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Old 06-24-2012, 12:14 AM
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Opcom Opcom is offline
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Join Date: Jun 2012
Location: Dallas, Texas
Posts: 51
X-Y applications

Some details about an unusual project. I am thinking there might be some magnetics gurus here who could find it interesting. Can anyone speak to the characteristics of electromagnetic deflection X-Y displays?

The summary is that I would like to use a 'sparkfun' or equivalent oscilloscope clock with a 10" round CRT. Rather than consume P4 TV tubes, I have a few radar CRTs that will do. Using a scope for the scopeclock is not good for the scope and the display is too small from across the room.

The old original "cathode corner" scope clock has far better, sine-generated, numerals, but it is out of production and the person in authority has not replied, probably busy. I sincerely hope he will release a new version because of the superior characters generated by his scope clock board. The sparkfun board is OK, but it draws points, not the cleaner looking lines.

The H and V signals (XY) from the sparkfun clock are much too fast for magnetic deflection CRT yokes commonly found in TV sets. There is a DAC write each 2.25us, but a custom firmware has enabled a 100-150us delay for each plotted point.

Even so it is too fast for a common display having a 20KHz response. Calculations showed that with a regular TV yoke, the amplifier voltage must exceed +/-300V and the amplifier must supply up to 3 amperes.

Therefore a yoke must be rewound using less turns of thicker wire so the inductance will be very low, and a simple lower voltage amp can be used. This is borne out from inspecting an old military XY display. Its yoke has only a few turns of quite heavy wire, and tiny inductance. The power supply for that unit was about +/-40V at 10A

Attached are some pics of what has been done so far with an unmodified Wavetek electromagnetic deflection scope. I also have a H/P electrostatic deflection scope, model 1300 but it is not working. I need the books on both if anyone has them.

I have not yet rewound a yoke to enable a suitable amplifier to be built and used. I previously estimated a 60KHz bandwidth to be adequate but now believe a 100KHz bandwidth is possible and reasonable with the right yoke winding.

There is a very good read on crts, deflection amps, and design of yokes here. It is tube-type stuff, but the ideas are what matters.
Attached Images
File Type: jpg sparkful.JPG (54.9 KB, 25 views)
File Type: jpg fullspeed-on-20k-em-crt.JPG (61.4 KB, 41 views)
File Type: jpg 100us-timing.JPG (54.5 KB, 41 views)
File Type: jpg 100_8685.JPG (56.9 KB, 36 views)
File Type: gif file_1.gif (40.7 KB, 21 views)
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