Samsung P2770hd strange backlight issue (stumped)
Good day folks. I've got this Samsung p2770hd with a strange backlight problem. The chap brought it in saying that it would only stay on for a few seconds then go dark again. I immediately knew what was going on and went straight to the backlight portion. When faced with faulty backlights, the first thing I usually do is bypass the protection of the driver IC (a SEM2005 in this case) by shorting the appropriate pin to ground (in this case pin 1). This is where the confusion began: normally, any display I've worked on would turn on and stay on without any complaints after bypassing its protection (even if something started getting hotter than it should), but this one no longer came on AT ALL after pulling pin 1 to ground (not even briefly like it did without the strap to ground). I read somewhere that I should try using a diode instead of a wire to make my jumper to ground - funnily enough it worked ! The monitor came on and stayed on ! Call me a dumbass, but I can't figure out why the diode managed to bypass the protection, while a small value resistor (2.2ohm) and even a dead short didn't !
The main issue however is this: after letting the display run for a few seconds, I noticed the HV transformer got way hotter on one side than on the other and also the display was dimmer at the top, so there was definitely something wrong going on. To rule out the possibility of a bad lamp, I tried another display and same thing happened, which confirmed (so I thought) it's the HV transformer to blame, and sure enough by measuring its secondary side, one set of contacts read 1. something kOhms, while the other read 4 kOhms (the latter being the defective winding, by comparison with a "good" transformer). Now it gets better: it just so happened that ANOTHER one of these exact same TVs had just come in with a faulty power connector, but otherwise fully functional, so I decided to pull the transformer off of that one and replace it on my faulting board - HUGE MISTAKE ! I did so and guess what: it still doesn't work ! When I plug the supply in (with no main board and ON pin connected to 5v via small resistor), the area around the transformer makes a slight pop, the backlight glows VERY briefly and then the whole thing shuts down...I thought "ok, maybe there's something else wrong on this board, let's revert to the original setup so I can try the whole supply on this display", so I moved the transformer back onto the "donnor" board where it came from and it got "even better": now the original TV would no longer turn on either ! At first I thought it was a friggin' joke: I now have two busted TVs to worry about, which is all the more annoying considering that the first one worked perfectly fine ! What the heck happened here ? Are those transformers so low quality that they can't withstand 2 rounds of soldering ? The resistance of the two secondary windings is identical and hasn't changed all along (1.x kOhms) ! :| I can't possibly blame it on anything else other than the transformer, since it was the only thing I removed ! Measuring the resistors and blue ballast capacitors on the HV side with my multimeter shows virtually identical readings on the ohm scale (I know that's not the proper way of testing capacitors and other such components, but please bare with me as I don't have very fancy gadgets on hand yet). I bought a new transformer which I shall try tomorrow, but it's not the exact same thing and doesn't even fit in the same holes properly, but the guy told me it's a direct replacement and it SHOULD work (which is kinda backed up by the fact that its windings indeed have that 1.x kOhms like the original). What do you guys think ? It's a mystery that I cannot solve ! Same board, same transformer, same panel (even tried 3 of those), but it still doesn't work !
The main issue however is this: after letting the display run for a few seconds, I noticed the HV transformer got way hotter on one side than on the other and also the display was dimmer at the top, so there was definitely something wrong going on. To rule out the possibility of a bad lamp, I tried another display and same thing happened, which confirmed (so I thought) it's the HV transformer to blame, and sure enough by measuring its secondary side, one set of contacts read 1. something kOhms, while the other read 4 kOhms (the latter being the defective winding, by comparison with a "good" transformer). Now it gets better: it just so happened that ANOTHER one of these exact same TVs had just come in with a faulty power connector, but otherwise fully functional, so I decided to pull the transformer off of that one and replace it on my faulting board - HUGE MISTAKE ! I did so and guess what: it still doesn't work ! When I plug the supply in (with no main board and ON pin connected to 5v via small resistor), the area around the transformer makes a slight pop, the backlight glows VERY briefly and then the whole thing shuts down...I thought "ok, maybe there's something else wrong on this board, let's revert to the original setup so I can try the whole supply on this display", so I moved the transformer back onto the "donnor" board where it came from and it got "even better": now the original TV would no longer turn on either ! At first I thought it was a friggin' joke: I now have two busted TVs to worry about, which is all the more annoying considering that the first one worked perfectly fine ! What the heck happened here ? Are those transformers so low quality that they can't withstand 2 rounds of soldering ? The resistance of the two secondary windings is identical and hasn't changed all along (1.x kOhms) ! :| I can't possibly blame it on anything else other than the transformer, since it was the only thing I removed ! Measuring the resistors and blue ballast capacitors on the HV side with my multimeter shows virtually identical readings on the ohm scale (I know that's not the proper way of testing capacitors and other such components, but please bare with me as I don't have very fancy gadgets on hand yet). I bought a new transformer which I shall try tomorrow, but it's not the exact same thing and doesn't even fit in the same holes properly, but the guy told me it's a direct replacement and it SHOULD work (which is kinda backed up by the fact that its windings indeed have that 1.x kOhms like the original). What do you guys think ? It's a mystery that I cannot solve ! Same board, same transformer, same panel (even tried 3 of those), but it still doesn't work !
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