Check here: How to replace Capacitor and Power Board on LG LED Smart TV 47LN5750 no Picture Repair - YouTube
Changed caps on a number of PC motherboards. One of them the some caps were obviously bad. Got a kit with the common ‘problem’ caps for that MB. Replaced the obviously blown ones. It acted exactly the same. Went back and replaced the rest. > Fixed. Could see absolutely no problem with those caps but they were evidently bad.
Had a TV with similar problems and assumed it was the same. No bad caps found. Turns out it was probably just one of the ribbon plugs had gotten high resistance over time and simply taking them out and plugging back in fixed it.