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Quasi-interesting CRT puzzler

Okay, this is kind of a retarded question, but here goes.

Let's say you want to convert you basic PC monitor from raster scan to vector.

You pull off the yoke connector from the main board and find out the it still runs okay, well except that you've burnt a little spot of phosphor in the center of the screen unless you were prescient enough to back off the brightness, contrast, and the three screen controls first.

Then you hook up a DC supply to the V and H yoke coils and find out it takes about 3 AMPS to do a full screen deflection in the vertical direction, about TEN amps in the horizontal direction.

(I think that's because the horizontal deflection has to happen faster, therfore lower inductance in the h coil, therefore fewer gausses per amp).

So we need some pretty hefty drivers, many amps plus many volts if we want fast deflection.

Not a huge problem so far.

But now we realize we don't want to burn the screen if we're drawing short or slow vectors, and we don't want the lines to be invisible if we're drawing them long or quickly.

So we need some sort of Z-axis brightness modulation, somehow proportional to the "writing rate", like in inches per millisecond or thereabouts.

Anybody have any idea how to compute this on the fly as it were?

Ideally I'd like to be able to draw 6000 full length vectors per second.

A quick web search didnt find anything promising.

Regards,

George

Reply to
Ancient_Hacker
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Write at a constant speed. Variable speed is trouble and will require manual adjustments to compensate for CRT variables.

Graham H

Reply to
Graham Holloway

To paraphrase Gene Kranz "Constant speed is not an option". I need to draw some straight lines, arcs, curves, all coming from different analog sources, each with its own deliberate and preset and in several cases, unchangeable sizes, lengths, and drawing rates.

I know it's doable as I just got a new $400 crown fitted to my third molar. Ouch.

Explanation: ( My dentist is a "somewhat less than perfect kids dentist" )

Further explanation: (He has an ancient "Asteroids" arcade video game juggernaut in his waiting room to keep the kids busy))

Further Explanation: (Being a bit off-, I had to go back for several refittings to get the crown to sit right so I could actually eat anything other than chocolate milkshakes (not that I was complaining), I spent lots of time in the waiting room.)

Much Further explanation: ( The game draws many different length but seemingly constant intensity vectors and arcs to draw the asteroids )

Reply to
Ancient_Hacker

In message , dated Thu, 17 Aug 2006, Ancient_Hacker writes

You can overcome the problems by retaining the raster scan at zero brightness and using the vector information to brighten the spot. I don't know exactly how it's done (comparators, I suppose), but I have a Rohde & Schwartz sweeper that does it that way, and the servicing information on it.

6000 vectors/s doesn't represent a very high video frequency. 
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
Reply to
John Woodgate

This is sort of an obvious answer, but since the write speed in each direction has to do with the rate of change of current, which is proportional to the voltage on the coils, couldn't you just use op amps to sum the absolutes of the suitably scaled rates of change of voltage and use that to modulate spot intensity? I recognize spot velocity isn't proportional to the two linear sum of the two coil voltages, but it may work well enough as an approximation. You could, I guess, get really fancy and via analog methods modulate the spot brightness in proportion to the square root of the sums of the squares of the squares of the voltages. That would be an entertaining and not too difficult a circuit to design.

Reply to
Tony

Thanks, I found a tek patent on exactly such a circuit. Not that I'd copy it exactly. But the general method is obvious once you hear it.

You just differentiate the x and y signals and that gives you a measure of the velocity. Ideally you'd square and square root the sum to get the actual distance per time info, but just adding them should be good enough for starters.

It does if you're trying to do it with a deflection yoke! To delta the current form -10 to +10 amps across a typical 2millihenry deflection coil 6000 times per second takes how many volts? Waay out of the SOA of most transistors. I wish there were a shortcut.

Reply to
Ancient_Hacker

You got a crown for only $400???? Where is your dentist. Here, in Taxachuesetts it's at least $1100.

Al

Reply to
Al

In message , dated Thu, 17 Aug 2006, Ancient_Hacker writes

Indeed; I meant as a video frequency for the brightness modulation of the raster.

