Small CRT Questions

Hey folks. I came across a portable 5" B&W TV which has some minor tuning problems, so I thought this could make a good platform to finally experiment with controlling a CRT on my own. It might be fun to try creating vector graphics and such from a microcontroller, or rigging up a simple oscilloscope since I don't own one yet.

My questions are mostly regarding voltages. What kinds of voltages should I generally expect to drive deflection coils/plates? And would those be in DC or AC? It seems like DC would do it considering it's basically an electromagnet driving them as far as I know, but I don't know enough about it to be making any premature assumptions. I would also assume that the voltage needed would depend on the tube size, so a larger TV would require more perhaps?

I would also need to be able to control the intensity of the electron beam. Or at least an on/off. So, again, I'm curious what type of voltage one should expect to apply, and AC or DC?

The TV in question just runs off of 12v. So I'm also wondering how much voltage it might potentially generate at the flyback for this small kind of tube? I'm afraid to put my dinky multimeter to the task in case it goes beyond the 750v setting (not that I'd feel comfortable even putting that much to it). This is more just out of curiosity, since I'm sure it's still plenty enough to give me a good shock. I'm still not sure what connection is what yet either until I do more investigating.

I'm assuming that neither the deflection nor the intensity requires very much voltage. But until I know more of what to expect, I'm afraid to go sticking a meter or anything on it just yet. Any info or pointers to a resource with information on such things would be much appreciated!

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Best start, trace the circuit board around the flyback transformer. Get = an idea for its pinout and what voltages it made. You can drive it = basically the same way to get the same voltages, then add your own = signals (video and deflection).

Video is typically applied to the cathode, in the 0-100V range, grid = grounded. Focus will be in the 800V range, and 2nd anode, maybe 8kV for = that size tube. You'll want at least 4kV. Current is under 1mA, so the = cathode is quite easy to drive, though you'll want a fair bit of bias = current if you want high bandwidth (NTSC is only a few MHz, not a big = deal).

The deflection coils will probably be low voltage, so you can drive = vertical from a +/-15V constant current amp. I tried this myself, and = discovered the horizontal coil is actually quite awful, much worse to = drive than vertical. I think it's primarily intended for the circuit = it's placed in, and doesn't really do good elsewhere. It's not a simple = RLC component.

Incidentially, don't let the fine wire and ferrite core fool you: the = vertical deflection coil isn't high inductance. Take a close look, the = windings are wired out of phase, hence producing a "total" fringing = field. The ferrite core acts like two pole pieces, with zero magnetic = field in the middle between the windings.

I once made a simple setup, using a comparator for sync seperation, RC + = comp for vsync seperation, and a pair of one-shot ramp generators for = sweep. Boring linear amps with current feedback were driving vertical = deflection, but as you can see, the slew rate was a bit weak at +/-15V = supply.


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HSweep waveform:
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notice clipping during retrace (coincident with the bright edge on the = picture), and the slow RLC slope in the ramp region.


--=20 Deep Friar: a very philosophical monk. Website:

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Tim Williams

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