1 LG 47LD450-UA -- Power Board or Mainboard?
To those awesome enough to give my “heavily budgeted” self an honest opinion and/or educated guess.
Picked up this LG TV for $20 on local buy/sell/trade. Guy was nice enough and honest about the condition. It worked great for years, then about a week before I bought it, it doesn't turn on — pressing the power button starts the status LED to fade dim to bright forever.
A preface to the following: I worked managing an SMT production line for small r/c and robotics for nearly a decade. I can get by if it has a circuit board, but this also means I might be making incorrect assumptions.
I did a thorough inspection of the power supply, checking electrolytics for the obvious signs of poopiness. Nothing apparent.
The voltages on the PSU terminal to the mainboard appear to be normal when off, 3.5V (3.481V) on the 4 conductors from PSU to MB, all else seem to float low.
Upon hitting the power button, all 12V are 12V and all 20V are ~22V (model dependant I've read). The power on pin, blue wire/pin 1, is the same 2.481V in this state. A-dim reads 2.967V and P-dim reads 85mV. Error reads 92.2mV.
Poking around the PSU with my trusty Klein meter, I am seeing a lot of 385V and some intermediary values like 80V and thereabout. What i don't see is what I believe to be the backlight stage outputting any voltage at all. There are two vertical voltage regulators screed to an aluminum ground/heatsink that start at 5.4V or so and wind down to ~4.5V then jump up again over about 10 seconds. There is another next to that at double that voltage. It appears the voltage is being transformed up properly, but not able to be piped to the backlight. Maybe…
Whatever the case, I have a choice to make. If it's the mainboard, this is going to the recycler. If it's the power supply, it's worth throwing $40-$45 at for a 120hz panel, but not $140 for the mainboard.
What do you guys think?
Esta é uma boa pergunta?
893 
2,3 mil 
@thenrz check the voltage on the power/on pin when you press the power button. Let us know what you get. Also, post some good pictures of your boards with your QUESTION. That way we can see what you see.
Adicionar imagens a uma pergunta já existente
por oldturkey03
Understood on the photos, I can fulfill that request later this evening. Just close-ups of the PSU and mainboard? Perhaps containing the part #s in frame?
As for the voltage check, you are requesting the value during the kinetic phase during power on? As in {steady state off | >> button press and measure << | steady state on}? I believe the Klein meter I have here at home to be able to react quickly enough with autoranging disabled.
I had considered getting the oscilloscope out, but I was hesitant to take it that far considering I have no reference by which to determine whether the waveform I am seeing is proper. There are so many models, and this one ancient by comparison at a decade old, I found quite literally zero data referencing that kind of detail.
Thanks for the reply.
por Alex LeFevre