LCD backlight problem? inverter problem? SHARP

[A Man]

the MOSFETs are made by Rohm Semiconductor, and are 500V 7A MOSFETs that use a 10V drive voltage. as for the Zeners, i can only GUESS that they might be 24V zeners (you will need a schematic to know for sure). poking around on the primary side of the supply really wasn't necessary if you had a solid power LED. if you get it running again, stick to the secondary side of the supply. you should have connectors running to the main board, and they usually mark the voltages on the connector pinout. if they don't you'll need a schematic

I cán't find this exact MOSFET but I think this Fairchild FDP7N50 should work. Any ideas on this? The main thing are the diodes... Really need some insight into this. I don't fully understand what is happening here and I was hoping someone a little more experienced could explain the theory and calculate/deduce/knew what diodes were needed. This is def part of the inverter circuit because now that the shorted components are removed (I replaced the fuse first and it immediately blew on repower) the tv will start to power on again but shut down immediately and blink 2x rapidly indicating a fault in the inverter/LCD.

 

[unclejed613]

you may need to add a small resistor in series with the gate. as i said, that's a MOSFET that uses 10V drive, not 3 to 5 volts like other MOSFETs. the zeners are probably used as a voltage reference for a comparator circuit. without the zeners, the pulse width of the MOSFET drive will not be correct, and so the secondary voltages won't be correct. i've seen quite a few designs that use 48-50V as the voltage reference on the primary side, so 24V is probably the correct voltage for each zener.

 

[A Man]

I'm not sure what you mean by adding a resistor since the 7N50 is a 10V 7A 50W MOSFET which is as close (slightly higher) to the original as I could find. I deduced from some of your blogs that you are a repair professional, which I can assure you, I am not. I'm a hands-on, learn by trial and error, school of hard knocks kind of newbie to electronics repair. I've had a few basic electronics classes but never finished because the theory behind how everything works wasn't what I wanted to know. Of course, now I believe that if I knew how the circuits were supposed to work, then I would be able to figure out why they did not work. By trade, I am a natural gas compressor mechanic so this is not my area of expertise and therefore I am grateful for all of your expertise and knowledge you are inclined to share. Just my two cents...

So, back to the subject at hand... I just ordered a replacement board for the time being to further diagnose the problem at hand of no backlight (and until I learn more on how to repair the broken one and then I will have it as a reference for voltage checks and such.) However, when I installed this board I got the same results from before I blew the MOSFETs. The set will shut down with a 1x blink indicating lamp issues. If I force it into service mode, the LCD only briefly blinks on but the set won't power off on a fault as before.

So I disassembled the LCD display. All ten bulbs are lighting but nearly all are discolored on the ends and several were a pink/purplish color. Any ideas on how to proceed? Where should I investigate? Is there a voltage check on the inverter output? A way to test the lamps? The inverter circuits won't switch on unless there is a load, correct? Nor will they switch on if the current/voltage draw is too high, correct? And this is why it's shutting down the backlight, correct?

 

[A Man]

So I disassembled the LCD display. All ten bulbs are lighting but nearly all are discolored on the ends and several were a pink/purplish color. Any ideas on how to proceed? Where should I investigate? Is there a voltage check on the inverter output? A way to test the lamps? The inverter circuits won't switch on unless there is a load, correct? Nor will they switch on if the current/voltage draw is too high, correct? And this is why it's shutting down the backlight, correct?

Anyone have any ideas?

 

[ben7]

Anyone have any ideas?

So the lamps have a purple-ish color on the ends, maybe they need to be replaced?
I think they might be the problem.

 

[Scoot_eruk]

Hi Guys, just came across this thread while searching for my problem. I have a sharp aquos lc32bt8ea (UK model). It powers off with one blink of the power LED. I cleared the lamp errors last night from the factory menu and then it started working but only for about an hour and then shut itself down again. Could someone please shed some light on what could be wrong. Also does anyone here have a repair manual for this particular model or a similar Sharp 32 inch model.
Many thanks and Regards
Ann

 

[Nigel Goodwin]

As I said previously in this thread, it's most likely one or more faulty CCFL tubes - replacing them all costs about the same as a new panel in a non-Sharp TV - not worth doing.

 

[Scoot_eruk]

Thank you for your reply. My question really is if I could still use the TV by bypassing the faulty CCFL tube. If so, then would simply removing thew faulty tube will wotk or is there a detection circuitry which needs to be fooled in thinking that the tube is OK etc

 

[Nigel Goodwin]

Thank you for your reply. My question really is if I could still use the TV by bypassing the faulty CCFL tube. If so, then would simply removing thew faulty tube will wotk or is there a detection circuitry which needs to be fooled in thinking that the tube is OK etc

No you can't, and if you run it with the protection disabled then the PCB's inside the LCD screen burn-up - I know because I've had a number of panels on long term test, with the panels in pieces, and the trip disabled - while trying to find which of the tubes might be faulty. I spent a GREAT deal of time trying to change just the faulty one or two tubes (you should always change in pairs, as they are in series pairs), my conclusion was that in most cases the problem was multiple tube failures - presumably all at the end of their life, as they are all the same age.

Of the few where I managed to get them working by only changing two or four tubes, they all bounced back a few months later as the other old tubes failed as well.