VHF Tuner Alignment: where is the overall oscillator adjustment?

[LukeSimon]

Aligning the burst amp in the chroma AFC didn’t fix the hue, but the tint control seems to have the ability to shift the phase by about plus or minus 20 degrees now. The problem is that the hue is 90 degrees out of phase, so the range of the tint control is not sufficient to correct a 90 degree phase shift.

Correct Test Pattern Hues:


Notice the 90 degree phase shift:




Here is my current theory. The burst and chroma signal travel through separate paths. The red path is the burst. The blue path is the chroma signal. It passes through A14, A13, and A12. These 3 coils are what are tuned during chroma bandpass alignment. They are causing the 90 degree phase shift. So I will first try to adjust A14 to get the correct hue, and then redo alignment but only changing A13 and A12.

[old_tv_nut]

Don't go ruining your bandpass to center the tint.

Is it correct that L31 connects to the bottom of A17 (doesn't jump over it?).
If so, this is the suspect area. I would first try setting the tint control to center range and tuning A17 to see if that will center the tint.
Also check if C138 (? can't read it clearly) on bottom of A17 is good.

Of course, ther is always the possibilty that the aligment procedure for A15 and A17 isn't right.

[old_tv_nut]

There are some inductors in this circuit that look like ad hoc fixes because the phase didn't come out right in the first design: L35, in series with the color control output; L32, in series with the CW feed to the phase detector.

Is the 4700 ohm from L32 to ground good? Are the supposedly equal-value components in the phase detector all good?

[LukeSimon]

Originally Posted by old_tv_nut

Don't go ruining your bandpass to center the tint.

Is it correct that L31 connects to the bottom of A17 (doesn't jump over it?).
If so, this is the suspect area. I would first try setting the tint control to center range and tuning A17 to see if that will center the tint.
Also check if C138 (? can't read it clearly) on bottom of A17 is good.

Of course, ther is always the possibilty that the aligment procedure for A15 and A17 isn't right.

I tried the entire range of A17 and it doesn’t get the hue to the right phase. The reason I am going back to adjusting the chroma bandpass is that there are many variations of the bandpass slugs that create the right bandpass shape. Before I did the alignment, the bandpass slugs were set to peak max gain at 3.58mhz. The bode plot looked like a very tall and skinny triangle. That had the right hue, but caused the horribly bad smearing. The Pb and Pr bandwidth was far less than the 0.5mhz that it should have been.

If I could observe the phase angle and the bode plot of gain at the same time, I’d be able to align the band pass and land on a phase angle close to that of the color burst amplifier output.

[old_tv_nut]

L32 and the 4700 ohm to ground look suspiciously like a phase shifter. What happens if you short L32?

[LukeSimon]

Top is before alignment. Bottom is after. The hue was 90 degrees out of phase when the chroma bandpass is optimally aligned. De-tuning it slightly allows for the tint control to get the correct hue when the tint capacitor is turned to minimum capacitance.

A better design would be for Magnavox to have chroma tint control after the chroma bandpass, so that alignment of the bandpass doesn’t impact the hue.

[old_coot88]

Originally Posted by LukeSimon

...A better design would be for Magnavox to have chroma tint control after the chroma bandpass, so that alignment of the bandpass doesn’t impact the hue.

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.

[Electronic M]

If it doesn't throw the set off too badly in response you want to have the control be correct at least a few degrees away from it's stop. Not all sources have correct or matching tint settings and if one of your sources needs a few degrees past the stop it will drive you nuts in the future.

[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.

[Yamamaya42]

Sounds so complicated and scary! IF /chroma bandpass alignments, where all NOOBs fear to tread!

[Alex KL-1]

Originally Posted by Yamamaya42

Sounds so complicated and scary! IF /chroma bandpass alignments, where all NOOBs fear to tread!

Well, is a great work! Difficult for sure; patience is a virtue.
And, for some people like me, that worked only with TV's from ceramic filter era ahead (probably some members here are from more recent era also), never touched alignment when at work so don't have the field expertise, will be scary, but a interesting thing to learn. IF the core from these delicate coils not end stuck or destroyed...
So, watching this with curiosity.

[old_tv_nut]

Originally Posted by Alex KL-1

Well, is a great work! Difficult for sure; patience is a virtue.
And, for some people like me, that worked only with TV's from ceramic filter era ahead (probably some members here are from more recent era also), never touched alignment when at work so don't have the field expertise, will be scary, but a interesting thing to learn. IF the core from these delicate coils not end stuck or destroyed...
So, watching this with curiosity.

Yes, the scariest thing to me is breaking a core!
I'd also note that the higher frequencies in the tuner and IF amplifier make lead lengths and physical probe design more critical, but chroma should be relatively insensitive to the connections of the alignment equipment.