Thursday, December 22, 2011

LCD Monitor Repair


Some time ago I was donated a non-working LG L194WT-SF LCD monitor for parts. After a couple of months of collecting dust with it I decided to have a look at it from the inside. Since the monitor has been at a repair shop and was declared beyond repair, I thought I had nothing to lose- best case scenario, I fix the monitor and can use it or give it away in the future, the worst case scenario, I see what components are inside in such a piece of hardware and I might be able to salvage some parts for my projects.

The problem

There was only one problem with this monitor: it didn’t respond to switching on. Absolutely nothing happened when the power chord is plugged in and the power button is pressed. Most LCD monitors fail at either the backlight module (inverter) and at the power supply. When the inverter or the backlight is broken the monitor is still fully functional, just the picture is very difficult to see. Since this monitor was completely dead (well, pretended, anyway J) I suspected that its built-in switch mode power supply was at fault.

In switch mode power supplies, most often, one (or more) of the capacitors usually give up due to thermal wear-and-tear. Since this monitor is quite a few years old, these capacitors were my primary suspects.

Dive in

The monitor’s enclosure is held together by only 4 big screws on the back. After removing them, and gently prying the front frame off, the whole enclosure comes off easily. Only remains the LCD panel and a big box of electronics.
Once the enclosure is gone, it gets interesting. First of all, I was amazed to see that the big box of electronics is only held in place by some sticky tape – this may be standard practice, but I haven’t seen anything like this before in a commercial product. Of course, when the enclosure is on, it also helps keep it in place, but still! J
To flip the big box of electronics some connectors must be unplugged from it, they are the power supply for the background light of the LCD panel.
Once the cables are free the big box of electronics folds out freely, providing easy access to the component side of the PCBs in it.
Switch mode power supply. Notice the standard AC inlet at the bottom and a cluster of similar capacitors in the top left corner.

There are two printed circuit boards in it, one is the power supply and the other is the display driver. I didn’t have to poke around long – once I saw the components on the power supply board I immediately noticed that one of four capacitors in a bunch had deformed – a clear sign of a faulty capacitor. Just what I expected!
One of these capacitors was my prime suspect
Here the problem is clearly visible: BUSTED!

This is the bad boy after desoldering it: even the bottom of it is out of place.

The culprit is 1000uF/16V. I had a look at my pile of junk and I quickly found a similar one, 1000uF capacitor, rated at 25V that I had salvaged from a broken motherboard. It was even smaller in size. Anyway, the soldering job was easy. When I soldered in the new capacitor I immediately tried if this fixed the problem – maybe other parts had to be replaced as well. Luckily enough, the monitor powered up straight away – success!!!
Power LED on + "something" on the screen = success!


Repair shop technicians (at least the ones in my area) either don’t have a clue about their job, or don’t want to bother with small jobs like this. For me it took about half an hour from start to finish, including taking pictures, figuring out how to open the enclosure (the most difficult part for me…). Cost of this repair for me was zero in materials, since I already had the replacement capacitor. Had I not, it would have been around €1, including electricity, solder, etc. I trust, anyone with the slightest affinity to soldering could do what I did. It’s just so much easier to buy/sell a new, shiny one than fixing the old…

Although this fix was done on a particular LCD monitor, other monitors, even very expensive TVs with similar problems could be fixed like this. It's very little effort and it's very economical.