I bought this LCD monitor on August 7th, 2007. (19 inch Hanns-G model HQ-191DP6). It worked well for almost 4 years until last Thursday July 21st, 2011. The screen flickered a few times and it went completely dark.
Over the weekend I opened it up to see if I can repair it. If you have any interest in LCD monitors, this post will be very helpful to you. In this case I was not able to repair it, but now I know what failure modes can be repaired, and what can not be repaired.
The first problem was opening the thing up without breaking it. Fortunately, somebody else had already documented this part, I just followed the instructions on this link .
Back cover off
It is hard to see in this picture, but the cables that go to the Cold Cathode Fluorescent Lights (CCFLs) are on the left side.
There are four connectors: power, DVI, VGA, and sound input connectors.
On the left side of the monitor you can see four cables. Each of those cables are connected to a separate CCFL. There are four sets of cables, so there must be four CCFLs. I thought those lights would be somewhat evenly spaced and placed horizontally behind the LCD panel. As you will see later, those lights are located at the top and the bottom of the screen.
Getting to Circuit Boards
I took the left and right brackets off, and the metal cage that protects the circuit boards. At this point you can clearly see the four separate power cables going to CCFLs.
There were two circuit boards inside the metal cage: the one on the right processes the video signals and drives the LCD panel. The one on the left is the power supply, it powers the signal board, and the CCFLs.
At this point, I put a piece of paper under the circuits so they don’t short circuit and powered up the monitor. Shortly after power up, I could hear sizzling sounds coming from the top set of CCFL cables. Clearly there was a problem with that set of lights, so I unplugged them (as seen here) and powered the monitor up only with the second set of lights. This seemed to work. I figured the monitor would not be as bright, but it still would be usable. I put everything back together and connected the monitor to my computer. It worked, but it was not usable. The display would come on for a few seconds, and then it would be dark for eight or ten seconds, and it would repeat the cycle again and again.
Here is a better view of the circuit boards. Video board on the right and the power supply on the left.
Power supply capacitors
Look at the picture above carefully. The vent lines are perfectly straight on the green capacitors, but you can see the tops of the three black capacitors bulging up. The failure mode of repeating the same sequence over and over pointed to a capacitor failure. I replaced all three capacitors.
Jumpers to set current
Since I was not using the set of lights at the top, I moved the jumper to change the maximum current from 7.5mA to 6.5mA to reduce the load on the power supply. By the way all four connectors provide 720 volts AC each.
Taking apart and putting the monitor back together for trying out something I changed on the power supply board was taking a lot of time. The new capacitors I put in also required more space. Because of these reasons, I decided to extend the cables and move the power supply outside the monitor. That would separate the power supply issues to outside of the monitor. The next three pictures show progress on that front.
Moving power supply outside
Moving power supply outside
Moving power supply outside
After I moved the power supply outside the monitor, I was still having problems with the second set of lights. I was starting to think that the problem is not with the power supply, but it is with the lights. Since I pretty much had lost this monitor, I continued to dismantle it to get to the lights.
Here is the LCD panel, it is extremely thin, looks dark, and it is rigid. The display consists of a series of layers, the LCD panel is the outermost layer. In other words, if you touch the display of an LCD monitor, your finger would probably touch the LCD panel. Just behind it, there are three or four milky white flexible plastic sheets. One of them definitely is a lens of some sort. When you look through it, you can see concentric circles on it.
All the layers that I mentioned above are visible inside the black bezel in this picture. There is a half inch thick glass behind these layers. It probably is plastic of some sort, but it looks like a glass with honeycomb shape structure in it. This part provides the structural support for everything else in the display, and also spreads out the light from CCFLs. Note that although we took apart almost all the parts in the display, we still can not see the CCFL lights. You can also see that it would be extremely hard to put everything back together now.
Side view of the same thing.
The picture is fuzzy, but you can see the burnt out lamp on the right hand side. The lamp diameter is no more than a quarter of an inch (a few millimeters).
There are two CCFL light assemblies, each containing two lights. One assembly is mounted on top of the glass support structure, and the other at the bottom pointing up. They are mounted horizontally as shown in this picture. You would think that they would build these panels such that we could slide these lights out and insert new ones. If it were so easy, why would anybody buy another monitor?
I still wanted to know for sure whether the problem was the power supply, or the light bulbs. The big transformer you see in this picture gives 700 AC volts at 90 mA, more then enough for these light bulbs. Just to be safe I put a 10K resistor in series so they don’t explode on me. I plugged in the power and tested each light bulb. The burnt ones did not come on at all, the ones that seemed to work would come on and go off at about the same frequency. Soooo, the problem was with the light bulbs.
These three pictures show the CCFL wiring. Two wires are connected to each bulb. One of the wires (black and white) runs along the length of the assembly all the way to the other side of the light bulb. The colored wires are connected to the closest side. Note that this is different than regular fluorescent lights which take four wires. Although they are color coded, since the supply voltage is AC, you could reverse them without any problems.
Could we use the LCD panel with ambient light? The pictures below answer this question. The credit for the pictures below goes to my son Kubi.
Linux mint screen, but the camera was not able to focus on the screen.
You can clearly see the honeycomb pattern in this picture. It is from the half inch thick glass panel. Note that the display is only the LCD panel and the glass panel, without the other plastic sheets.
In this picture there is a desk lamp behind the screen. The light goes right through the screen.
Here is the google page. Note that any point that is white on the screen becomes completely transparent so you can see the other side. Normally there is a sheet metal behind the screen so you don’t see through it.
CCFLs from a different monitor
The connectors to CCFL light bulb seems to be somewhat standardized, but not completely. We opened up another dead LCD monitor and took out the CCFL lights. In this picture you can see that they combined two white wires to one connector, and the two coloured wires into the other connector. In this case the thinner white lines run along the light assembly to the other side.
To summarize, if the problem is with the CCFL lights, there isn’t much that can be done to repair the monitor. If the problem is with the power supply, then there is a good possibility that the problem can be fixed.
To increase the lifetime of your monitor, make sure that the CCFL lights are turned off in the screen saver mode. If the monitor is not turning the lights off, make sure you turn off your monitor when not in use. Note that the screen saver was meant to protect CRT screens from permanently burning an image on the screen, I don’t know what use it would have for LCD screens. CCFL lights don’t last long, and in my case they lasted only about four years. I kept my monitor always on, with power save mode enabled.
For my next monitor I will definitely be considering LED monitor.