[Top Cap]

Could be the sound stages are powered from a supply derived from the LOPT with its own smoothing components so immune at this time from ripple on the main supply.
The problem you show reminds me of TV I had fun with, hum bars upsetting the video and sync. Spent ages looking for the fault until I discovered that the steel frame that encircled the circuit board had numerous dry joints and was meant to be the main ground bus. Once re-soldered all the way round the set worked great. I had another set, same make and model, with similar fault and only took a few minutes to fix this time.