#16
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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. |
#17
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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.
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#18
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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. |
#19
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Quote:
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#20
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Quote:
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. Last edited by LukeSimon; 09-05-2023 at 03:10 PM. |
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#21
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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.
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#22
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Quote:
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#23
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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. |
#24
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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
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#25
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Quote:
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. |
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#26
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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.
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#27
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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)? |
#28
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Quote:
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.
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#29
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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. |
#30
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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 ˝ 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.
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