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TFT backlight shuts off after 2 seconds..

I have a more than one tft screen where the backlight shuts off after ~2 seconds. By turning the monitor off and on again I get the backlight to work for another 2s. The screen functionality for the TFT matrix seems ok. And because the tubes actually gives light even if only for 2s I assume they work. And conclude it's the backlight inverter pcb.

My general understanding of this pcb is that it will take DC (usually

12V?) and chop it up and then transform it up by some means of smps. I have access to multimeter and oscilloscope. Considering this there should be a at least one power semiconductor acting as a switch. Controller circuit and an output transforrmer.

The switch should have pulsed dc input and a pulsed voltage should occour over the emitter-collector (or source-drain etc..). The transformer primary should have pulsed dc aswell. Any break in this chain should limit where the fault might be. The output of the transformer (1300 kVrms?) is proberbly not a good idea to measure as it's likely to blow the measurement equipment.

What strategy should one use..?

(I have been thinking about using a CCFL driver used for computer decoration, but those lack brightness control..)

Two of the screens with these symptoms are: NEC LCD1810 (Backlight HIU-391C) ViewSonic VE710b

Reply to
posedge52
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The inverters may be shutting down as a result of detecting an abnormal load - such as a defective lamp - or because the system is telling them to, as it would if going into power save. It could also be that the inverter itself is faulty. You could start by looking to see if there is any input line to the inverter board, which changes state when the lamps go out.

Many parts available from

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Arfa

Reply to
Arfa Daily

The lamp seems to work (from what I remember) if just for 2s. And I have not spotted any change in voltage on the input/connections to the ccfl-inverter with the multimeter on board before and after shutdown. The monitor doesn't indicate any powersave mode. I think I even could see very dimly the stuff that's supposed to be on the screen. So I'm looking for a fault search on the inverter board.

lcdparts.com are nice, but they charge 70 USD, and with freight, vat etc.. it doesn't make sense.

Reply to
posedge52

That particular model has a habit of blowing one or more picofuses for no apparent reason. Check all the fuses on the inverter - likely one is blown. I fixed several of those with the same problem.

Reply to
JW

CCFL backlight driver IC's are usually configured to allow an initial surge of current to get things started, then current limiting kicks in via a capacitor charging up. The symptom you describe is usually caused by worn-out tubes that will no longer stay lit at the current limit levels designed into the circuit.

Reply to
Ol' Duffer

If it has more than one bulb, then one of them is probably not lighting at all. Most inverters will shut down unless all the bulbs are working. I've never seen a bulb fail completely and cause this, but one of the inverter sections often fails. To figure out which one it is, power it up with only one bulb at a time connected until you find the one that's not working. Then you at least know which part of the inverter to focus on. To check the bulb, simply plug it into one of the inverter's other outputs. Typical failures include failed transformers, shorted transistors and diodes, and open fuses. Andy Cuffe

snipped-for-privacy@gmail.com

Reply to
Andy Cuffe

Check to make sure both ccfl lamps are working.. If only one is working it will usually do exactly what you are describing.

Mike

Reply to
Michael Kennedy

I just fixed a similar problem in a Sony KLV-L32M1. The service manual did'nt have a schematic of the invertor section, so i pulled up datasheets of the driver ICs and the main controller. The main controller had a selectable option of reporting CCFL and peripheral circuitry failure by either turning off all CCFLs together, or (by removing a ) ohm to ground jumper on 1 of its pin thereby lifting the pin high) running it in IND (individual) mode where only the faulty drive circuit and CCFL would be turned off. Hence the tv would now switch on with all lamps glowing while the defective one would fire and then quit. Replacing the faulty lamp (which mind you seemed to be fine, it glowed if forced, slight blackening at one end observed) solved the problem. Returned the ic to group mode by re-installing the grounding jumper.

Reply to
jango2

This page

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was my inspriation when I fixed my Sony LCD. I didn't replace the bulbs, but I did use an external invertor PCB to drive the bulbs. I used one from a cold cathode case mod kit.

Reply to
Anonymous

I have considered this path aswell, but the catch is that then you can no longer adjust brightness.. Any idea on how to solve that ..?

Reply to
posedge52

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