Crazy idea - Could a rear-projection Oscope be built from TV parts? (Nope)

[WolfmanTech]

So, long story short- I have a rear-projection assembly (3 tubes/lenses/circuit-boards) but one tube is broken, could I build a rear-projection (or projector?) Oscilloscope with just one of the tube units? The service manual lists 31Kv for the flyback supply, how does this compare to what oscilloscopes run? (I have a range of various CRT TV/monitor bits laying about for parts!) I also have (somewhere) a couple small portable CRT sets that I'd like to make into oscilloscopes one way or another, and those might be something to attempt before the (theoretical) rear-projection project. Thoughts, questions, answers or advice?
(service manual is at https://www.manualslib.com/manual/896116/Sony-Kp-53hs10.html#manual )

[Ian.M]

No. Magnetic deflection doesn't support the bandwidth and sweep rates needed for a usable scope.  The scan coils have far too much inductance to permit fast enough changes in the current through them.  At best, you might be able to get enough bandwidth for audio, and somewhere between 50 and 100 sweeps per second.

*ALL* practical analog CROs use electrostatic deflection which must be built into the tube, directly in front of the electron gun.

[TheoB]

Yes. But only if you make a sampling scope. The CRT simply acts as a display for the controller. Probably not what you had in mind

[WolfmanTech]

Alright, thanks! Maybe I'll try making them into projection units like I've seen putting dancing snow-flakes on the sides of houses instead.

[Metatronic_Mods]

I don't understand, why nope? Audio rate seems fine enough. I mean if you're even thinking of converting a rear projection tv into a colored oscope projector, surely it's with entertainment purposes in mind, and surely audio rate would be just as interesting if not more interesting than hf.

Anyways, I'm browsing around for any info that will help me to do the same. Just had a friend donate a rather nice, fairly new rear projection tv that's still working great. I'm hoping it'll prove an easier starting point than they maybe busted? crt rack pulled out of a maybe working? 25 year old tv.

Just out of curiosity, was your interest in this piqued as a result of Hansi Raber's project?

[james_s]

It's possible to make an audio frequency oscilloscope or vector scope with a TV tube, you're not going to get much bandwidth but it will go into the kHz with some effort. You could also make a vector monitor as used in some classic arcade games. In the one I built I rewound the vertical deflection winding to reduce the inductance but projection TV yokes have somewhat lower inductance already so just changing the two vertical windings from series to parallel might do the trick.

[Ian.M]

Yes, it can be done, but CRT horizontal defection circuits are often closely coupled to the EHT supply and are usually heavily optimised for fixed frequency operation at one or a very limited choice of 'spot' frequencies.  Unless you find that rare beast - a complete working multisync monitor with wide range HSync, you are SOL for anything more than a fixed timebase  'art' piece, and if you do have such a monitor, its far more valuable as-is for displaying legacy computer and arcade machine graphics.

[james_s]

Well it's a given that you'd want to make a separate HV supply, that's quite easy to do with a surplus TV flyback transformer though. It's what I did for the monitor in my 5" mini Asteroids game which I built out of a cheap portable CRT TV. Rewound the vertical yoke for reduced inductance, HV came from a random salvaged module I had lying around but I have a working prototype using a 555 to drive the original flyback/LOPT from the TV. Lighting up a CRT is not difficult, the voltages are not super critical.

[Metatronic_Mods]

Yes, it can be done, but CRT horizontal defection circuits are often closely coupled to the EHT supply and are usually heavily optimised for fixed frequency operation at one or a very limited choice of 'spot' frequencies.  Unless you find that rare beast - a complete working multisync monitor with wide range HSync, you are SOL for anything more than a fixed timebase  'art' piece, and if you do have such a monitor, its far more valuable as-is for displaying legacy computer and arcade machine graphics.

What is the EHT supply? I had noticed apparent coupling on the horizontal coils; it's hard to say for sure because of the plastic housing around the winding terminations, but I think I had it disconnected and was still seeing some fairly high voltages across it.

As far as bandwidth/display quality, I've gotten some barely passable shapes using it like a vector monitor. There's definitely a good bit of rounding off of any sharp edges. But I've only been using a voltage amplifier to amplify a vector image signal from my computer's sound card to drive the coils, and I've read, and it makes sense intuitively, that a current source is needed instead.

I'll look more into the asteroids circuit and driver circuits for vector monitors. But do you have any advice, am I on the right track with the current amplifier?

[james_s]

EHT is the high voltage that feeds the accelerating anode of the CRT. On TV sets and some monitors the HV transformer is part of a resonant tank circuit along with the horizontal deflection coils. It's a very elegant design because it results in fairly high currents in the deflection coils without a lot of dissipation because the energy is sort of bouncing back and forth, passing through the coils as it goes.

A vector monitor needs a current amplifier for the deflection. If you download the manual for the Electrohome G05 for example you can see how this works. Since beam position is proportional to current through the coil, the deflection amp translates the voltage input into current through the coil. The way this works is it has a differential amplifier with the input signal going to the positive input. Instead of sampling the voltage output, there is a current sense resistor connected to the deflection coil and the voltage on the top of this is proportional to current through the yoke and this voltage is the negative feedback for the amplifier. A differential amplifier with negative feedback tries to keep both inputs at the same potential, so the current through the yoke will track the voltage signal applied to the positive input.

[stevelup]

Alright, thanks! Maybe I'll try making them into projection units like I've seen putting dancing snow-flakes on the sides of houses instead.

I think you're massively overestimating the potential luminous output from one of these tubes!

[CatalinaWOW]

Another application that should be possible and maybe of interest would be as an automotive diagnostic scope.  I can just see a big screen display in one of those reality TV shows doing restorations on old cars.

[rstofer]

In years past, we sent video output from computers and video games to a TV set.  This often required RF modulation to get the signal into a frequency range the TV would accept.  In the end, a TV set displays a certain number of vertical lines (usually interlaced) and has a certain time period where each line is active.  Within limits, anything you can stuff in that line interval will get displayed.  See timing diagrams elsewhere (Google).

So, build a digital oscilloscope that has enough memory to hold the entire screen image (in bits, fortunately) and create a timing generator to create all the required TV signals (blanking, etc) with the memory shifting out data for each line.

There are lots of examples of TV based games so it shouldn't be hard to find something that will work.

[Metatronic_Mods]

I figured out last night that the CRT rack I'm working with has a separate pair of focusing coils for each CRT. These cause the beam to form a sort of small lissajous pattern with no input (wide blurry traces with input) to the deflection coils. I'm guessing this helps compensate for the fact there's 3 tubes with different offsets and angles to the screen.

If I disconnect the focus coils from their drivers, I get a singular point (sharp lines with deflection input), but I can also smell something (probably the driver) overheating. I'm guessing the driver must be acting as a current source for the focus coils, so the proper way to disable it would be to short the outputs rather than leave the circuit open? But so far I haven't found any information about the focus coils to confirm or refute that idea.

I never bothered to worry about the focus coil (if they even had one) for the other TV's I've hacked to bits. I guess because those were single tube TV's and didn't have/need any additional offset compensation from the focus coils.