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Opcom 06-24-2012 12:14 AM

X-Y applications
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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.

Opcom 06-29-2012 12:21 AM

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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.

old_tv_nut 06-29-2012 10:39 AM

Just blue-sky thoughts:

Is the yoke winding you need closer to a TV vertical or TV horizontal? Could you take apart 2 TV yokes and then combine the two best windings at right angles for X-Y deflection? (yeah, might be a mechanical impossibility, just trying to brainstorm)

old_tv_nut 06-29-2012 10:44 AM

more: not sure about vertical windings, but I know horizontal windings for transistor sets are usually lower inductance/ higher current than horizontal windings for tube sets. Sacrificing a couple of cheap $20 black and white portables for parts would not be a crime. (don't know what neck size you need, tho)

Opcom 06-30-2012 12:38 AM

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The yokes I need are just regular tube type yokes, but I will take ss ones that have the proper diameter and degree geometry. The tube data is below.

It is possible to place a set of four V windings or H windings staggered and interleaved around the tube if the core is abandoned, but the magnetic field gradients are malformed and beam position is no longer linear to yoke current. Not that it ever is, but new errors are introduced.

The ferrite can be abandoned and there are air core yokes, but in the volume 22 it discussed that, why it is not a great idea.

I have to rewind a bad yoke here. Bought from ebay NOS in the box.. as is of course and with evidence of having been tried.. I learned from a yoke from a government XYTron display that one secret is to use few turns of heavier wire. That yoke has about 14 turns in each winding, so 28 V and 28 H. I can't examine more closely because it is two ferrite shells with windings inside and glued together. However, from the MIT volume 22 referenced above, I see how a pegboard can be made to allow the making of distributed windings that can be formed into the proper shape. If I can take the windings from the bad yoke as a template, I should be able to devise a suitable sized winding jig.

Many yokes have inductances in mH. Even the horizontal windings from solid state sets have more inductance than needed. If checked in a ss TV set, the voltage across them is 1KV peak, so it is suitable for higher inductances. Tube yokes - I never messured the voltage across the H yoke on a color set - probably pretty high. A vertical winding has lots of inductance, even on low voltage sets. It's the pulse applied to the yoke that is the true measure of the actual deflection voltage required, and we see 50-100V pulses on a ss vertical yoke for 60Hz sweep.

With a practical low voltage amplifier, the windings should be more like 50-100uH. One way to describe a yoke winding is in volts per amperes per microsecond. e.g. 100V for every ampere of current increase per microsecond. Another thing is the current, but the ampere-turns is the key to the magnetic field just like a choke or transformer. So if the yoke with 200 turns needs 3A to position the spot at the edge of the screen, then theoretically the yoke with 50 turns needs 12A. The fast yokes from that XYTron display can take that kind of current, or maybe 15A anyway. The project has to move the yoke current from -3A to +3A in 16us. well that is the goal anyway.

One cheat is putting the windings in parallel to allow more current basically cutting the voltage requirement in half. I have not experimented to see if the compensation network, the RC dongle across half the yoke in TV service, can be eliminated by this.

This said I do not intend to destroy good yokes, I need them for display of NTSC video on most of those tubes. Crappy condition yokes might get a rewind, but it is not necessary to ruin the old windings to remove them, and replace with low turns count windings for these experiments.

The yokes I am after are 50-60, or 70 degree types and fit a neck of 1 7/8" (1.875"). I can use 70 degree yokes if necessary but prefer the 50-55 degree types as well as focus coils and/or magnets for same tubes. Some older ones had a focus coil or magnet in the set as well.

Opcom 07-24-2012 10:57 PM

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I pulled apart the 'bad' NOS eBay yoke.. It was not hard to take apart but I cracked a ferrite a bit. I don't think it will harm anything if I super glue it back with no voids.

I'm looking at the windings and can see how a pinboard template could be made up using the original windings, flattening them out as they might have been during some manufacturing stage, but they are made with a lacquer so they might not want to flatten easily.

Once a pinboard template is made that defines the inner and outer size of a particular winding (H or V), then a few more pins can be added so that I could wind a few turns of heavier wire on the board for the new coil. The method is shown in MIT volume 22, p. 717 and described in the text. It looks easy but probably won't be.

lnx64 01-28-2013 02:45 PM

I've been playing with my oscilloscope and did something like this a little while ago.

What I did was driven by the sound card on my computer though.

zenith2134 12-10-2013 03:58 PM

AWESOME vector graphics there, lnx64! This type of thing has always been a passion of mine as well.

Opcom 12-11-2014 10:19 AM

Wow it has been a long time. been busy, work etc. Inx64, those are interesting graphics from the PC. How did the PC make those?

What I found out is that the best EM deflection result with low voltage solid state amps is going to be a rewound yoke with relatively few turns of heavy wire. High current low voltage is easier for solid state. The military yoke looks like 10-20 turns, and the current on that was about 10A.

For those looking to the tube diagrams, OK those yokes have thousands of turns, just beware the challenge of lots of turns and high voltages against the wire insulation.

I am working with an additional person/company now to help put forward the scope-clock hobby. The other party has been doing most all of the work and it has been with electrostatic CRTs. I am trying to help by working with electromagnetics and going to help them to work with that technology. So, I started with the O-clock and this showed me the bandwidth needed, approximately. It'll work if the bandwidth is high enough. 60KHz minimum, is hard for a deflection coil using a reasonable amplifier that hobbyists can make.

Based on previously designed vector displays, it looks like the method of drawing that is best for EM deflection is incompatible with the way the sparkfun clock works. The vectors must all have no more than a certain di/dt, otherwise the amplifier will have to make excessive voltage in order to reposition the spot at extremes, and there may be overshoot or ringing as a result. Controlling overshoot and ringing caused by too-small di/dt puts even more stress on the amplifier as it must 'eat' that energy.

Username1 12-14-2014 01:28 PM

X64, just run that other channel through a single transistor amp with no real gain
to flip it.....

Anyway.... Pretty dam cool ! !


Tube Radio 07-15-2015 11:00 PM

Old topic, but what about using a vector monitor for the scope clock or would that not even be enough?

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