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Old 06-28-2012, 11:21 PM
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Opcom Opcom is offline
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Location: Dallas, Texas
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It is not television just a weird project.

The second image and the 7904 oscilloscope image above are that of the scopeclock XY display at a normal refresh rate.

The third picture above was done with a 6 or 8 Hz refresh by having the scopeclock delay for a set time after each point was plotted, to allow the deflection system to catch up. That's about 10-20K points per second.

It is interesting that the XY video games like asteroids produce slightly better drawings but they use many fewer lines, more of a "connect the dots" where most of the lines are seen but the dots, where the beam stops, are mostly blanked.

The scopeclock blanks the beam most of the time while it is in motion, and dwells on the dots, plotting many of them close together to make the appearance of a line. In this way the game designer has taken advantage of the slow writing speed of an electromagnetic display to draw using ramps and sines, while the scopeclock designer has expected a high speed display.

As an aside things like this are done with lasers in laser shows but the fastest servos are 15k or so, maybe some are faster. It is entirely possible to do a simpler clock face with a common laser projector, and that is a mechanical device. The better ones use a current feedback system controlled by a moving vane on the shaft of the mirror servo.

Surely, the next step being an electromagnetic yoke, can be faster than a mechanical system. The electromagnetic XY display also uses current feedback amplifiers so that the current and therefore the magnetic field in the yoke is at any moment proportional to the input voltage of the amplifier.

I've done some study of these amps. almost every one of them is just like an inexpensive audio amplifier except that it uses a resistor in the return lead of the yoke as the feedback voltage rather than the voltage sample as used in hi-fi amps.

Compare this to the linearity-related resistor found in the return lead of the vertical yoke on a TV set or monitor.

In some older cases using tubes, a push pull amp drove a push-pull yoke with current feedback deriving from the cathodes of the power tubes. The writing speeds for these radar applications was rather low considering a 400H to 1800Hz repetition rate. Those yokes had very high DC resistance and behaved more like resistors than inductors, so at low speeds a decent amplifier needed little feedback.

Images below show current feedback power amps suitable for deflection. A commercial XY game display channel, a tube type amp from a radar application, and a simplified circuit that takes advantage of an op-amp and is suitable for as high of voltages as the output stage can take.

The reason for higher voltages is straightforward, faster writing speeds. It takes less time to build up a yoke current to 3A with a +/-100v supply than with a +/-35v supply.

Is it considered wrong to use NOS round tubes in such a manner? - -I hope this is not offending TV collectors. But they were going to be pitched. Found in the rafters of a closed surplus store, Nortex Electronics. No doubt they came from local defense contractors, even the P4's. Time for some fun.
Attached Images
File Type: gif XY_amp_amplifone.gif (90.6 KB, 18 views)
File Type: gif deflection balanced DC amplifier.gif (27.1 KB, 18 views)
File Type: gif basic DC amps.gif (61.9 KB, 19 views)
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