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#1
07-09-2014, 05:23 PM
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Flagging on top of screen only when used with agile modulator
I am having a strange issue with some equipment. I run three Blonder Tongue agile modulators in my place. They all have both DVD and VHS players used as their input. On one of these channels, however, the VHS tapes play with flagging at the top of the screen. I figured it was a tracking issue, but it wasn't . Next, I swapped out the VCR with two other known-good ones, and the problem persisted.
Now, I am aware that these B-T modulators can develop hum-bars due to poor power supply filtering action; but this issue is a new one on me. On the other rigs, all my VHS play perfectly. Yes I have also tried a number of TV receivers to eliminate that as the source of the problem. Any thoughts ? 
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#2
07-12-2014, 11:39 PM
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If I understand right:
-Any VCR, fed into one specific modulator, shows flagging, on any TV connected to it. -Move any of those VCRs to one or more other modulators, and there is no flagging, on any TV connected to any of them. In that case, I would try adjusting the video level on the modulator as an easy test. You could also put the DVD player on the suspect modulator and see what you get.
__________________
Chris Quote from another forum: "(Antique TV collecting) always seemed to me to be a fringe hobby that only weirdos did."
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#3
07-17-2014, 11:11 AM
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I have determined the problem. Something I overlooked. It is the tapes themselves causing this. It is just more or less prominent depending on which player I use.
Some tapes have it bad and others don't have it at all. Some are used/second-hand and that is likely the reason. What causes this anyway? None of my older tapes have it, it's mostly on the newer ones. And I assume nothing can be done about it?
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#4
07-17-2014, 01:00 PM
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A good time base corrector (TBC for short) can fix VHS flag waving.
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4
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#6
07-17-2014, 04:02 PM
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Good thread here about VCR flagging on early TV sets:
http://www.videokarma.org/showthread.php?t=258501 jr
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#7
07-18-2014, 03:00 PM
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Follow that other thread on flagging. In summary, sets made before VCRs came into use have slow horizontal AFC to reduce the effects of noisy (snowy) signals, but don't respond well to the timing jump at the VCR head switch point. Later sets had the time constants reduced so the flag waving would die out very quickly while the scan was still above the top of the visible raster (but horizontal sync then had more jitter on noisy off-air signals). It is possible to change the time constants in an older set to be more like a later one and get rid of visible flagging.
Whether you get a lot of flagging or a little from a particular tape is totally at the mercy of how tightly it was stretched over the head drum when recorded and how tightly it is stretched when played back. The mistiming jump can vary anywhere from zero to 1/2 a horizontal line.
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#8
07-18-2014, 08:00 PM
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I think I'll simply invest in a timebase corrector. The only tv set I will look into modifying is my 1979 Zenith system 3 25", because I run an old VTR right into it, rather than over the air with an agile modulator. I have also seen professional VHS decks with a skewing control..There's a chance that adjusting the tape tension that way could also improve the flagging issue.
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