TV Backlight Turns On For a Moment Only
This is my first ever TV repair, I usually repair smaller electronics, so be gentle on a tv-rookie please :) I’m good with data circuits and DC power circuits, but I never had real experience with power supplies. Seen some vids, but never had one on hands.
TV: Hisense H55MEC3050
Docs I found:
Power supply schematic RSAG7.820.6106 Power Supply
Main board rsag7.820.6392 (haven’t looked for schematic)
I have a multimeter and an oscilloscope. I watched a few vids and read a few guides on how to troubleshoot the stuff, so this is what I’ve done. Once I turn the TV on:
1) Backlight Enable line between power supply and main logic board - always 3.7V (driver IC datasheet says it needs 1.7V for enable or higher and below 0.8V to disable)
2) Backlight dimming signal is 98% Duty cycle 4.7V (according to driver IC datasheet, should work). So pretty much stable 4.7V there. Like this:
3) Measured voltage on the backlight connector WITH connected backlight - on power on the voltage immediately jumps to the correct 170V, after ~400ms of stable flat 170V, it suddenly decays exponentially as if someone just cut the power to it or disabled backlight. I manage to notice backlight flash for a brief moment from the back side of the TV. After it decays, it stays at 1.3-1.5V forever. Looks like this:
4) Measured voltage on backlight connector WITHOUT backlight cable. It overshoots target voltage and then quickly decays too. Like so
5) There is no short circuit in the backlight itself, so it shouldn’t be some safety killswitch. Also, I would have voltage with unplugged backlight cable then, but I don’t (see 4).
6) Before I did most of these measurements, I replaced the power supply board RSAG7.820.6106 ''with another one from ebay (I thought 1.3V on backlight circuit and no short would mean power supply is dead. I was wrong.). Behavior of old and new power supply is 100% identical.
Tbh I have no clue what it is, but there isn’t actually much left. It’s not TCON board since the image is ok and it has nothing to do with backlight, then it could only be the main logic board with all the connectors, right?
Assistance would be much appreciated
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