Here is one cute little TV and one of mine famiclones ;D
Te picture and sound are fantastic, but unfortunately do not mach at the same channel watch the video :
https://www.youtube.com/watch?v=r4Fs468lTwg&feature=emb_logo
I can get fantastic picture with no sound, or fantastic sound without picture.
What else can you expect from a modulator that has absolutely no shielding and thru-hole components with long legs.
Actually this modulator give very good image and sound at channel 10 and I am surprised by that :D , the TV is out of tune in this case for picture and sound, as you can notice :(
Isn't the problem caused by the gap between video and audio frequency in modulated signal? When you change the value of trim coii, you change the frequency of both. Maybe some of the components in modulator decereased its parameters.
This famiclone have normal behavior on my daily CRT TV set from 90's vie RF, the picture and sound are good and in synchronous.