Video input mod and vertical poor interlacing

[Alex KL-1]

Hello folks of VideoKarma,
In my experiences adding direct video input to old TV's, I noted that is very common to vertical intelacing resulting poor, resembling a 240p video instead of a 480i. Is more common on simple transistorized TV sets, and is absent on the TV's having "vertical logic" TDA IC's. Over the air I'm never seen this effect. Sometimes occur with my digital TV converter, and is very common when I use a PC with S-Video output (using the pure luminance signal). For supressing this bad interlacing, I need to critically adjust the vertical freq pot on some TV, on verge of desync or when linearity starts to be affected. I'm wondering, what is going there? Someone know about this phenomenon? And possible interventions to mitigate this (without resorting to things like the LM1881)?

If a thead about this subject exists, I'm happy if someone point to me