Plague LCD backlight

[Rachie5272]

I found a pair of LCD monitors, both with an identical problem: when turned on, the CCFL backlight lights for about a second, then goes out.  The LCD itself remains working, but of course is nearly invisible.

These are plague monitors: the low voltage "samyoung" caps were bulging from both ends.  I replaced them, and even replaced the high voltage cap, but it didn't work.  The power rails all read steady at their rated voltages, even during the attempted backlight startup.  I didn't replace the smaller low voltage electrolytics, but they appear fine.

The CCFL tubes themselves seem fine.  I connected a different CCFL to the inverter, which does the same thing.

Any ideas?

[GreyWoolfe]

Remember, appearances are deceiving.  They can look physically fine, but the ESR will be way off.  Best to replace all the caps and remember to use good quality, brand name caps.  No eBay specials.

[mariush]

The inverter will turn off if one or several of the CCFL lights isn't connected properly .. for example when the wire at the end becomes loose (desoldered) which can happen from repeated warming-cooling cycles. You don't see the loose wire because it's usually behind a silicone cap meant to prevent arcing to the metal bits of the lcd display.

The CCFL lights also age and as they age the current they use increases. It can go up to a point where the inverter will stop itself so that the small transformers on the inverter board won't overheat.

So inspect the ends of the CCFL lights again, that would my recommendation if you already tried another inverter.

Much cheaper than replacing all capacitors pointlessly.

[wraper]

Check secondary windings of CCFL inverter transformer (black rectangle). Both windings should measure about 1k. They usually fail open. If transformers are OK, likely one of the CCFL tubes is dead.

[ccayser]

Also sometimes some part of the transformer secondary wiring becomes short, and you are not able to measure if the secondary winding is faulty. I have been testing the well being of transformer with known good pair of CCFL lamps and usually the faulty side is dimmer. You should be able to find the replacement for the transformer on ebay. I also remind you that the primary and secondary side has high voltages. Think twice before you do anything. Good practice is to measure if primary side cap has any charge in it or not.

[Rachie5272]

Well, this is weird.  I tested the inverter again with random CCFL tubes I had laying around, and it stayed on indefinitely.  Then I tested the monitor's tubes with a different inverter, and all four worked.  I also measured the resistance of the transformer: both secondary windings read almost exactly 1 k?.

I'm not sure what this means.  I guess I'll try examining the tube wiring for tiny gaps which the known good inverter wouldn't care about.

Don't worry, I have plenty of experience with high voltage.  I always short out the primary capacitor before touching anything.

[kjn4685]

I had the same problem with a hp 1740 monitor and found the ccfls had a loose connection. I hope this will help you.

[wraper]

Then, probably, some other electrolytic capacitor(s) is dead. Could be even small one on the primary side. I always replace them all except big high voltage one on the primary side as it rarely dies and is expensive. Even slightly lower load can make the difference between does inverter work or does not. If full capacitor replacement does not work then, probably, some tube is good enough to still barely work but bad enough to trigger particular inverter protection.

[Rachie5272]

Getting at the CCFL tubes is turning out to be a right pain.  They're glued in place, and it feels like they'll break at any moment.

The good news is the fuse works.  The bad news is discovered this by trying to discharge the primary capacitor while it was still plugged in.  At least that was an easy fix.

[Riotpack]

The good news is the fuse works.  The bad news is discovered this by trying to discharge the primary capacitor while it was still plugged in.  At least that was an easy fix.

That sounds safe.... Before I even touch a DUT I have the mains cord fully visible to the plug on my bench. Sometimes you only get one chance.

[Rachie5272]

My standard procedure is to discharge the capacitor before I touch the device with anything other than well insulated tools.  Forgetting to unplug it only means frying the device, not myself.

I still can't figure out what's wrong.  I replaced all the electrolytics, with no change.  I tried plugging in the CCFLs from the monitor's identical twin, and still nothing.  Then I tried different combinations of the tubes.  So, how likely is it both monitors developed faults in all tubes at the same time?

I wouldn't think plague caps would take out other components, especially when the power rails are all fine.

I could try replacing the caps in the other power supply, but I have a feeling nothing would change.

[wraper]

Try to connect lamps from the both LCD panels to one board in 4 possible different combinations. Therefore if there in one lamp dead in each panel, you should find working combination.

[ccayser]

Lets say your monitor has 2 lamp connectors on each side, so 4 total. Lets call them A1 A2 and B1 and B2
1) connect A1 to external and others to internal lamps.
2) connect A2 to external and others to internal lamps.
3) connect B1 to external and others to internal lamps.
4) connect B2 to external and others to internal lamps.
Just try to find any combination that it is working. It is good information that with external lamps it stayed on.
Usually if you connect like that the external CCFL will be lit half way if you find the faulty side. But to find it out, you need to connect one internal lamp and one external lamp.

[fable]

Hi.You just need to remove (bypass) OVC protection which sometimes kicks in too early.Look at the back side of board for driver chip download a datasheet for him and read it.or just take a picture of back side of board and il will tell what to do next.I done this on many times and never had problems

[Rachie5272]

I tried various combinations of CCFL tubes, but nothing works.  Even all external tubes no longer works.

Disabling the protection sounds like a good idea.  The driver chip is:
MPS 0815
MP1038EY
789448.59

It looks like I simply tie the FT fault timer pin to ground, instead of the timing capacitor.  While I'm at it, I might as well force the chip enable high, so the backlight is always on when plugged in.

[wraper]

Protection circuit could be faulty too. Or some other fault in the inverter. Check blue high voltage capacitors. IIRC they could be about 3-15 pF.

[fable]

Just tie FT pin to GND ...u dont need to do anything else.

[Rachie5272]

I replaced the bad caps on the second power supply, and now it works fine.  The first power supply still doesn't work, and the lights don't even flicker on powerup anymore.  I'll probably give up on it.

Now I need to figure out what to do with the monitor.  I thought it was cracked, so I intended to use the backlight for another project, but it turns out to be perfectly fine.

[wraper]

Probably you burned F301 fuse while making experiments. Check that.

[Rachie5272]

You were right: F301 was fused open.  It blew because the 18 V rail of the inverter section was shorted out.  It looks like the culprit is MOSFET U303.  I don't have a proper replacement, but I'll try jamming any old FET in there and see what happens.

The circuit already looks like Frankenstein's monster with all my ugly hacks, but I'm not in the mood for an elegant fix.

[Rachie5272]

It worked!  Two little SOT23 FETs replaced the SOIC one, and the backlight is glowing away.  Of the two dumpster sourced monitors, one is now fully working, and the other will become a light source.