Re-using a Broken Television as a Monitor

or

What To Do With a Broken Television when You Can't Fix It

I was given a broken television.  Technically this television was doubly broken: once when the HDMI inputs stopped working and once when everything stopped working. After the first fault it was taken to a television repair shop (yes, they do still exist) and they couldn't fix it as the parts are not available. I had a look at it on that occasion and found that some RGB inputs did still work, so we fed a signal in there and that allowed the television to live a little longer.

After the second failure nothing appeared on the screen at all, and the backlight did not switch on, so I was given the television to see if I could fix it or do something with it.

The television is an LG 39LE4900 and looks like almost every other television on the planet. Once opened I found a power supply PCB

 and a control PCB:

I also found an inverter PCB, under a metal cover, which powers the backlights:

and what I later found out was a TCON PCB, also under a metal cover:

I had a quick look at the boards and nothing seemed to be obviously wrong. When I powered the television up nothing appeared on the screen and the backlights were off. The remote control worked, in that it turned the television on, but nothing appeared on the screen. I used a torch to see if the LCD was displaying something even though the backlights were off and it wasn't.

I managed to find a service manual on the internet for a similar model, but the control PCB was the only board that the manual had schematics for. I thought this was a bit odd at the time, but it actually makes sense, I'll explain later. unfortunately, as the service manual model wasn't exactly the model I had, all of the component numbers were different to my PCB. I could work out that there was basically one large, presumably custom, chip that controlled almost everything. I checked voltages of power supplies and everything seemed fine.

I also tried to find a serial output from the board that worked, but no luck.

The next step was to get some replacement PCBs from ebay. I did this and eventually replaced all of the PCBs and found that the television still didn't work. This was a bit odd. I then took the step of buying a complete new screen (and a couple more PCBs) to see if that would work with the old and new PCBs. It didn't. This was a bit surprising as I had expected to get something working at this point. 

I then started looking at the PCBs in more detail and found that the TCON board and inverter were made by AUO, and there was actually a datasheet for them. Well, sort of. There was a datasheet for the LCD and backlight and TCON and inverter PCBs, all bundled as a module. AUO seems to sell this as a module that manufacturers can use in their products. LG have done this and then added a PSU and controller PCB. This explains why the controller was the only PCB with schematics in the service manual. The  rest of the electronics was bought in. (Except maybe the PSU which I think is a standard LG board).

The LCD/backlights/TCON/Inverter can be thought of as a larger version of a 16x02 LCD module. That also has a display, backlight, and driver circuit. The only difference is the size of the LCD and the interface used. With the AUO datasheet I now had details of the two connectors that the module provides, one for data and one for backlight. The data input is LVDS.

After another search, I found that the schematic for the PSU PCB was also on the internet, which was useful as I could then hardwire the supply to the LCD backlights. After having done this, I found that the backlights worked fine on both screens. This was good news.

 I then decided that fixing the television wasn't really practical, but I might stand a chance at turning it into a monitor. I'd recently fixed the display on my Novena

 

 laptop so had played around with LCD panels and knew I could get cheap driver PCBs that would take VGA or HDMI and drive an LVDS interface.

I decided to make a hacked together VGA to LVDS adapter PCB attached to a flat cable connector adapter PCB. The VGA/LVDS adapter needs +5V, whereas the PSU PCB in the television only provided +24V or +12V so I had to also use a DC-DC converter to power it. I hardwired the LVDS signals from the adapter PCB to the flat cable PCB using wires. I cut the connector that the adapter PCB supplied off the end.

Once this monster was all wired up and attached to the television, power was applied and a laptop supplied a VGA signal. And it worked. the picture was upside down, but the laptop could flip that and once done the television now displayed a nice stable picture. Well, it did when I could accurately and firmly attach the flat cable from my PCB to the TCON board. I'd been unable to get the connector that the LG control board used, so I'd bought the one that the TCON board used. This connector rather than being the FPC type connector that attaches to the end of the flat cable, attached to a connector that fits round the end of the cable. 

 

This meant that I had to use a piece of folded paper to hold the bare flat cable in ot the connector. I also tried 3D printing a flat spacer to wedge in the socket, but it wasn't ideal. In the end I bought a couple of new cables that were the same length as the original but had the connector on both ends rather than just one end. These work perfectly.

 With this mess of wires and boards working I decided to make a PCB. This would mean that the electronics and wiring could be tidied up and I could use one PCB rather than two or three. 

 

The VGA LVDS adapter was mounted on the new PCB, as was the DC-DC converter. The footprint I used for the converter was a very generic one, which lets me wire almost any DC-D converter I can find on to the board. The wiring holds the converter in place.

I also put a footprint for a Blue Pill on this PCB so I have the option of using that to perform housekeeping functions like turning the supply on and off and providing a timer to turn off the screen after a certain time. I haven't used the Blue pill to do this yet, I just turn the monitor on and off at the mains.

I can also use this PCB as a general Blue pill breakout PCB. I have five PCBs and will probably only ever use two, so the other three can be repurposed.

The PCB fits nicely in on the mounting holes that the AUO module provides, and the VGA connector is accessible once some metalwork has bee cut away.

I've been using the monitor for a few weeks now and it seems ot work fine. I'm making a PCB for the second screen as well, so I'll have that up and running too, I hope. I checked the screen with the first PCB I made and it does also work.

Why the televisions didn't work with the new control PCBs I don't know. I have tested every other PCB I bought and they all work fine. Are the controllers broken? Or do they need some magic setup? I don't know. I'm going to strip them for parts.

Details

The connection between the TCON PCB and the VGA-LVDS adapter:

The 12V supply was used to power the TCON board. The VGA-LVDS PCB needed +5V so the DC-DC converter was used to provide that. I used a variable DC-DC onverter and set it to +5V running from the +12V supply. I measured about 500mA drawn by the adapter so used 1.5A or 2A converters.

The PSU was hardwired to power on. This was done with a wire link on the PSU PCB. Using the schematic I managed to find an appropriate supply and signal to use. (PWR_ON connected to 3V5).

Connections between TCON and LVDS adapter:

VGA-LVDS                TCON

O0+    O0-                CH1_0+    CH1_0-

O1+    O1-                CH1_1+    CH1_1- 

O2+    O2-                CH1_2+    CH1_2-

GND                         GND

OC+    OC-               CH1_CLK+    CH1_CLK-

O3+    O3-                CH1_3+    CH1_3-

E0+    E0-                CH2_0+    CH2_0-

E1+    E1-                CH2_1+    CH2_1- 

E2+    E2-                CH2_2+    CH2_2-

GND                         GND

EC+    EC-               CH2_CLK+    CH2_CLK-

E3+    E3-                CH2_3+    CH2_3-

                                  VDD  +12V

                                LVDS_SEL: Open (HS)

(Closed is JEIDA, which didn't seem correct from the AUO datasheet)

 

Videos

I have a series of videos that show the work I did.