I have an esoteric question I probably should know the answer to

[zenith2134]

As mentioned in another thread of mine, I am curious as to the behavior of the anode's h.v. during blanking state. I seem to recall reading how the high voltage raises with a blank raster but cannot verify this barring a quick check of one of my tvs. I realize that beam current drops to zero in this state...but what happens to hv.
It sure seems like there used to be more technical articles on the net when I first joined A.K./V.K. but that they're tougher to track down nowadays...
Should hit up a library to brush up..

[bandersen]

Interesting question. I would imagine it does rise, but the HV filter cap + dag coating cap will still load it down somewhat.

[zenith2134]

sorta like a tuned system I guess, once it reaches equilibrium w/ the aquadag capacitance. goodpoint.
these kind of queries are the hardest to factually determine imo.

[old_tv_nut]

Originally Posted by zenith2134

As mentioned in another thread of mine, I am curious as to the behavior of the anode's h.v. during blanking state. I seem to recall reading how the high voltage raises with a blank raster but cannot verify this barring a quick check of one of my tvs. I realize that beam current drops to zero in this state...but what happens to hv.
It sure seems like there used to be more technical articles on the net when I first joined A.K./V.K. but that they're tougher to track down nowadays...
Should hit up a library to brush up..

You mean a black raster, right? It doesn't change during the brief horizontal blanking due to the filtering of the dag capacitance. The question of how much it changes from black (or dark) scene to all white scene depends: on the older style shunt regulator sets, the HV is essentially constant, because the regulator works by making the total current constant. In later sets, where regulation comes from adjusting the drive, there can be some variation, which normally would cause raster size to breathe, but is compensated by the variable loading of the B+, which affects the scanning current. (All the above about later sets is IIRC!)

[zenith2134]

Well, at the time I was trying to determine whether or not the ultor volts in a '76 color solid state tv were going to vary excessively at zero beam current or full white screen. basically, can they be increased with a bank of oil filled capacitors. That way, one can bend the horiz and hv circuit in such a way that the x-ray protect can be defeated. If one was to attempt high-contrast imagery onto films.

[old_tv_nut]

By the way, a correction to what I posted, from another thread :

the older type sets with a regulator tube measure the boost voltage as a substitute for the actual high voltage, rather than measuring the beam current. The result is the same - the HV stays constant when beam current varies.

As you know, the '76 vintage solid state set works entirely differently. Any variation in HV would be: higher voltage on dark or black scene. I don't remember what the overvoltage circuit is actually measuring in these sets.

These sets often also have an beam current limiting circuit. again, I can't remember what is actually measured. The control effect would be made by adjusting brightness or contrast. In discrete transistor sets this was usually done by turning down the "brightness" (black level) to prevent excessive current. In some later models with analog ICs, the "picture" level (what used to be called "contrast") could be controlled to limit beam current. The beam limit is what would limit the picture brightness for photography.

I don't think there is a real need to boost sets of this vintage for film photography. You need an exposure time no faster than 1/30 in order to capture a full frame, and with modern film speed of 400 or so, you can get there with a reasonable lens aperture. Even film speed of 200 is probably quite workable.

For best image quality, you do want more HV, but it's risky to push a set. The other way to improve the spot size is to keep the beam current low by turning down the contrast and brightness.

One more comment: using film for this seems like something you would do just for the reason of wanting to play with film. Digital cameras can give better results these days, if your purpose is simply to document the picture on the set.

[zenith2134]

Thank you for that great information. I will look into quality digital cameras, and not rule them out now.