On Saturday, February 27, 2016 at 12:59:31 AM UTC-5, Aleksandr Motsjonov wrote:
John-Del, I've looked a bit more. I found labels on back side. One of the inputs from FRC Board is "ON_BACK", and there is also "P_ON" both are zero V when turned off, and when I press power the both get up to 3.2V
"EN" and "ADJ" that goes to power supply board are at GND level at all times.
The backlight voltage shows 25.8 when turned off, and 32.3 when turned on.. I don't have a oscilloscope to see what kinda of shape this signal is.
I still don't understand how does power supply "decides" when to change backlight voltage if "EN" and "ADJ" don't seems to be changing at all ... or maybe I measuring wrong.

First you need to determine if you have a floating ground on the LED strips.. You can do a resistance check from the ground plane of the power supply or metal frame of TV to either side of the LED connector. If one side reads zero ohms, use the frame or ground screw for your LED voltage. If not, locate a large electrolytic near the LED output section. This will have a voltage rating of 100 to 450 volts and the neg side of the cap will make a good ground. There are several ways the LED voltage is varied on these. Sometimes they actually vary the DC output, other times they use a mosfet on the low side of the LED strips and modulate the gate to provide the proper voltage drop across the string. Your reading is odd. It clearly is changing voltage but there doesn't seem to be enough difference between off and on to go from LED cutoff to full bright, but I could be wrong on this.

To answer your other questions takes some more guessing (again, no SM). In standby, the SMPS will put out either 3.3 or 5v typically for the main. The P_ON command from the main turns on the other outputs of the SMPS, such as 9, 12, 24 as required for the rest of the circuitry. In some TVs, this also turns on the PFC circuit (if equipped). When the PFC turns on, you'll see a big bump of voltage on this cap. Other boards will use a command from the inverter drive IC to turn on the PFC (rare). Still others use the successful power up of one of the other secondary supplies to turn on the PFC..

Not all configurations use the adj and enable lines, particularly the real cheap TVS. Is your "EN" line also the "ON_BACK" line? You can try floating the adj line to see if the backlight voltage increases.

Most times if a TV's status LED reports a start when the button is pushed and successfully turns on the power supply, the main is OK (but not always . I've seen some Vizios that will turn on everything but the backlight because the PWM (brightness) is missing.

The secret to all of this is datasheets. If you can find the datasheets for the LED drive IC (and PFC if it's equipped with one) you can go a long way with the proper pin outs, typical voltages, sample circuits, and descriptions of signals the ICs use.