the tv entered the workshop with the QM801 mosget 09N60 transistor blown and the input fuse open. I made the replacement of these components in addition to the integrated smd S3310 of the source. When I turned on the TV I noticed that the 5v voltage at the source output is pulsating but the other voltages are apparently normal. When I disconnect the CNM803 cable that goes to the main board, the LEDs on the bar light up and flash in the same way that the 5v voltage fluctuates.

BN44-0604A power source

the other tensions are also fluctuating. I can't find voltage on the DM851C diode

Try to disconnect the main and the backlight LEDs from the main power supply, and turn on, if the voltage is normal, then the backlight is faulty.
Try to check and replace CM874, C9102, CM871, CM870, ICM803, PS801S, ZDM803, CM806, RM811, RM801, RM802.

СКАЧАТЬ BN44-00605A+PSLF770S05A+ : BN44-00605A+PSLF770S05A+.pdf

today I decided to replace the photocoupler 817 and the regulator TL431 connected to the photocoupler. I turned on the tv, now the voltage on the DM855c diode appeared but instead of having 5v it has 6.8 even with the board connected to the main board

Overestimated voltage can occur due to optocouplers 817 or tl431. Replace them with others.

the friend is absolutely right, the problem was in the 817B photo coupler and the TL431 regulator

TV repaired and working

I was very happy after I managed to repair the source plate, I had even closed the TV and showed a video of the TV working for the owner. When he arrived at the workshop, when I turned on the TV, I noticed that the channels changed by themselves, the volume indication also happened the same thing. I noticed that the integrated processor of the main board was a little hot, I ended up discouraged by the situation of the TV.

Try turning off the local TV keyboard and see if the channels are switched.
If the channels are switched, then look for a malfunction in the mainboard.

I even turned off the rear button on the TV, the one that switches channels and changes volume without success (still changing channels alone)

Could it be the problem SENK13 1337-CB PRM94200 (IC901) chip?

I'm hoping he isn't the problem.

The strange thing is that the TV was working perfectly until I took a photo and video to send to the owner of the TV.