VHF Tuner Alignment: where is the overall oscillator adjustment?

[LukeSimon]

Originally Posted by old_coot88

Phase of the burst signal is what the tint control has to regulate, thereby the phase of the 3.58 oscillator. Its output is then referenced to the wideband chroma bandpass signal, to feed the demods.

I got the oscillator to the point where it had its full range of plus or minus 30 degrees. The problem is that the difference between the color burst phase and the chroma signal after bandpass ends up being 90 degrees different. So the 30 degrees is not enough.

The chroma demod acts as a differential amplifier and so it is the difference in phases between the oscillator and the amplified chroma that matters. That difference cannot be more than plus or minud 30 degrees. As the chroma bandpass is adjusted, the chroma signal’s phase is shifted. So having tint follow the bandpass would have made alignment not impact tint since both the burst and the chroma signal’s phases would be shifted together.

[old_coot88]

Originally Posted by LukeSimon

...having tint follow the bandpass would have made alignment not impact tint since both the burst and the chroma signal’s phases would be shifted together.

Only thing that approach is gonna do is change the shape of the bandpass curve, ie, its swept amplitude curve - not its phases. The chroma carrier is not a single, fixed-amplitude sine wave like the reference/burst signal is.

EDIT. Just for the heck of it and an experiment, can you reverse the connections to the primary of A17/L30? That would reverse the phase of the burst signal.

[old_tv_nut]

Originally Posted by old_coot88

Only thing that approach is gonna do is change the shape of the bandpass curve, ie, its swept amplitude curve - not its phases. The chroma carrier is not a single, fixed-amplitude sine wave like the reference/burst signal is.

Incorrect. The chroma bandpass is a tuned circuit whose phase shift is affected by the amplitude response tuning and not necessarily zero at the subcarrier frequency, and thus can affect the tint if the burst takeoff for the oscillator control is before the chroma bandpass. However, if the chroma bandpass response is nominal, the phase as well as amplitude should be close to as-designed. A 90 degree shift is much more than woud be expected from a slight bandpass misalignment. That's why the search should be for something that shifts the relative phase of the signal and oscillator paths, and it is more likely to be in the oscillator path. This could occur in the feedback of CW from the oscillator to the chroma phase detector as well as in the burst takeoff path. That's why I suggested investigating the L-R circuit in the path from oscillator output to phase detector. The problem could also be caused by the oscillator output transformer being mistuned.

[old_coot88]

Thank you for the correction, OTN. So noted.