I happen to get hold of two highly professional 80 inches LCD TVs that were going to be scrapped since they only displayed a black screen. Curious as I am I couldn’t help wonder if I was able to repair them, e.g. use spare part from one TV in the other so that I can get one working TV. That would be an excellent contribution to my home-cinema setup!
I spent countless hours on trouble shooting: exchanging capacitors, measuring all voltage regulators, supervising signals to the external flash, I found a debug port which actually did output some text messages (it was running some sort of Linux version) but no help etc. I even bought a service manual for the TV but no matter what I just couldn’t get it to work again. However, there was some light in the tunnel: The backlight did work, the symptoms on the two TVs was the exact same and the general feeling was that it might not be neither the LCD panel itself or the backlight that was broken. It seems to be located to the mainboard. There are mainboards for sale on eBay but they were expansive and since I still haven’t found the root cause I hesitate to buy them.
I found the name of the LCD panel in the service manual and googled the datasheet for the LCD panel. The datasheet mentioned a recommended driver for the LCD and I got this idea to try and talk to the LCD panel directly, bypass the mainboard. I just needed someway to convert a HDMI signal to the required RGB parallel interface used by the driver. Luckily Adafruit come to the rescue with their TFP401 board. I quickly made a schematic and PCB board for the driver IC and ordered the TFP401 board. The TFP401 outputs RGB using 8-bits but the LCD is actually a 10-bits display so I loose 2 bits of color information but as a test that worked for me. I still couldn’t be 100% sure that the LCD panel was working and I don’t want to spend neither too much time or money on things that cannot be repaired. The TP401 board and a carrier board for the LCD driver IC seems to be a nice compromise and my final effort to get the TV to work again.
After a few weeks the boards finally arrived and I quickly solder and connected everything. But the LCD panel remained black. Darn it! At this stage I almost gave up. The LCD driver IC comes with a lot of options and I went through them one by one but with no success. As a last effort I started desperately checking the EDID. The EDID is an EEPROM that a connecting device to a TV uses to get information about the TV, like e.g. the model and best resolution to use.
I tried to reprogram the EDID (Extended Display Identification Data) (the TFP401 has a EEPROM onboard that carries the EDID) but with little success. Still a black screen. However, during my adventure in how to use EDID, I discovered that it is possible to override the EDID settings in Linux and I could much quicker come up with correct parameters. This site is a good starting point.
And voila! I got the TV to work! I have never been more satisfied to see a Linux prompt on a 80 inches screen as I was then. I don’t want to think how many hours this project took and frankly it doesn’t matter: I got my 80 inches TV work!
Side note: I got both TV to work but I only have place on the wall for one TV…
- Datasheet for LCD panel
- Service manual
- Datasheet for recommended LCD driver
The adventure is not quite finished. The panel uses 10 bits color resolution and it is a shame not use take advantage of that. There are also some features in the TV that I would like to get working e.g. ambient light sensor and IR receiver. It would also be sweet to add it as a device in my home automation network. Stay tuned for an update in a near future!