Tube burn spots, white dots, and green splotches

[Mondomark]

Advance apologies if this might be in the wrong thread as these issues affect consumer & prosumer tube video cameras from the 80s.

During the filming of an experimental doc, a burn spot occurred using a Canon VC-50 Pro. I've created a short video on YouTube (http://youtu.be/QtnYkZpvWJ8) that documents the spot on a saticon tube, plus related flaws on other cameras for which any help in remedying (or minimizing) the issues would be greatly appreciated.

The additional flaws include what appear to be dead pixels on another Canon, a black spot on a Quasar Newvicon camera, and green stains that warps the colour registration on both Quasar and Sylvania (JVC) Newvicon cameras.

Cheers,


- MRH

[Dave A]

Back to my tube days. If we picked up a burn on an I/O we would point it at a white card and manually iris up and try to burn out the spot by basically burning the whole surface.

As tubes do not have pixels per se, the white dots may be defects in the tube. Black spots are probably dirt somewhere perhaps, the tube face if you can get to it.

The blotches are misadjustments in the camera. Sort of like shading adjustments in older broadcast cameras. You need a schematic for this problem.

[old_tv_nut]

Saticons are constantly burning in the image, even of ordinary scene areas that are not overly bright. The three-saticon high def cameras used early in HDTV could not be left staring at a fixed scene without it becoming visible as a ghost image. Fortunately, if pointed at a white card/wall for a while (like 20 minutes), the burned image would gradually disappear. Your ceiling light may have overdone it and become permanent - the only way to find out is to do the white card treatment and see if it disappears.

The tiny sharply defined white spots are likely internal particles from pointing the camera down and will not be fixable.

I agree with Dave A that the black spot is likely dirt that could be cleaned off the tube face.

The green blotches are a problem of single-tube cameras, which depend on sensing the carrier frequency caused by color filter stripes on the tube faceplate. If the beam focus and/or astigmatism varies across the target, it changes the amplitude of the red and blue signals compared to the green. Usually it is just the focus (more than astigimatism), meaning that both red and blue are reduced in the poor focus areas and green predominates. If the astigmatism gets bad, it could preferentially attenuate red more than blue or vice-versa. The Quasar VK747 seems to have good spot focus in a ring half-wayout from the center (where the reds are strong), and poor focus in both the center and edges. When new, all these single tube cameras had some color imbalance near the edges, but they should not be as bad as your example. The camera needs a good adjustment of beam alignment, and also you would need to check if there is a dynamic focus voltage in this design, in which case it may not be correct due to aging of components or misadjustment.

The vertical stretch at the bottom is caused by poor vertical scan linearity. The fault in the camera is a shrinkage of the scan near the bottom, which produces the opposite effect in the image. I'd suspect a leaky capacitor somewhere in the vertical sweep circuit.

[Mondomark]

Hi Gentleman -

Firstly, thanks for taking the time to watch the video and provide helpful comments. I'll give the white card/open iris a try for the Saticon burn spot, and maybe it'll fade. After the incident, I left the camera alone for a week or two, and when initially turned on, it wasn't as prominent, but as shooting continued, it became stronger, if that makes any sense. After not using the camera for 1.5 months, the burn seemed even less severe, but I wonder if the activation of the tube also warms up the coating and 'brings out' the scarring. As for the three white dots, pity it isn't something like a bad capacitor, as I gather that could be fixed.

The green blotches are a little amusing to me because I've only encountered them on single Newvicon tube cameras. I found a second Sylvania VK747 in better shape, and yet it has the same green misalignment; same goes for a JVC GX-N70U which is a much better camera. (Would love to be able to fix this one, as it's a nice camera.)

Is a beam alignment something that must be done by a technician? I've also heard of one incident where an open iris set to a brightly lit green background worked after a few hours, but perhaps that was a fluke, or only fixable with a higher grade tube.

Have to say I'm not impressed with Newvicon tubes as they seem rather cheap in quality. Some base-level consumer cameras with single tube Saticons still work great, and I'm impressed by the resilience of single tube vidicons in older cameras like a Panasonic PK-200 (although the size and weight of these monsters is extraordinary).

Cheers,


- MRH