Author Topic: LCD Monitor troubleshooting  (Read 19942 times)

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Offline TorArneTopic starter

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LCD Monitor troubleshooting
« on: October 31, 2014, 11:27:58 pm »
Hi,

I'm new here, so this is my first post.
I have a HP LA2205wg 22" screen which has gone black.
I have tested that the CCFL tube is working, and it seems to be working ok.
I can see a really faint picture, as if the CCFL tube isnt working.. but it is. (atleast it lights up when i power the monitor on)

I have measured most of the caps, and none are bad.

Anyone got ideas on what to measure/check next?
I actually got 3 or 4 of these monitors, all with the same error..
So if i fix one, i fix all :)

Tor Arne
 

Online wraper

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Re: LCD Monitor troubleshooting
« Reply #1 on: October 31, 2014, 11:43:49 pm »
It lights up and works or switches off after a second or two? Is there only one CCFL tube or two of them? Post a picture of the PSU PCB.
 

Offline TorArneTopic starter

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Re: LCD Monitor troubleshooting
« Reply #2 on: November 01, 2014, 12:08:04 am »
The screen it self doesnt seem to light up at all.
The CCFL turns on.
Its just a stable very very dim picture.

Edit: I will actually do a recheck that the CCFL actually does light up
 

Offline SeanB

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Re: LCD Monitor troubleshooting
« Reply #3 on: November 01, 2014, 05:31:30 am »
Check the fuse in the 5V rail ( F851 next to the red 5V wiring) but your most likely problem is a dead main board.

I had one with the same power supply board, and that is what died. As I got it in a roadside pickup it was no loss to scrap it and only keep the CCFL tubes and driver.
 

Offline fluxcapacitor

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Re: LCD Monitor troubleshooting
« Reply #4 on: November 01, 2014, 07:26:20 am »
You say youve tested most of the capacitors. Did you test the main cap , its been changed at some point, by you or someone else .
 

Offline TorArneTopic starter

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Re: LCD Monitor troubleshooting
« Reply #5 on: November 01, 2014, 07:42:41 am »
You say youve tested most of the capacitors. Did you test the main cap , its been changed at some point, by you or someone else .

I replaced the main cap, as i had it off and had a spare. (old one was'nt bad, but changed anyway)

Check the fuse in the 5V rail ( F851 next to the red 5V wiring) but your most likely problem is a dead main board.

I had one with the same power supply board, and that is what died. As I got it in a roadside pickup it was no loss to scrap it and only keep the CCFL tubes and driver.

Checked the fuse, works like a charm.
You didnt manage to fix the dead main board?

Tor Arne
 

Offline SeanB

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Re: LCD Monitor troubleshooting
« Reply #6 on: November 01, 2014, 06:00:14 pm »
Plain VGA input. Tiny board with a single chip on it with a hole in the top. Was worth more as scrap metal.
 

Online wraper

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Re: LCD Monitor troubleshooting
« Reply #7 on: November 01, 2014, 07:04:26 pm »
So what with the lamps? Saying dim image, I understand that monitor is showing the image from the computer, just barely visible? If that is the case, main board cannot be dead. It can be only non working backlight or dead LCD panel. Second variant is very unlikely to be present on 4 monitors being exactly the same. On all monitors there is protection which shuts down backlight after a moment is something is not ok. Most common are: 1. one of the two (or more in the large panels) CCLF lamps is bad or 2. one of the secondary windings of the transformer(s) is faulty.
 

Offline TorArneTopic starter

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Re: LCD Monitor troubleshooting
« Reply #8 on: November 02, 2014, 11:27:49 am »
I took some pictures.
1 from a working screen (a samsung)
1 from the broken screen (hp)

There is also some flickering on the screen. unsure why.
 

Online wraper

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Re: LCD Monitor troubleshooting
« Reply #9 on: November 02, 2014, 11:41:47 am »
Strange. Backlight is definitely working, at least partially. What happens if you change brightness in the settings from min to max, did you reset settings to factory default? Might be that backlight inverter works somehow, but below lowest PWM duty cycle limit.
 

