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#1
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RCA Victor 21-T-8205 White line in middle of picture
Hello all, thank you for the help providing awhile back on this set! It has had a full recap and is working like a champ. I've been able to run it for 4-5 hours continuously without any issues other than the occasionally user control adjustment. The one thing that is persistent before and after the re-cap is the white line in the middle of the picture. It is more noticeable in images with lots of blacks or darker colors. It moves with the horizontal hold adjustment as well. Is this a fixable issue, or just something to get used to given the age of the set? I'm not certain what it is called specifically, so googling hasn't given much insight. Thanks again!
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#2
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Did you adjust the horizontal drive according to the service manual?
http://www.earlytelevision.org/pdf/r...202_manual.pdf jr |
#3
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I believe I did, but I will go back and re-do the procedure. Do you know what is meant by the term "motorboating" in the service manual?
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#4
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Quote:
https://en.wikipedia.org/wiki/Motorboating_(electronics) |
#5
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The line on screen looks like what is called a horizontal drive bar.
__________________
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 |
Audiokarma |
#6
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I had a Motorola with a drive bar similar to that; with careful adjustment of horiz drive & horiz hold & could make it better but never made it completely go away. In my case I suspect there was some capacitor in the circuit that was a little off.
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Bryan |
#7
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It does not appear to be a horizontal drive line. The line is most likely due to insufficient horizontal retrace blanking. During horizontal retrace there may be a positive overshoot of the rear of the sync pulse.
If it was a drive line, horizontal scan linearity would be affected with evidence of picture foldover across the line. I see no evidence of this in your photos. To confirm, adjust the horizontal sync phase by adjusting the horizontal hold control. If the line moves, it is due to a blanking problem. If it stays fixed, it is a drive line. You should be able to minimize it by adjusting the fine tuning. But there are means to improve retrace blanking if that is the problem which I believe it is. Last edited by Penthode; 06-10-2018 at 11:13 PM. |
#8
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Penthode's 100% correct.
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#9
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Quote:
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#10
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There are no adjustments necessary unless you check the RF/IF alignment. The horizontal drive adjustment, if the set has one, will make no difference.
What may be useful to post the schematic of the horizontal sweep/ video amplifier section to see if there is a blanking circuit present. It may also be due to RF radiation of the horizontal retrace spike being picked up by the tuner. It may help selecting another channel to feed the set. It also may help to move or shield the wires leading to the deflection yoke. Other things to consider are consumer box sync generators which exhibit too fast a fall and rise time. You may wish to try another set top box. Also alignment of the IF where the video carrier is to low on the Nyquist slope or a faulty IF tube (check by substitution one-by-one ensuring the original is replaced in its original socket). |
Audiokarma |
#11
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Very interesting, certainly explains those stubborn "drive bars" that cannot be adjusted out by the usual horizontal drive adjustment. I posted a link to the schematic in post 2 of this thread... do you see a horizontal blanking circuit?
jr |
#12
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As it doesn't change with tuning, it's some baseband pulse in the video/CRT.
Also, you said the H drive adjustment doesn't fix it. There are probably dozens of small ways this can occur. I see this set uses PC boards. It may be worth checking all grounds on the IF, Video, Horizontal (and who knows where else). The horizontal output current and voltage is hundreds of times larger than the video signal, so even a small amount of it can be visible. Also as mentioned, try lead dress. Take any of the flying leads and move them around to see if it makes a difference. Make sure the tuner/IF cable is solidly grounded. When this kind of thing showed up in a chassis design, the engineers would try random extra grounds and variations in lead dress until they found the fix. Any of those fixes could be disturbed over time by repairwork or grounds going bad. |
#13
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As Wayne suggests, a bad circuit board common ground connection to the chassis could well be it. As it is a baseband video problem it would be well to check the lead dress and the soldered grounds especially around the video amplifier. And wouldn't hurt to examine the decoupling-bypass capacitors around the video amplifier e.g. screen bypass and even the 100uF cap on the +260v line.
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#14
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I don't think the added grid pulse is a treatment for a drive bar, which, if I understand correctly, occurs during trace, not retrace. The grid blanking pulse acts during retrace.
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#15
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Does it move more than the picture and in the opposite direction of the picture, when you turn the horizontal hold? If it does, that indicates it's happening during horizontal retrace, so not a drive line.
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Audiokarma |
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