDs and were lit, and the wacom touch surface worked, but the display wouldn't turn on.
I popped it open and the problem was that an 0603 decoupling cap had failed short (it was covered in cracks under the microscope), and that had caused the very small mosfet (SOT23) which switches on power to the entire display to literally explode. I replaced the cap and the mosfet after using my multimeter to measure the display current (500mA@12V, I used a mosfet which can take 4A at that voltage). The display came alive and all was well.
Looking at it more closely over the past few days, I'm getting strange vertical lines/bands down the display. It's hard to explain how it looks, so I have attached pictures. I can only describe it by saying that the green and red sections of the pixel seem to be unable to completely turn off; the problem is invisible with a white background but very clear on black.
The strangest thing is that if I leave it off for 24h and turn it on again, the good and bad areas of the display have randomly moved and are different in size! Sometimes it is mostly good with only a few stray lines, sometimes (like when I took these photos) it is mostly bad. On top of this, if I leave the display on for 30 minutes, it mostly goes away (sometimes completely). And if I turn the display on and off quickly or unplug the display cable, the lines still show up on the OSD (see pic) in the same place.
My guess was that this is a thermal problem somewhere, but I've tried the can of cold air on just about every IC I can find. My little thermal camera reports nothing hotter than 50C except a 1.8V linear reg which is running at something like 70-80C.
Some more info:
The display is made of two PCBs; a DVI/VGA to LVDS board, and then an LVDS receiver board on the LCD panel itself made by NEC (PCB says NEC 213 pw 303). The first board has an
ST display controller and the switch mode power supply for the entire device. The second board has a Doestek dtc33ln08brh (google returns no hits, I assumed LVDS recevier) as well as a
Sharp IR3E11M1 which seems like a high current voltage reference. Basically everything else is passives and small switch mode power supplies. I'm pretty sure the DVI board generates the OSD.
The panel is connected by hot bar soldered FFC cables on the LVDS recevier board. There are about 8 or 9 of these along the top of the screen and about 6-7 down the left side. I initially suspected there might be a bad joint which was warming up, but after having a gentle go with a 100C air gun on these, I couldn't see anything change on the display.
Things I have tried (all with the wacom touch panel disconnected):
-> Continuity testing the LVDS cable: all good
-> Changing the resolution of the display: lines remain in physically the same place
-> Use VGA instead of DVI: no difference
-> Change DVI cable: no difference
-> Use HDMI to DVI cable: no difference
-> disconnecting and reconnecting cables: no difference
-> cold air spray on the chips mentioned above and bar solder tabs: no discernable difference
-> 100C hot air gun on the above chips and bar solder tabs: no discernable difference
-> poking and prodding the tabs with my finger: no discernable difference
Any ideas here folks? I'm actually considering trying to build an LVDS recevier with an FPGA to see if it is the first PCB or the second which has the issue. And I'm guessing this isn't a power supply thing, but if anyone can think of a good hypothesis as to why it could be, I'm all ears.