[Penthode]

I have had 5 decades aligning these early sets with split sound like yours. The distortion cannot be just the ratio detector because the sound IF in a split sound set changes with the fine tuning. Remember it isn't an intercarrier sound set where the sound carrier beats with the video carrier to maintain sound over the video tuning range. In a split sound set you adjust for best sound and the picture should and must be set at that point optimally. The sound tunes like an FM radio and when tuned you accept the video as is. This is why you need the proper equipment to do the alignment.

You need a sweep generator and accurate marker oscillator to align the video and audio IF. There is lots of literature on this. Follow the manufacturers notes.

I have aligned split sound Westinghouse, Dumont and RCA sets. (I currently have 4 pre-1950 RCA split sound sets). They all work superbly with full 4MHz video bandwidth and excellent sound. The alignment process is less forgiving but the picture quality is second to none when done right. And I prefer them as you do not get that intercarrier buzz!

Remember that the video is transmitted vestigial sideband and so the video carrier must be at 50% down on the nyquist slope. The picture when tuned correctly will be sharpest not brightest.

Seems to me you are almost there and hope to see you cross the finish line.