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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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LCD monitor inverter/backlight repair
Hi all,
I am trying to repair a 19" Samsung 191T monitor that appears to have been damaged due to a storm-related power surge over the weekend. The problem with the monitor is clearly related to the backlight: the four backlight lamps appear "burnt" and if I shine a bright light into the back of the panel, the image on the screen appears perfectly fine :-) The monitor includes two main circuit boards: a big one that the DVI input and power connects to, and a smaller LCD inverter board. The LCD inverter board is an Ambit BN44-00060A, which appears nearly identical to this one: http://www.lcdpart.com/images/AMBIT_K021036.gif There are no fuses on either board, as far as I can tell. The LCD inverter board is connected to the main circuit board via a 12-pin connector. The voltages output by the LCD inverter board are very close to zero, so obviously something is wrong with it :-( If I understand correctly, the cold cathode backlight bulbs need something like 1000V AC, is that right? I would like to be able to verify that nothing is wrong with the power supply part of the MAIN circuit board as well... does anyone know what the voltages should be on the 12 input pins to the inverter board? Since the image is apparently fine, there is no problem with the display electronics on the main board, but there could be a problem with the power supply on the main board... I would like to know if there is a way to tell this. Can anyone give me any advice on how to test the LCD inverter and main power supply boards? There are no fuses. Thanks for any advice!!! Dan Lenski |
#3
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LCD monitor inverter/backlight repair
James Sweet wrote:
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 |
#4
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LCD monitor inverter/backlight repair
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#5
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LCD monitor inverter/backlight repair
James Sweet wrote:
You can get CCFL lamps from lots of places, they're not expensive and would work for testing. 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: http://www.xoxide.com/dual-white-cold-cathode-kit.html 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. 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 |
#6
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LCD monitor inverter/backlight repair
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 |
#7
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LCD monitor inverter/backlight repair
wrote:
James Sweet wrote: You can get CCFL lamps from lots of places, they're not expensive and would work for testing. 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: http://www.xoxide.com/dual-white-cold-cathode-kit.html How do you know the lamps are burned out? They can and do fail, but they normally last quite a long time. |
#8
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LCD monitor inverter/backlight repair
James Sweet wrote:
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 |
#9
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LCD monitor inverter/backlight repair
On 5 Sep 2006 12:53:26 -0700, wrote:
Hi all, I am trying to repair a 19" Samsung 191T monitor that appears to have been damaged due to a storm-related power surge over the weekend. The problem with the monitor is clearly related to the backlight: the four backlight lamps appear "burnt" and if I shine a bright light into the back of the panel, the image on the screen appears perfectly fine :-) The monitor includes two main circuit boards: a big one that the DVI input and power connects to, and a smaller LCD inverter board. The LCD inverter board is an Ambit BN44-00060A, which appears nearly identical to this one: http://www.lcdpart.com/images/AMBIT_K021036.gif There are no fuses on either board, as far as I can tell. The LCD inverter board is connected to the main circuit board via a 12-pin connector. The voltages output by the LCD inverter board are very close to zero, so obviously something is wrong with it :-( If I understand correctly, the cold cathode backlight bulbs need something like 1000V AC, is that right? I would like to be able to verify that nothing is wrong with the power supply part of the MAIN circuit board as well... does anyone know what the voltages should be on the 12 input pins to the inverter board? Since the image is apparently fine, there is no problem with the display electronics on the main board, but there could be a problem with the power supply on the main board... I would like to know if there is a way to tell this. Can anyone give me any advice on how to test the LCD inverter and main power supply boards? There are no fuses. Thanks for any advice!!! Dan Lenski 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 |
#10
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LCD monitor inverter/backlight repair
wrote in message oups.com... 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 Could be getter? and when I try to measure the resistance across them, it's infinite. I'd expect that. |
#11
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LCD monitor inverter/backlight repair
Andy Cuffe wrote:
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. 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? 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 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 |
#12
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LCD monitor inverter/backlight repair
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 "Andy Cuffe" wrote in message ... On 5 Sep 2006 12:53:26 -0700, wrote: Hi all, I am trying to repair a 19" Samsung 191T monitor that appears to have been damaged due to a storm-related power surge over the weekend. The problem with the monitor is clearly related to the backlight: the four backlight lamps appear "burnt" and if I shine a bright light into the back of the panel, the image on the screen appears perfectly fine :-) The monitor includes two main circuit boards: a big one that the DVI input and power connects to, and a smaller LCD inverter board. The LCD inverter board is an Ambit BN44-00060A, which appears nearly identical to this one: http://www.lcdpart.com/images/AMBIT_K021036.gif There are no fuses on either board, as far as I can tell. The LCD inverter board is connected to the main circuit board via a 12-pin connector. The voltages output by the LCD inverter board are very close to zero, so obviously something is wrong with it :-( If I understand correctly, the cold cathode backlight bulbs need something like 1000V AC, is that right? I would like to be able to verify that nothing is wrong with the power supply part of the MAIN circuit board as well... does anyone know what the voltages should be on the 12 input pins to the inverter board? Since the image is apparently fine, there is no problem with the display electronics on the main board, but there could be a problem with the power supply on the main board... I would like to know if there is a way to tell this. Can anyone give me any advice on how to test the LCD inverter and main power supply boards? There are no fuses. Thanks for any advice!!! Dan Lenski 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 |
#13
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LCD monitor inverter/backlight repair
wrote in message oups.com... snip I will look more closely for fuses. Is there any particular label used for fuses? As in "C3"=capacitor, "R24"=resistor, "U12"=IC? Fuses are commonly marked with F or FU. |
#14
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LCD monitor inverter/backlight repair
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 |
#15
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LCD monitor inverter/backlight repair
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#16
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LCD monitor inverter/backlight repair
Are the inverter fuses quick action or slo blo?
Thanks, --E Andy Cuffe wrote: 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 |
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