EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: marzanoalfonso on December 10, 2016, 02:47:33 pm
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Hi everyone,
My secondary screen, an Asus VH222 LCD (CCFL) is having a few issues.
It turns off after a few minutes of use (interval seems to be random) and then it will start a loop where it turns on and immediately off infinitely. If i turn the power off for a few minutes it starts working normally but will eventually end up doing the same thing.
The Power Supply board sports Elite brand capacitors, and all seem to be Ok, with no visible signs of failure.
Tested other components (diodes/resistors) with my cheap meter and all seems to be ok.
The only other (visible) board is the VGA board, holding a few Elite caps (all ok).
Backlight is also ok.
Has anyone ever encountered the same problem and can maybe provide tips on how to fix this issue?
Thanks in advance,
Al
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Electrolytic caps don't need to be bloated to fail.
That being said, i don't trust Elite caps as far as i can throw them. Ditto for CapXon, Fuhjyyu and countless other Chinese brands and no-names.
I wouldn't be surprised if it worked just fine again after a full re-cap. Don't neglect the small caps on the primary side.
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My bet is bad tact switches which became conductive between the contacts, especially happens when high moisture or corrosive chemicals present in the air. Not necessarily power switch itself, they likely are all connected together to one line through resistors of different value. Disconnect a small board with switches and see how it goes. If it helps, replace all of the switches as all of them will be in around the same bad condition.
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Replace the caps :)
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About those recapping recommendations. First of all, Elite caps are pretty good. I yet to see one which failed. By how fault happens, it does not seem cap related. When capacitors fail, usually monitor fails to start or backlight goes into protection. But it may start to work normally once warms up (ESR drops with temperature increase). But this defect happens in completely opposite way. If I still was after replacing capacitors without giving much thought, I would try replacing only this one for starters.
(https://www.eevblog.com/forum/repair/asus-vh222-turns-onoff-randomly/?action=dlattach;attach=277669;image)
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That's why i pointed out that one shouldn't neglect the small caps on the primary :)
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My bet is bad tact switches which became conductive between the contacts, especially happens when high moisture or corrosive chemicals present in the air. Not necessarily power switch itself, they likely are all connected together to one line through resistors of different value. Disconnect a small board with switches and see how it goes. If it helps, replace all of the switches as all of them will be in around the same bad condition.
I second the tact switch recommendation, I've had issues with a Benq that seemed to be "possessed by a spirit" due to buttons going conductive (reading kOhms when not pressed).
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Have you repaired your screen? I have the same model. Last days the backlight started going purple and today it simply blacked out. When I turn it on it powers off for about two seconds and then goes black again. I repair man told me it's typical ccfl fail. Do you know where I can find new ccfls? Below are pictures at the ends of both ccfl.
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You need to check resistance of secondary winding(s) of CCLF transformer first, should be around 1k and equal for all secondary windings. It's much more common than faulty lamps. Bad caps in PSU also cause similar symptoms.
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Total noob question but can you mark on the board which are the secondary windings of the CCFL transformer? I already changed some capacitors (I circled them in yellow) but the problem is still the same.
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After I changed all the electrolytic capacitors on both boards the monitor powers on for 1 sec and then goes blank.
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Total noob question but can you mark on the board which are the secondary windings of the CCFL transformer? I already changed some capacitors (I circled them in yellow) but the problem is still the same.
On the right side of the PCB there is CCFL transformer. On the right side of transformer there are 4 pins connected to two secondary windings. 2 pins on the top side (according to photo) are connected to first winding, 2 bottom pins - to second winding.
(https://www.eevblog.com/forum/repair/asus-vh222-turns-onoff-randomly/?action=dlattach;attach=584111;image)
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Thank you so much for your answer. I just noticed that only the backlight goes off, the monitor remains on. If there is a problem with the secondary winding, do I have to order the exact same one or a generic one? Do you know any other site than eBay that can ship to Greece? I have attached the schematic of the power board if it's any help...
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If there is a problem with the secondary winding, do I have to order the exact same one or a generic one? Do you know any other site than eBay that can ship to Greece?
There is no such thing as generic CCFL transformer. You should order the same, in some cases when original cannot be found it's possible to use something similar. It's easiest to buy them from aliexpress or ebay. On your photo one lamp looks dark on one end, if transformer measures OK, then you can be pretty sure that lamp is faulty.
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from my experience going pink/violet before turning off = bad lamp, as you said typical ccfl fail
Easy to test, get another old/cracked LCD monitor and connect lamps to this ones inverter.
You could try finding cracked 21.5 panel to swap lamps from, or even attempt LED backlight mod, but this is just a poor 10! year old TN screen, worth maybe $50 fully working.
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You could try finding cracked 21.5 panel to swap lamps from, or even attempt LED backlight mod, but this is just a poor 10! year old TN screen, worth maybe $50 fully working.
It's not worth $50 fully working because you can buy a brand new monitor for a little bit more than that.