Without any meaningful information about how the IC is meant to work, I'm just guessing, it might not tell you much but depending on your mood it might be fun to poke about. End of day you will be looking for a reason as to why it is misbehaving over time and the waveforms will just say it is. Since it is faulty then you need to take a guess as to which component has gone wrong and my pick would be C803 because it is an electrolytic, it is small and it is dangled off the end of a boost convertor. That means it suffers from high ripple currents. If you are boosting 18V to 35V then that's about 50% duty cycle and at full brightness it will see 0A to Twice the average output current at the switching frequency. If you knew what the average output current for four strings of LEDs was you might guess how happy it is but someone has designed in what looks like at least 1A diode with what appears to be an incredibly meaty mosfet. Have a measure with your scope across R812/R813 in the source of Q801 as current sense resistors, 300mR 150mR in parallel to find out what the peak current is. Your scope probes should have come with a springy thing. If you take the end clippy, tit, thing off you will expose a ground ring that you slip the springy thing on so you can make what is known as a short probe measurement. You can do the same across C803 and you will be looking for excess ripple voltage. I can't say what excess will be but if you notice that it is varying, increasing, over time it is something that is going to upset the IC.
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Oh... If you have a close look on the bottom at the solder joints on C803 you might notice that they are different from the other through hole components. Different shade with a hint of flux and the leads have not been bent over onto the board prior to soldering. It looks like it has been replaced in the past... The silicone blob also looks like it used to accommodate a bigger component.
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I replaced all the electrolytic s , C803 (4.7 uf 100V) included , it is only 1 row of around 20 leds which i was a bit surprised about , they run up just one side of the screen , im still not overly confident to connect the scope , or would the springy thing ,short probe measurement mean i dont connect the earth clip ?
I really need to get an isolation transformer
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i have found the same supply board for sale taken from the same monitor with a broken lcd for £16 , i think i will get this , seems a cheap way to go to get the monitor up and running again ..
I really need to get an isolation transformer
... i have found the same supply board for sale taken from the same monitor with a broken lcd for £16 , i think i will get this , seems a cheap way to go to get the monitor up and running again ..
Ah... I'm afraid that without a circuit diagram or proper application note for the control IC it's not really possible to give much more guidance. If you're happy to poke about the bored with your multimeter probes it's not much more of a leap of faith in your steady hands to use the scope probe. Yes if you use the springy thing you can ignore, remove the ground dangle. My arms aren't long enough so the £16 option sounds good.
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