Steve’s Hacking Blog

Yet another broken LCD TV came its way to me, this time for free. This particular model has a dead backlight. Since I have a slightly larger TV with no image panel and a nice working backlight, is it possible to swap them, knowing they are incompatible?

NOTE: Do not attempt to use an inverter from a smaller screen. It will not have enough power to drive the lamps, and will end up frying itself. Always try for a similar size, or slightly larger.

DANGER: We are working with electronics that generate thousands of volts at a very high frequency. The risk of electric shock is very high in this project. Do not fire up any inverters unless all high-voltage outputs are properly insulated. I am not responsible for damage or personal injury.

About LCD TV Backlights

Just about every LCD television backlight is the same, with varying dimming controls, a different number of outputs, and slightly different output voltage and frequency. However, they are close enough that swapping them out is relatively easy. For example, they all run on 24 Volts DC. They are all designed to run an array of lamps, with two lamps in series on each output. They all provide feedback to the mainboard so the TV can shut down in case of a problem. They all use a single 3.3V Enable signal to turn the backlight on and off at will.

In my case, I swapped a Sharp Aquos 32″ inverter into a Westinghouse 27″ TV. Here’s how I did it:

1. Find the problem

The failed backlight would turn on with the TV, but then go dead after a couple seconds. The TV would stay on, indicating that it didnt know there was a problem. Checking the inverter, it still had power, and its Enable line was still high. This means that the TV itself didnt shut down the inverter, but rather the inverter failed. I found that interrupting the Enable line momentarily will cause the inverter to power on for another seconds and die again. This means there is something wrong with the inverter, causing it to kill itself for safety reasons. Usually that means a bad lamp, but they were all okay. I threw it in the trash at this point.

A picture of the old, failed inverter

2. Find all the pinouts of both inverters

Each inverter was two boards connected by a small ribbon cable. Each board had its own connection from the power supply, and they both shared a single connection to the mainboard. The power supply connections were just 24VDC’s and Ground’s. The mainboard connection was simply an Enable, a Ground, a Feedback, and a Dimmer. This will be way too easy if it works at all.

3. Identify possible problems with swapping

Both the working and non-working inverters had similar configurations, but with incompatible connections and pins. On top of that, the dead inverter had 7 outputs, matched with the 27″ screen, whereas the new inverter has 8 outputs, for the 32″ screen. This will be a problem. If there is an open output, the inverter will not work. It will either shut down, or burn out.

Unplugging the lamps from the inverter

Old inverter removed. The thick colorful cable on the left has both power and control lines in it.

4. Perform the swap

The first step is to connect the outputs of the donor inverter to the lamps in the screen. I cut all of the plugs off the old 32″ TV, and soldered them to the lamps in the new one, so they would plug in and fit nicely with the inverter. Since there is an extra output, I wired it to a separate neon light, which I call the ‘balancer’ light. This is to prevent the inverter from shutting down due to an open output. The next step was to wire up the 24VDC supply lines, and grounds. This was fairly simple, its just DC power. Then I connected the Enable line from the mainboard to the new inverter. This gives 3.3 Volts when the TV is on, and tells the inverter to fire up.

Testing the lamps individually using a 12V CCFL Driver. Dont touch these wires while they are powered! There is an obscene amount of power here! THOUSANDS of volts at a very high frequency.

The new inverter, connected to the lamps, with the extra output connected to the balancing lamp, and connected to the power and control lines. Ready for testing! (If it works I can pretty it up later)

5. Test it!

At this point I’m getting anxious. I really want this to work. I make sure all the high voltage wires are properly insulated (this is important!), and press the power button. The TV clicks for a moment, then my balancing neon lights up, along with the rest of the screen. ITS ALIVE! I wait eagerly for the whole thing to shut down in a puff of smoke, but it stays on! However there’s a big problem. All I see on the screen is colored lines. Oh god don’t tell me I destroyed the panel during all this.. Oops.. actually I forgot to replace a small ribbon cable that connects the horizontal panel drivers to the vertical ones (hehe). I connect that, and it looks beautiful again. I cycle inputs, I cruise the menus, I attach a video camera so I can get some video going, and the TV just putters along like it was never broken. At this point I dont have any dimming control, but who cares. The TV works! I dont need a stinking dimmer! I’ll keep it at full brightness forever! Muahahaha! As long as you dont hear any high-pitched screaming, its fine. (Screaming means a bigtime inverter shorting problem! Shut it down!)

Regarding making the inverter fit and preventing shorts, I wrapped the whole inverter in stiff plastic, and cut air holes over the front for ventilation. Once the board is wrapped, it can be mounted however you please. Just make sure your grounds are good and there wont be a problem.

6. Post-wartime thoughts

The TV is obviously not new. Its 5 years old, and now it has a new life as a desktop screen. I did notice that when the screen is a solid color, I can see bars across the panel where the lamps are, and this probably means the lamps are too bright. This can’t hurt the lamps, but it can bother you if you pay attention to it. A simple solution is to wire the dimmer and set it appropriately, but I found it easier to insert an extra diffusion screen between the lamps and the panel. Problem solved.

Not a bad TV for zero dollars and a few days of tinkering. This turned out to be a relatively rare television panel, with a native resolution of 1280×720, which you almost never see (usually a 720p TV is actually 1366×768). You will also notice that I took the speakers off. This is to make it look more like a computer monitor and less like a 5-year-old TV.

7. Spare parts

Since I scrapped my LCD light table for this TV, I have some spare parts. I have 16 CCFL lamps for a 32″ LCD TV, as well as a mainboard, power supply, and TCON for a 32″ Sharp Aquos just laying around. I will probably sell them on ebay for a couple bucks.

This was a triumph.