True vector displays date from the valve/tube era, with higher impedance coils and BIG voltages. The horizontal output valve/tube would typically work with 3 kV pulses on the anode.

--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
Reply to
John Woodgate

I one worked on a spiral scan CRT system for radar. That was fun !

Graham

Reply to
Eeyore

"Eeyore" wrote in message news: snipped-for-privacy@REMOVETHIS.hotmail.com...

Tuned deflection coils, more efficient use of power?

I worked on a radar display in the early 70's that used deflection coils of around 10uH total. We used a class A push-pull output stage with centre-tapped coils and a series inductor. We got around 5MHz small signal bandwidth. The energy stored in the inductor provided the high voltage needed for fast transitions of the beam. The system could display a 625 line TV picture and write all over it with vectors and characters in the vertical flyback period. Problem - 800 watts per axis!

Graham H

Reply to
Graham Holloway

"Ancient_Hacker" schrieb im Newsbeitrag news: snipped-for-privacy@m73g2000cwd.googlegroups.com...

Hello,

So let's do the math.

V = L * di/dt

Now I assume the worst case. The factor two is for forward and backward movement so that we are back at the starting point.

V = +/-(2e-3 * 10*2*6000) = +/-240V

This requires a +/-300V supply with 10A.

I would say possible, but mission useless. Ok, it's a hobby project.

Best regards, Helmut

Reply to
Helmut Sennewald

Ancient_Hacker wrote in sci.electronics.design:

[...]

Adding the absolute values, I assume.

I could see a problem with differentiating input that is switched from one analog source to another. Unless the transition is smooth you'll see big derivatives you may have to control and/or ignore.

Anno

Reply to
anno4000

On Thu, 17 Aug 2006 12:37:56 GMT, Al wrote in Msg.

Not a very good one it seems.

robert

Reply to
Robert Latest

Get a few schematics for the old WellsGardner Color XY monitors used in the likes of Atari Tempest and so on. They basically had a deflection coil that was symetrical for the X and Y, used identical X and Y deflection circuits and used the amplitude of the drive circuit to handle the bigger X than Y (yes it could drive the guns off the edge of the phosphor). The guns were driven just like they are in a raster scan system just with the addition of a "Spot Killer" circuit that would kill the guns if there were no X or Y deflection, keeping you from burning a hole or line in the phosphor. The X and Y amps look a lot like a beefed up audio amp.

Jim

Reply to
James Beck

talk to the guys at zector graphics.

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a commercial unit. schematics are/were on their web page.

you got it easy, being a laser show guy, we have to use high speed moving mirrors!

Steve Roberts

Reply to
osr

George,

Here's a web site you might find useful:

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---Joel

Reply to
Joel Kolstad

a better link to zektor

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scroll past the pro audio stuff, its on page 3 of the online catalog.

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Steve Roberts

Reply to
osr

This is really the only practical way to do it, other than to buy an actual vector monitor. Converting one would be a nightmare - for one thing, the yoke is dramatically different from a raster scan monitor's yoke - both sets of coils are the same (in a vector monitor) with about two dozen turns of about #18 wire. And you'd have to take out the deflection amps, while somehow keeping the HV supply, and replace them with some hefty power amps.

Having worked on both kinds of monitors back in my video game repairman days, I'd say it's a non-starter.

Thanks, Rich

Reply to
Rich Grise

Next time you're in there, ask him if he'll open up the game and let youi look at the monitor yoke. That should give you some idea of what you'd be up against.

Sorry, RIch

Reply to
Rich Grise

Yipes! Luckily I overstated the actual situation-- we only need to draw about 20 full-screen lines 60 times a second, so we only need 10 amps at 1/5th of that, or 60 volts I think. In addition we need to draw up to 100 1/4 inch high characters at 60Hz, so applying the fudge factors, hmmmm.... 60 volts may be plenty. Well within the capability of a HV op amp with a couple booster transistors. And a heat sink And a fan.

Crazy project!

Reply to
Ancient_Hacker

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