tscheucher
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Jul 8, 2009, 3:57 PM
Post #4 of 4
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Re: [XxNinja] Tv Turns On And Goes Blank
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It sounds like a frequent LCD problem. Three words: LCD backlight inverter!
To make sure that the assumption is right, get a flashlight and place it directly in the front of your LCD panel. I assume that the inverterīs transformers arenīt shorted, cause the PS unit starts up without difficulties and keeps on at stable operation without flickering front led according to your explanations.
Now illuminate it and figure out if you can see at least a few fragments of the pictures content. If itīs the case procced as following:
Replace the opto-coupler(s) and the TL431 (shunt regulator) which are supporting the inverter voltage. Most of the issues for a non working backlight is a logic ic (inverter controller ic) failure, which can only be fixed by replacing it. The inverter ic is checking out the monitorīs "health status" (current consumption of external components and possible short curcuiting) and gives back a feedback signal to the inverters driver stages to keep them "on" status on by producing a biasing voltage to "hold" their mosfet gates or transistor bases on status open.
The noise you here is produced by the power supplyīs flyback transformerīs mechanical oscillation due to wrong supply frequency and clock rates because of a bad opto-coupler. If youīve fixed the supply issues, try this:
- Best case: The inverterīs SMD fuse has blown - Replace it with a new one from a delay type (somewhere around 3,15 or 4A)
- Replace the backlight inverter by using a piezo type inverter in order equalizing "tired" lamps power (current) consumption. That type of inverter is self adjusting and you only have to make sure, that you correctly connect it to the main circuit. Take measures at the old one in order to figure out which cable has got which function (exept the gnd - cable). They only have 3 major tasks:
1) Constant 12 or 24V voltage for operation
2) Startup - cord (leading starting voltage in a range from 5 to 12V depending on the brand and model)
3) Brightness control voltage (adjustable somewhere between 3 and 5V at the on screen menue)
The connections of the new inverter are marked with small numbers from 1 to xxx. - You can read their functions in the manual coming with it.
If you donīt want to spend money for a spare part, you can "manually" operate the inverter by aborting some of itīs tasks. That means, that after these modifications, you arenīt able to control itīs startup point (it starts up the same time the main power supply is switched on) and brightness control is disabled in hardware.
Have a look at the components on the inverter - board and seek for the driver stages. Mostly power mosfetīs or high current npn - transistors in cheap devices. Desolder their Gate or Base Pin from the board and try to find a 5V source somewhere in the PS unit. If youīve found, connect the Gates (Bases) to that source and insulate them from their former biasing source on the inverter board. Try to startup the TV again and hopefully the image is back. You can control brightness manually by using an adjustable voltage regulator for biasing the driver stages.
Good luck!
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