LCD monitor inverter/backlight repair

These sort of things can be difficult to track down, it could be something wrong in the supply to the inverter, as well as many inverters will shut down if they detect a lamp fault. It really helps if you have access to another identical monitor to swap parts around, though you can do some work without. The inverter itself is my primary suspect, if it has any electrolytic capacitors check those.

Thanks James! From the healthy market for inverters, it does seem like the inverter is a likely culprit.

Is there any way I can safely mimic a working lamp, e.g. by shorting the leads with an appropriate resistor or something?

I will check the electrolytic caps... though I'm not sure how I can actually measure their capacitance while they're connected to the circuit, any trick to that?

Dan

You can get CCFL lamps from lots of places, they're not expensive and would work for testing.

You don't measure the capacitance, what you need is an ESR meter, or you can try just replacing them. Surface mount lytics are the most suspect.

Right... I guess I'll need CCFL lamps anyway to replace the burnt-out backlight lamps. Do you know of a source where I can get them cheap? The cheapest I can find for 390 mm lamps like this monitor has is $13 / each, at lcdparts.com ... would it be a terrible idea to use these cheap 310 mm ones:

Gotcha. Don't have an ESR meter, but there's only one electrolytic cap on the board, AFAICT, so I'll figure out some way to test it.

Thanks! Dan

I've just measured the 12 input pins to the inverter board with a voltmeter. As far as I can tell, 8 of the pins are at ground, and the other 4 are at 14V. Does this sound reasonable?

If so, I suppose I could just replace the whole inverter with the inverter from the cheap kits at xoxide.com that I mentioned above.

Dan

How do you know the lamps are burned out? They can and do fail, but they normally last quite a long time.

The ends have what appear to be burn marks inside them... and when I try to measure the resistance across them, it's infinite. Not sure if this is a good way to check the lamps :-/

Dan

The back light inverter is the most common failure on LCD monitors. Normally you should see +9 to +12 v going to the inverter. You will also probably see +5 and one other voltage to control brightness.

Chances are, the inverter is the problem since it usually shares its power supplies with the LCD. Look for any fuses on the inverter board. They will probably be small surface mount components near the

12 pin connector. That orange component to the left of the big green capacitor could be a fuse. There may be some fuses on the bottom of the board too. If you find any open fuses, there's a good chance that a new fuse will fix it. About 3/4 of the inverters I see with blown fuses have no other problems. If a new fuse doesn't fix it, you will probably need to replace the inverter board. Schematics are rarely available and parts are impossible to find. Andy Cuffe

snipped-for-privacy@gmail.com

Could be getter?

I'd expect that.

Thank you, Andy! This is exactly the info I'm looking for. On this board, the input is most definitely around 14V. The power brick is labeled 14V/3A as well. Perhaps the 14V is to add a couple diode drops to the 12V?

I will look more closely for fuses. Is there any particular label used for fuses? As in "C3"=capacitor, "R24"=resistor, "U12"=IC?

If I have to replace the inverter, is there anything wrong with using the ones that come with the cheap case-mod kits? The backlight lamps certainly do appear to be fried, since they have a dark gray haze around one electrode that I don't see in any of the photos at lcdpart.com.

Thanks a lot, Dan

Andy's advice is right on. I'd bet that it is your inverter that went South ... There is usually at least one surface mount fuse on the inverter board and this can be tested with a simple ohmmeter. But, if the fuse blew, it is usually due to a fault elsewhere on the inverter board so simply replacing it will not likely solve your problem.

Bob

Fuses are commonly marked with F or FU.

There are usually 4 different lines on the power connector:

1. 12 or 14 volt main power source.
2. Ground (usually 2 or more wires)
3. On/Off control line (zero or 5 volts)
4. Brightness control (zero to 5 volts).

Every inverter board I have worked on had a fuse on the main power source. Usually located right near the input connector and the larger electrolytic capacitor. Trace your 14 volt line. The fuse us normally marked F1 or something like that. They are surface mounted and may look like a black resistor. Usually rated about 2-1/2 amps.

If the fuse is open, check the driver transistors (or FETs) that drive the primary winding of the step-up transformers. They usually become shorted.

Hope this helps.... John

Thanks! As far as I can tell, all the lines on the inverter connector are at 0 or 14 V :-(

There was indeed a (tiny) fuse, I finally found it. It is not blown, so that wasn't the problem.

After messing around with this thing some more, and accidentally shorting 14 V to ground in the power supply, I've made the whole thing worse. Now I can't even get an image to show up by shining a bright light into the backlight tubes. Ugh. I think I'd better cut my losses and stop messing with this thing. Oh well. It's been a good learning experience!

Dan

Are the inverter fuses quick action or slo blo?

Thanks,

--E

Andy Cuffe wrote: