VHF Tuner Alignment: where is the overall oscillator adjustment?

[LukeSimon]

If I fine tune into a mildly wormy picture, the color smearing is reduced a lot. It is still there, but reduced. Here is the test pattern when fine tuned to be a bit wormy.

I wonder if I can adjust the 4.5mc trap to reduce the wormy picture.

[Yamamaya42]

It DOES say to de-tune FT until a beat pattern is seen (herringbone pattern ) , then adjust for minimum, so it can't hurt to try, I'm more curious about the slight mirroring seen to the right of the characters/ letters seen on the BW screen, mine also has the same to a lesser degree, no idea of the cause however.

[old_tv_nut]

What is the source of the test pattern?
NTSC transmitters had a phase pre-distortion filter meant to compensate the IF phase/delay response of early color sets. The effect was to delay the luminance a bit compared to the chroma. Sets soon began using different IF responses, so there was a mismatch to the standard FCC NTSC predistortion, corrected by picking the correct delay for the luminance delay line in the receiver. BUT, consumer video sources did not/do not have the predistortion filter, so the chroma may appear positioned a bit to the right of the luma. How much of this effect occurs depends on the particular receiver's IF design.

[Electronic M]

Originally Posted by LukeSimon

I tried fine tuning again. Turned color gain off, so picture is black and white. Then rotate fine tuning clockwise until I start to see wormy sound in the picture, then I slowly rotate counterclockwise just a tiny bit until the picture stops being wormy. The black and white picture looks sharp, but when I turn color gain up to a normal level, the color smearing becomes visible. Fine tuning counterclockwise makes the color smear much worse. The tint control has very little range, and color gain almost has to be set to its lowest setting, otherwise the picture is too saturated.

All tubes have been replaced with tested NOS tubes, except the 25XP22 picture tube which is used but tests very strong cutoff, very strong emissions, and no shorts. Also, I did the Sams chroma AFC alignment, which tunes the chroma oscillator, so it locks in the color signal even on a weak signal. I have not performed the chroma bandpass alignment or the 4.5mhz trap alignment.

What should I try next? A chroma bandpass alignment? I have a B&K 415 and an oscilloscope, but I have never used the B&K 415 before.

I think this is either ghosting in the signal path between the generator and antenna input, or an IF alignment / video peaking problem. The picture above on the top numbers you can see that the number has synchronized luma and chroma, but then next to the numbers there's (EDIT lumina and) chroma ghost numbers where it should be dark. This is some kind of ghosting/ringing effect somewhere.

[LukeSimon]

Originally Posted by Yamamaya42

It DOES say to de-tune FT until a beat pattern is seen (herringbone pattern ) , then adjust for minimum, so it can't hurt to try, I'm more curious about the slight mirroring seen to the right of the characters/ letters seen on the BW screen, mine also has the same to a lesser degree, no idea of the cause however.

Remember, I have two Magnavox’s: a 1967 T931 and this 1968 T933. The differences between the two chassis versions are very minor. The T931 doesn’t have this issue, and when I look at how the core of the 4.5mc trap is screwed down into the adjustable transformer in the T931, and how it is poking up outside of the T933, I am starting to wonder if I should fine tune into a very wormy picture and then screw the core into the 4.5mc trap so that the picture isn’t wormy. The color smearing goes away if I fine tune to a very obnoxiously wormy picture.

Smeary T933’s 4.5mc trap core is poking out of the coil.


No smear T931’s core is screwed down into the center of the coil. According to the Photofacts, this part number is identical between both chassis versions. In fact, this part of the circuit (video detector) is identical in both chassis versions.


T933’s color smear goes away if picture is tuned to be wormy.


Sams says that 4.5mc trap should be aligned by fine tuning to a very wormy picture, and then adjusting the core util the wormy interference is minimized. So once that is done, I may be able to fine tune to zero worminess and near zero smear.


Since I was fine tuning CW to a wormy picture and then fine tuning CCW until not wormy, I may simply be not using the proper fine tuning because the misaligned 4.5mc trap makes the worminess excessive too early in the fine tuning CW rotation.

[Phil]

I would check the resistors around the delay line. Perhaps a series resistor at the input (or not) and usually a resistor to ground at the output. If these aren't presenting the delay line with the proper impedance the delay will not be correct. I had that problem in an old RCA.

[old_tv_nut]

Originally Posted by Phil

I would check the resistors around the delay line. Perhaps a series resistor at the input (or not) and usually a resistor to ground at the output. If these aren't presenting the delay line with the proper impedance the delay will not be correct. I had that problem in an old RCA.

Ditto. Coils around the delay line also.

[LukeSimon]

Originally Posted by old_tv_nut

Ditto. Coils around the delay line also.

Wouldn’t that cause smearing in the black and white picture, and/or the color and luma signal not being aligned. I don’t see smearing in the BW picture, and the color signal is not shifted to the right of the BW signal. It seems aligned with luma but is heavily smearing.

I think another clue is that the color burst has smearing, and the color burst doesn’t pass through the chroma bandpass. The smearing of color burst is causing the tint control to have a small range, but after aligning the chroma AFC, the color oscillator is still able to lock instantly and without issue.

These clues seem to suggest it is not delay line and not chroma bandpass. It is RF or IF that is not peaking enough at 3.58mhz.

[Yamamaya42]

I think that they are referring to the shadowing / echo seen on the BW picture.
http://videokarma.org/attachment.php...3&d=1693860184

this should be cleaner , not sure of the cause

[old_tv_nut]

Originally Posted by LukeSimon

...
Sams says that 4.5mc trap should be aligned by fine tuning to a very wormy picture, and then adjusting the core util the wormy interference is minimized. So once that is done, I may be able to fine tune to zero worminess and near zero smear.


Since I was fine tuning CW to a wormy picture and then fine tuning CCW until not wormy, I may simply be not using the proper fine tuning because the misaligned 4.5mc trap makes the worminess excessive too early in the fine tuning CW rotation.

Sorry I didn't see this before.

Yes, with the core so obviously misadjusted, it makes sense to try adjusting it. But if you can;t improve it by adjusting the core, you will need to look for something else.

[LukeSimon]

Adjusting the 4.5mc trap and then fine tuning again didn’t help much. Another symptom that may hint at the issue is the sharpness potentiometer has no effect. This may be unrelated to the smearing, or it may indicate a peaking coil or the tube is not correctly peaking at 3.58mhz. The chroma signal is output from the tube’s plate and luma is output from the tube’s cathode. There appears to be peaking coils on both cathode and plate.

[old_tv_nut]

It should be easy to check the coils for opens and also check that the connection to the sharpness control is not open.

BTW, I'm scratching my head about how that sharpness control is supposed to work. Where does the luminance signal go next (which tube and what electrode of that tube)?

[Yamamaya42]

Originally Posted by old_tv_nut

It should be easy to check the coils for opens and also check that the connection to the sharpness control is not open.

BTW, I'm scratching my head about how that sharpness control is supposed to work. Where does the luminance signal go next (which tube and what electrode of that tube)?

I find it a bit confusing too, but it DOES do something on my set.
http://suzaku.live-evil.org/maggie-video.jpg
however the control WAS very dirty at first, and needed 2 cleanings before it would work on my set.

you could try to short the top (input) point to ground to see it that makes any change.

[old_tv_nut]

You don't have to believe me, but I suspected the next stage was a grounded-grid amp.
With the sharpness turned to minimum, current from V5A cathode is divided between the 39 ohm resistor and the cathode impedance of V5B at all video frequencies, so the circuit is actually lossy, current-wise, while R66 provides voltage gain. When sharpness is set to max, there is 139 ohms impedance at the resonant frequency of C48 and L15 (maybe tuned to about 2 MHz), which causes more of the current to go into the cathode of V5B. The bandwidth of the peak depends on the Q of the circuit, which is determined by the ratio of R4 at max (100 ohms) to the inductance L15.

[Yamamaya42]

I just find it strange that R4 would have no effect at all, unless it was bad/dirty and not working correctly, if there was a fault with L15, which is only about ½ ohm, the cathodes of V5 A/B would lose ground point and drift way above 1v, and video would be way more messed up then he seems to be getting, I would assume.

R4 should be easy enough to check, it's on the back of the set on the top.