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#16
04-07-2014, 07:44 PM
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OK, here's the effect I'm seeing (that background is a saturated blue in colour).
Also... looking at the text in the first image... is my picture tube under-focused (i.e. focal point behind the phosphor face)? The scanlines seem to expand to a larger "blip" when the brightness is turned up. (clickable for full-size)  And another with a "real" image - visible on the hand/glove  It's a little fuzzy since my camera doesn't like the TV screen and wants to use the flash but I won't let it. I'm pretty sure this isn't audio carrier as I experimented with tuning off-channel and I can see what old_tv_nut is talking about - especially on channels with just speech since there are naturally periods of silence and the effect will come and go with the sound. Looks a decent match for what Bob posted earlier but not as pronounced, so if anything I suspect either IF alignment is a bit off, or the bandwidth is just barely wide enough to let some of that effect bleed through. Last edited by VintagePC; 04-07-2014 at 07:47 PM. 
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#18
04-08-2014, 08:51 AM
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Quote:
Regarding focus: All electron guns make a larger spot at higher current ("bloom") to some extent. The effect is more or less non linear and somewhat different in different tube types depending on the gun design. Color tubes for a long time were run with a cyanish white point in order not to have the red current be way more than the blue and green(maybe 2:1 max - still unbalanced!), because the blooming in the red gun would produce red edges around small bright white highlights. Blooming gets worse as the cathodes age and the settings are cranked up to pull current from a larger area of the virtual cathode (the electron cloud in front of the cathode). TV receiver guns are carefully designed to eliminate a worse effect - a large circular halo around the spot, which reduces detail contrast. You can see such halos in some oscilloscope CRTs when the beam current is turned way up. The compromise may be that you may see more of the TV CRT central spot blooming if it is driven hard, whereas the oscilloscope spot stays sharper but gets foggier. By the way, too sharp a spot just makes the scan lines more objectionable - there is an optimum range. Tektronix once had a project to build a professional NTSC monitor with essentially high definition guns and phosphor dot pattern, and they didn't go to production because the video professionals told them it was impossible to judge video quality with those fully defined scanning lines flickering at them.
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#19
04-23-2014, 03:44 PM
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I have a case of --something--like this at the moment.
I have an old Bush 405 line set Ive modded to run at the UK's 625 line, and feeding video compo direct into the video-amp stages, via a small gain stage. --I should point out--the set is a 14" Bush TV62 monochrome set and from 1955-7 vintage, when 405 line and VHF was the only show in town. Ive potched about with the video amp to remove an overshoot issue--that has also extended its bandwidth probably to over 4 meg according to test-card images Our chroma sub-carrier is 4.433MHz here.... The effects I get using a Composite Video signal are the herringbone-patterning on parts of the picture of high colour content, but also have an odd vertical strip approx 1/3 picture width that can only be noticed on dark/low contrast scenes, and can be shifted sidewise by use of line-osc setting. It looks exactly like a honeycomb or 'eye pattern' a CD player laser if scoped shows... Switching the compo video from a PAL 4.43 signal to NTSC 3.38 has the effect of changing the pitch/coarseness of the herringbone and the honeycomb Surprisingly, the set is just as happy to run at 525 line as 625 line, although its not so keen on 60Hz frame-rate, and has somewhat reduced height, and hum on picture becomes an issue. 405 line UK VHF transmissions were frame-frequency locked to 50Hz mains freq so maybe are not as tolerant of hum issues as later 625 line sets ... Most odd! Really need to try the set out via an S-Video source of just the Luma signal, see if I can banish that darned honeycomb. The herringbone I can live with... Sure the colour-burst is doing summit odd when it gets in there....
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#20
04-23-2014, 09:41 PM
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#21
04-24-2014, 06:38 AM
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#22
04-26-2014, 11:47 AM
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The Audio carrier--IF I remember correctly for VHF/405 was 3.5MHz, although I may be wrong....
Ive completely disconnected the input to the video stage from the detector/IF strip, but guess there could be some sound carrier leaking in somewhere.... The Burst visible on flyback/retrace sounds very possible though--I really will have to drag the set back out and try with straight luma signal. I keep getting diverted to do these damned new-fangled flat-screen lcd things.....
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#23
04-26-2014, 02:25 PM
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Linear Mode