Online wraper

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Re: LCD Monitor troubleshooting
« Reply #10 on: November 02, 2014, 11:53:07 am »
Do you have oscilloscope?
 

Offline TorArneTopic starter

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Re: LCD Monitor troubleshooting
« Reply #11 on: November 02, 2014, 11:53:16 am »
I opened the monitor so you can see the CCFL.
And its on.. not extremly bright.. but its on. see picture.

I have tried to change brightness and all.. also tried factory setting.. no go.
 

Offline TorArneTopic starter

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Re: LCD Monitor troubleshooting
« Reply #12 on: November 02, 2014, 11:54:05 am »
Do you have oscilloscope?

I do have a cheapass usb one.. not very good.. but should work for simple stuff.

Can the CCFL tube partly be broken? or is it working.. or none working?

Edit: I can get my hands on a working monitor.. same model.. and just switch parts around to figure out whats broken.. If that helps.
Cant do that untill Monday thou (monitor at work)
« Last Edit: November 02, 2014, 12:04:33 pm by TorArne »
 

Online wraper

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Re: LCD Monitor troubleshooting
« Reply #13 on: November 02, 2014, 12:04:44 pm »
If CCLF would be dead, inverter protection should shut down. As you have oscilloscope, would be worth to hook it to transformer primary winding (between GND and terminal where transistor switches are connected) and post waveform here with min and max brightness setting. Just be very careful as there is something like 600-700V on secondary winding.
« Last Edit: November 02, 2014, 01:05:00 pm by wraper »
 

Online wraper

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Re: LCD Monitor troubleshooting
« Reply #14 on: November 02, 2014, 12:23:32 pm »
BTW just noticed that you changed some capacitors. If you fix this monitor, throw those green Chengx junk away and solder original ones (suppose they were Rubycon) if you still have them, or get something better. Those Chengx are not only poorest quality capacitors but also have huge ESR. I once bought few, didn't check the brand they gave me at the local shop because they usually had Jamicon WL in stock which are green too. So when I measured, they had ESR like 2-3X bigger than general purpose capacitors with the same capacitance and voltage specifications. Moreover, they even don't write series on them, just LOW ESR which they obviously are not. There seems to be series marked on yours, but they are too tiny to be good compared to original ones anyway.
« Last Edit: November 02, 2014, 12:40:22 pm by wraper »
 

Online wraper

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Re: LCD Monitor troubleshooting
« Reply #15 on: November 02, 2014, 12:39:11 pm »
As schematic don't seem to be available, write here IC model which is soldered on the small PCB near the transformer. I'll check datasheet and see where to measure PWM control signal. Photos of the Small PCB and back side of the big one would be useful too.
 

Offline TorArneTopic starter

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Re: LCD Monitor troubleshooting
« Reply #16 on: November 02, 2014, 04:14:35 pm »
If CCLF would be dead, inverter protection should shut down. As you have oscilloscope, would be worth to hook it to transformer primary winding (between GND and terminal where transistor switches are connected) and post waveform here with min and max brightness setting. Just be very careful as there is something like 600-700V on secondary winding.

That didnt go too well.
Blew a fuse, and pc is dead.
 

Offline TorArneTopic starter

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Re: LCD Monitor troubleshooting
« Reply #17 on: November 02, 2014, 04:16:34 pm »
BTW just noticed that you changed some capacitors. If you fix this monitor, throw those green Chengx junk away and solder original ones (suppose they were Rubycon) if you still have them, or get something better. Those Chengx are not only poorest quality capacitors but also have huge ESR. I once bought few, didn't check the brand they gave me at the local shop because they usually had Jamicon WL in stock which are green too. So when I measured, they had ESR like 2-3X bigger than general purpose capacitors with the same capacitance and voltage specifications. Moreover, they even don't write series on them, just LOW ESR which they obviously are not. There seems to be series marked on yours, but they are too tiny to be good compared to original ones anyway.

I will change the capacitor back.
Wasnt actually Rubycon on the big one, was a Elite brand.
Is that any good?
 

