Quote:
Originally Posted by VintagePC
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. |
Yes, these are chroma dots - quite normal. As discussed, level depends on IF and video response. What you have may very well be in spec - no realignment needed. In a B&W set with a "sharpness" control, it affects them too.
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.