Switching sources on a CRT's Coils? - Page 1
Forum section: electronics / beginners

author: Tumerboy
author_rank: Newbie
timestamp: November 22, 2022, 05:17:39 am
content: Hey all, I'm an old fart, but still pretty newb regarding electronics. I've got an old 5" portable B&W TV, and I would like to do the Oscilloscope/Vectorscope thing with it. I've seen lots of tutorials, and it seems simple enough to snip the wires and hook em up to an audio source (or whatever).What I'm curious about is if I could more permanently hook each coil's wires up to an SPDT switch that flips between the original connection, or the exterior (audio) source, such that when the switch was in one position it acted as an oscilloscope, and when flipped the other way, it would restore the original coil connection so the TV could still display an image as it has previously.To my noob brain, this seems like, "yeah, that should work."So tell me why I'm wrong, or why it's a bad idea?If it would work, is it the kind of thing where the TV needs to be shut off when the switches are flipped? or would doing that "live" not be terrible?Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 22, 2022, 05:51:59 am
content: The first two priorities are deflection (huh?..) and blanking.Horizontal deflection integrates the yoke coil into the flyback (HV converter) system. So you need to sub in an equivalent inductance, probably with resistance, maybe some capacitance as well. The RLC parameters should match, say within 10 or 20%, over the range of harmonics -- say 10kHz to a couple MHz. So, set up the yoke as part of an impedance divider, and measure its response at various frequencies (R divider and measure amplitude + phase, RC divider and measure resonance, a proper RLC meter at various frequencies, etc.).Probably, the TV should be off during switchover, so, add an interlock for that if you can.Vertical is usually driven separately, and may not be used to generate anything else; but, you'd have to trace the schematic / find service docs to know for sure. Potentially it can just be disconnected and no one's the wiser.Needless to say, worst case it just blows up, end of experiment. So, check around carefully before trying anything.Blanking: once deflection goes off, the beam is frozen in the middle. The beam is intense. ~1mA might not seem like much (or maybe it's less for B&W that size, but in that ballpark at least), but it's concentrated in a very tiny point. That, at some odd kilovolts, is a lot of power to deposit in such a tiny spot. It's like a laser beam. It might even be worse than a laser beam (or, some beams anyway -- I suppose there's a chance the e-beam in a TV can focus tighter than a 10um CO2 laser for example). It's no accident e-beams are used for welding and such. So, you need to keep the beam OFF as soon as deflection is gone. And only, and carefully, light it back up once it's moving again -- or only very dimly when stationary. (If it looks painfully intense, it's probably too bright. TVs shine MUCH brighter than o-scope tubes (electrostatic deflection) do -- you'll see.)That's not hard to do, but neither is it trivial. You'll want to inspect the video output path and see where the CRT is driven from. Usually the cathode, from a common-emitter or cascode video amp. The intended program material is video with periodic blanking, so AC coupling and DC restore networks are commonly used. You will require DC coupling. So, design accordingly. Probably tie it off to a BNC jack or something, with enough amp stages for a reasonable input level -- much of the video strip can probably be reused, but will have to be significantly rewired for DC coupling. (As a bonus(?), perhaps the stages can still be fed from the internal composite video signal coming from the IF detector -- maybe with a "background" (DC offset) adjust to account for the DC coupling.)Then you can use current-source amplifiers for precision deflection, but don't expect any favors on vertical; the high inductance and resistance mean it's not good for more than some 100s of Hz, even with lots of voltage to spare. Horizontal will be fine for audio frequencies of course....I suppose there's an outside chance you can find a yoke from a high resolution monitor and put that on; but then again, that's going to be a, what, T5-1/2 or so neck? Even a tiny (color) monitor is going to have a much larger neck... nevermind. Anyway, those yokes sometimes have similar winding designs for the two axes, so can be driven similarly, and to reasonable sweep rates. (Late generation Trinitron monitors supported refresh rates up to, I think 192Hz or so?!)(You could still try stuffing it inside a much-too-large yoke anyway and see what it does, but don't expect any linearity on the image. The excessively large field might also muck up beam physics inside the gun itself, giving poor focus, or diverting beam current to an unintended electrode.)So yeh, it was never a very good project in the first place, and that hasn't changed in the intervening, hm, half a century isn't it...Don't expect miracles from it, or... frankly much of anything. But if a curiosity is what you're after, a curiosity it will certainly do.Tim

author: Terry Bites
author_rank: Super Contributor
timestamp: November 22, 2022, 09:26:49 am
content: I'm an old fart too!