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#16
03-31-2022, 08:14 PM
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I have no idea if that's deliberate or accidental, but it seems non-critical because it's the last two video lines at the bottom of the picture, hidden by overscan even more than the switching transient is.
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#17
03-31-2022, 08:18 PM
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Comparing to your other plots, it looks like one has clean black video and all the others have some bursts of noise in those lines. Doesn't make a lot of sense since the preceding video lines are full amplitude. Maybe it's an artifact in the original source, don't know.
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#18
03-31-2022, 08:24 PM
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Looking again, it seems those two lines have the video blanked during recording, maybe deliberate in VHS machines for some reason? The noise bursts are possibly because the tape is not making the best possible contact with the heads near that edge. Since this is likely due to the particular tape and not the machine (note you have one clean tape), I would not try to chase it.
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#19
03-31-2022, 08:28 PM
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A professional analog video monitor would have a pulse-cross display to show what's going on in/near the blanking intervals.
https://en.wikipedia.org/wiki/Pulse-cross
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#20
04-02-2022, 09:35 AM
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Thanks for the additional information; I agree with your assessment so will not be touching the switching point or anything related to it at this point.
I tried some graphite lube for key locks last night on the ground strap contactor and it didn't make a lick of difference. The white streaks were still present at the same location on the screen. I see the grounding contactor has a divot worn in it and it's not rotating on center, but if I pick the strap completely off the head while it's spinning it doesn't make any difference either. What is intriguing me about this defect is the white streaks appear in just about the same exact horizontal line no matter what tape I play. All other references I've seen for white streaks in playback exhibited randomness in their appearance. Admittedly I'm not sure where to go here, but I think I either need to check every capacitor on the board and see if one is bad, or take the upper drum off and see if anything jumps out at me.
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#21
04-04-2022, 07:22 PM
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There are two magnets mounted on the flywheel of the lower drum. One is a north-pole magnet and the other is a south-pole. As the drum is spinning, those two magnets pass over the Pulse Generator sensor and thus the PG signal is formed which is that square-wave signal you see on your oscilloscope. The PG signal is used to tell the servo circuit which head is in contact with the tape. If the relation/alignment of the magnets vs the position of the video heads on the upper drum is messed up, the switching point will occur at the wrong timing, causing the symptom seen here.
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#22
04-04-2022, 11:56 PM
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Quote:
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#23
04-07-2022, 07:29 PM
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OK sounds good, I will work on getting those this weekend. I've been coming home late due to work, but can set it up again to scope it.
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#24
04-10-2022, 07:29 PM
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Alrighty, as promised please see that attached images. I took one of two fields of video, and then one of about four to give a larger picture. This was taken while playing "Apollo 13", a commercial movie.
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#25
04-10-2022, 08:07 PM
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Just to get terminology staight, you mean 2 frames or 4 frames of video, not fields.
I see nothing in the 2-frame capture, but see some tall narrow spike in the 2nd, 4th, and 6th field of the video in the 4-frame capture. I see nothing in the switching waveform of either. Do you agree?
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#26
04-10-2022, 08:10 PM
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The odd thing is that the timing seems so precise. Is that true when viewing the picture, that it is always the same exact distance from the top of the frame? This does not seem likely to be caused by an intermittent rotary contact, but I have no idea what in a VCR circuit could behave like that.
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#27
04-10-2022, 08:18 PM
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I didn't think of something: is your scope capturing the two traces alternately or chopping between the two inputs? If alternate traces, you wouldn't see the same glitch on both traces. Even if chopping, the glitch might show up in one and not the other. Try looking at the switching waveform a few more times just to make sure it's absolutely clean.
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#28
04-10-2022, 09:21 PM
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Hey thanks for the additional input. I agree with your terminology, you're correct since I am still tying what I see on scopes to NTSC video terminology. Did I capture what you were asking for correctly? I do see the spikes in video you mention.
To answer your other questions, yes the streaks/tearing are always in the exact same line down from the top of the screen. I agree that an intermittent contact is unlikely but I too have no idea what else would fail at one line only. The rest of the picture is perfect: no noise, clear image, good color, and even HiFi tracks every tape with aplomb. At least it sounds like I'm not the only one confuzzled.  I am using a Rigol DS1054Z, a digital scope, so I don't believe it would alternate between the inputs like an analog scope. I can take a closer look at the waveforms again to be sure they are clean. I will try to see what the other test points may show me, there are a few near the head output board and a bunch on the video board. Last edited by technoman9; 04-10-2022 at 09:27 PM.
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#29
04-10-2022, 09:50 PM
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Yes, these are good captures.
Still confuzzled! You could try looking at power supplies, I guess.
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#30
04-12-2022, 07:39 AM
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Quote:
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Linear Mode
