Electrostatic plates

[SGiard]

I would like to turn an old small monochrome CRT into an oscope, and I keep reading it's a bad idea because a scopes CRT is driven by electrostatic plates inside the tube instead of coils. What is an electrostatic plate anyway and could such plates be built and put outside the tube?

 

[MikeMl]

In a word; NO!. The electron beam must pass between the plates, and there must be an electrostatic field between the plates. The glass in the CRT is sprayed with a conductive metal film (for electrostatic shielding). The electron beam is effectively inside a Faraday cage, so any externally induced electrostatic field is prevented from influencing the stream of electrons.

 

[SGiard]

I knew there had to be a good reason, thanks....

 

[Hero999]

I've never tried it but it might be possible to apply a constant current to the coils rather than a constant voltage. The trouble is you'll need quite a high voltage to get a high frequency.

 

[Sceadwian]

It'd work with lower voltages, you just wouldn't get nearly as much deflection. Doesn't seem really worth it in my opinion, the phosphors of monitors aren't designed to have beams run across them like a scope, either so you'll burn the phosphor in short order crippling any image. You could feed it signals via the H/V and RGB inputs, those are very low voltage video signals.

 

[SGiard]

I am sorry Sceadwian and Hero999, you have completely lost me (not that hard to do...) but it sounds promising. First I know how to control the intensity of the beam so I don't think burning the phosphor is a problem and it would be all right if it worked at audio frequencies.

But I do not understand what you mean by constant current. You need a sawtooth waveform on the horizontal side, so I guess you are talking about the vertical deflection being fed a constant current? But then won't that just producer a flatline?

 

[Hero999]

The deflection of the beam is proportional to the filed which is proportional to the current flowing through the coil. The coil is inductive as well as resistive so varying the voltage across the coil will result in a low cut off frequency. If a voltage to current converter with a high voltage output is used, it should be possible to increase the roll off frequency considerably.