Online wraper

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Re: LCD Monitor troubleshooting
« Reply #18 on: November 02, 2014, 04:27:10 pm »
For high voltage capacitor elite is completely ok, order of magnitude better than Chengx anyway. On capacitorlab.com elite is mentioned as crap like all brands except japanese capacitors and samxon, but that list is about LOW esr capacitor failures what is not the case. IMO they are generally ok, especially if used where is no high ripple current.
« Last Edit: November 02, 2014, 04:31:03 pm by wraper »
 

Offline Rick

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Re: LCD Monitor troubleshooting
« Reply #19 on: November 02, 2014, 04:30:34 pm »
If it were me I would find a couple of ccfls and blindly replace the ccfls one by one till I find the defective one before doing anything else. I think the flickering may be due to one of them. Now we have cheap led strips sold with the driver here, but I think the ones sold on ebay are not that cheap.
 

Online wraper

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Re: LCD Monitor troubleshooting
« Reply #20 on: November 02, 2014, 04:42:37 pm »
If it were me I would find a couple of ccfls and blindly replace the ccfls one by one till I find the defective one before doing anything else. I think the flickering may be due to one of them. Now we have cheap led strips sold with the driver here, but I think the ones sold on ebay are not that cheap.
As if it would be an easy job to tear apart LCD panel and you need to buy a new CCFL in the first place. With a huge risk to completely destroy LCD. BUT, as I said, if CCFL failed, inverted will shut down, end of the story. What I see here, for some reason inverter is not regulating PWM duty cycle. It may be inverter failure or no PWM control signal coming from the main board. I tend to think this is the first variant.
 

Online wraper

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Re: LCD Monitor troubleshooting
« Reply #21 on: November 02, 2014, 04:50:32 pm »
Now we have cheap led strips sold with the driver here, but I think the ones sold on ebay are not that cheap.
I wonder about all those people who modify their monitors with the LED strips. Are they idiots or clueless? Even if it kinda works ok, color reproduction of the monitor will be completely fucked up. It will be impossible to get any acceptable color accuracy even if using calibrator.
 

Offline Rick

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Re: LCD Monitor troubleshooting
« Reply #22 on: November 02, 2014, 05:39:24 pm »
If it were me I would find a couple of ccfls and blindly replace the ccfls one by one till I find the defective one before doing anything else. I think the flickering may be due to one of them. Now we have cheap led strips sold with the driver here, but I think the ones sold on ebay are not that cheap.
As if it would be an easy job to tear apart LCD panel and you need to buy a new CCFL in the first place. With a huge risk to completely destroy LCD. BUT, as I said, if CCFL failed, inverted will shut down, end of the story. What I see here, for some reason inverter is not regulating PWM duty cycle. It may be inverter failure or no PWM control signal coming from the main board. I tend to think this is the first variant.

You don't need to tear apart you lcd panel. There is a way to pull the ccfls out without destroying the unit, you open the metal frame at the four corners of it with pliers, and pull the ccfls out, it's easy. But it may be different for each monitor.
 

Offline TorArneTopic starter

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Re: LCD Monitor troubleshooting
« Reply #23 on: November 02, 2014, 08:02:59 pm »
As schematic don't seem to be available, write here IC model which is soldered on the small PCB near the transformer. I'll check datasheet and see where to measure PWM control signal. Photos of the Small PCB and back side of the big one would be useful too.

Here you go.
Oscilloscope dont work anymore thou..
Its dead, docs says max 600v.
 

Online wraper

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Re: LCD Monitor troubleshooting
« Reply #24 on: November 02, 2014, 08:19:59 pm »
Its dead, docs says max 600v.
Did you probe secondary of the CCFL transformer  :scared:? Well, I warned  :phew:. Or considering blown fuse, did you probe something connected to the mains? Well, I asked for primary winding, guess you probed wrong primary as I meant inverter?
I asked for different board photo. Small PCB that sticks out of the PSU PCB near CCFL inverter transformer.
« Last Edit: November 02, 2014, 08:32:38 pm by wraper »
 


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