LCD monitor backlight - inverter board problem - Page 1
Forum section: electronics / repair

author: Shivram10
author_rank: Contributor
timestamp: July 15, 2014, 08:41:26 am
content: Hello everyone,I am a hobbyist in electronics, my neighbour has an LCD monitor of make ZENITH (Indian Company) bought back in 2008, the problem with the LCD is the display goes dimmer & dimmer until he cannot see anything on the screen this has been happening for last 2 weeks now, he turned off the computer and after some time when he turned it on the LCD used to work, Now there is no picture on the screen but the power led is glowing. I connected my LCD screen to his computer to see if the fault is with the monitor or the CPU and my monitor is working giving clear display. So the faulty is with his LCD monitorWhen I opened the monitor i checked the inverter board & main board there was no sign of any burnt components both boards were clean.Then I connected it to the mains AC and checked the output voltages in circle marked 1 it gave 12v & 5v DC for the main board then I checked the voltage in circle marked 2 it gave me 14v AC for the LCD backlight,then the backlight was glowing and giving me display, so i thought it was a loose connection so I turned off the monitor and after 15 mins I turned it back on (my PC was on) now it was not giving any display and the backlight was not glowing then I checked output voltage at circle marked 2 now it was not giving any voltage. then I disconnected the main board, backlight from the power supply and checked the voltages it was giving output of 12v & 5v DC but was not giving any voltage for the backlight of the LCD (Circle no 2).The LCD panel is made by LG Philips model - LG LP171WE2Power board is - HKC pi560it was assembled for Zenith Computers (India) model - Z787Then I googled and found some articles online for inverter board testing, most of them suggested to check the inverter transformer (circle No. 4) and transistors near the transformer, to check the feedback circuit, so I removed the Schottky barrier diodes (2 Nos) circle no. 3, part no - SBLF10100CT (attached datasheet as this is the only website providing me the information as google suggested me equivalent MBR10100CT with same as SBLF10100CT). They are Common-Cathode High Voltage Schottky Rectifier Diodes, I checked with my DMM - diode function they tested out fine.Kindly help me troubleshooting this monitor display problem. I have checked all the capacitors with ESR meter they all are good.PS - sorry for such a long post, the image of the inverter board is of the same model from internet as I didn't have camera with to take picture.

author: mij59
author_rank: Frequent Contributor
timestamp: July 15, 2014, 01:51:03 pm
content: Hi,Not sure whats going on, but since the back light get dimmer very slow it could be the CCFL lamps.If the lamp current is to low the inverter could shut down.Do not measure the output of the CCFL (no. 2), it's high voltage, serveral kV to startup the CCFL.You could check the transformer (no. 4)The transistors to the right of ( no. 4)

author: Stonent
author_rank: Super Contributor
timestamp: July 15, 2014, 02:48:54 pm
content: Technically Zenith is Korean since they are a division of LG.

author: Shivram10
author_rank: Contributor
timestamp: July 16, 2014, 03:57:46 pm
content: Quote from: mij59 on July 15, 2014, 01:51:03 pmThanks @mij59The backlight was getting dimmer (normal display to dark in 15 seconds) now there is no backlight.I don't know how to test the transformer (No.4), I googled for transformer test most of them said a multimeter test is not accurate and I don't have a Blue Ring Tester. So is there any other method to test the transformer.The transistor in big red circle are Schottky barrier diodes SBLF10100CT (tested both, working) & 2 small circle are C5706 NPN power transistors both working & found STK0765 Power Mosfet faulty.I will replace the the Power Mosfet and let you know how it turned out.Regards,Shivram10

author: mij59
author_rank: Frequent Contributor
timestamp: July 16, 2014, 04:24:12 pm
content: Quote from: Shivram10 on July 16, 2014, 03:57:46 pmThe secondary side of the transformer has one, or probably two winding's, if the winding's are not open its o.k. for now.Check the components in the circuit of the mosfet, power resistors etc.Test the power supply with all cables connected.

author: Stonent
author_rank: Super Contributor
timestamp: July 16, 2014, 06:53:39 pm
content: If you have an oscilloscope, volt-alert style probe (that has audio feedback), or a network cable trace probe (with audio feedback) you can wave the end near the transformer that drives the backlight and listen for the sound or look for the wave form. Many times they run at a higher frequency than the 50/60Hz AC that feeds the TV.I would be interested if the oscillations change or stop when the backlight starts fading.

author: Shivram10
author_rank: Contributor
timestamp: July 18, 2014, 09:01:16 am
content: Quote from: Stonent on July 16, 2014, 06:53:39 pm@StonentI don't have an oscilloscope, I do have a volt alert probe that sounds a buzzer when it is near 60v or more AC. I will try your method as when I checked the other components I found STK0765 Power Mosfet faulty and I have ordered the mosfet I will get that in 2-3 days and after replacing I will post more about the repair.Ragards,Shivram