DIY scope

[Rick Jansen]

In message <3.0.5.32.20000111193745.00d38900 at mailin.uni-bonn.de> you write:
> Hi all!
> 
> Recently I acquired two 30mm oscilloscope tubes.=20
> (Made in GDR, 1971, the warranty card was still in the box ;-)
> 
> Now here is the synth content:
> I toy with the idea makeing a small display for waveforms. =20
> I think I don't need a timebase, X-Y operation will=20
> suffice, maybe a luminance input.=20
> I figured out the pinout and heater voltage, and I've been able to get an
> unfocussed spot on the screen.=20
> 
> Now my questions are:
> =20
> How do I best get the required voltages? (The tube has cathode,=20
> luminance electrode (g1), a focussing electrode (g3) and the anode.=20
> Two deflection caps.) =20
> A small switcher perhaps? Can I get away with a single supply of=20
> say 800V and make a tap for the focussing electrode? How much current=20
> will the anode and the focussing electrode draw? Do I have to ground the=20
> anode, or the cathode? =20

I built a simple oscilloscope 15 years ago, with a very big oscilloscope
tube (22 cm across), especially just to "see" the music I was listening to.

You definitely need the data for your oscilloscope tube, unless you feel
like experimenting quite a lot. Such data lists the voltage ranges you need
for focus, luminance etc and the pin-out. My tube was made by AEG, I called 
them up and apparently they still had a book with the data, they made a xerox 
for me!

I generate the high voltage with a diode and capacitor "ladder". The
high voltage drives a resistor ladder with potmeters (with plastic shafts)
at the bottom for the focusing and luminance. You need the tube data for
the right voltage ranges.
The currents in scope tubes are very small actually. If your tubes are
small you may not need very high voltages.

Couldn't find a 6.3V filament current transformer either, used a 6V
standard resin encapsulated black plastic "cube" instead. Works fine. Do 
check that the secundary and primary windings are insulated well enough 
against the voltages you use, the cathode and filament are at a high 
negative voltage!!

I drive one X and one Y plate with voltages derived directly from the net,
thru an R/C network for phase shift, the R is a potmeter. Result is an 
adjustable ellips. The other X plate is driven from a small audio amplifier 
and another one of those very small black plastic resin insulated transformers. 
The voltages to drive a big tube like this are quite substantial, 50V/cm
isn't rare. I drive the secundary of the transformer with the audio signal, 
the "primary" drives the other X plate. Result is an ellips with an audio 
signal imposed on it vertically.

You could also drive the X and Y plate with a stereo signal, to monitor
phase differences between left and right :-)

You may find that even when you applied all the right voltages the spot isn't
in the center of the screen, or even not on the screen at all. In that case
you need to find a little magnet that you position carefully somewhere between
the accelleration/focusing electrodes and the screen, to deflect the beam so 
that it really hits the center of the screen.

Rick Jansen
__
rick at bpa.nl rja at euronet.nl   http://www.euronet.nl/~rja/
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