The PWM led of the laptop backlight is buggy. Where to dig?

Question

Do it yourself

Good afternoon! Ultrabook samsung np900x1b. The PWM of the led backlight is buggy:
- when powered by a battery, the backlight blinks wildly depending on the brightness (random rhythm), something (the choke probably) crackles to the beat of blinking
- when powered from the mains, the backlight and its adjustment work fine. The crackling changes to a squeak, the frequency of which depends on the brightness (probably the same throttle). The squeak is quite loud - it is difficult to work.
The owner says there was a mechanical impact (the child opened / closed the lid many times with force), after that it began.
There is a desire to try at least to find the cause yourself.
But the fact that the adjustment works from the network is confusing. Why so can be and where to dig?

Answer 1

change key and throttle