Humpty Dumpty sat on a wall…
Humpty Dumpty had a great fall…

…and so the great nursery rhyme goes! But here at Oscilloclock labs, we’re not talking about an egg (which, one theory goes, represented the defeated King Richard III). We’re talking about a beautiful old CRT, savagely shaken and shattered during international shipping. What a waste. But oh, what a great chance to see the insides close-up!

The Guts of a CRT

Many reference materials explain the bits and pieces of a CRT’s electron gun assembly, something along the lines below.

But here we have such an assembly exposed, up-front and personal!

Special Treatise – Deflection Blanking

This CRT is a bit unusual – it has a deflection blanking electrode. What on earth does this do?

Well, in most CRT applications, the beam is blanked by applying a negative voltage to the grid, which literally shuts off the beam. This is all fine and dandy, but there is one VERY annoying thing: the grid is actually at a very negative potential with respect to the deflection plates – often up to 2kV ! Translating a blanking pulse from a low-voltage microcontroller onto such a dangerous potential is tricky. Doing it at high frequencies is even more of a challenge. (Of course, the latest-revision Oscilloclock Power Board solves the challenge, offering fully isolated blanking up to 2.2kV at multi-MHz bandwidth.)

The deflection blanking electrode, on the other hand, blanks the beam by bending it so much that it hits the electrode wall. This seems like a bit of a waste – the beam is always on, and much energy is dissipated in the electrode. BUT this electrode is at roughly the same potential as the other deflection plates!! O happy day! There are no high voltages involved, so direct drive at very high frequencies directly from the microcontroller or low-level amplifier is possible.

For a more precise treatise, I defer to G. N. Patchett, who writes thus:

Aren’t CRTs dangerous?

Before you take your own CRT and hack it apart, a few words of caution. DO NOT TRY THIS AT HOME until you have at least read this, and please do be careful!

1. High Vacuum

CRTs are devoid of air, to allow electrons to flow readily, just like in space. This means they will implode. The electron gun might just get sucked through the screen and shoot into you, causing massive injury or death. One tried-and-proven way to safely break a CRT is to wrap it in several layers of towels or rugs, go outside, and whack it with a long board.

Or, as was my case, simply ship it to someone with very little packaging!

2. Broken Glass

Not much to say here. Don’t cut yourself!

3. Leaded Front Glass

Some CRTs, particularly those driven at 10kV and above, employ leaded front glass in order to stop harmful X-rays. Some people may feel this lead is a problem, dangerous in some way. But remember that this is the same material as leaded crystal, used in glassware for aeons – and even today. You can dispose of this glass in the same way that you would a broken crystal wine glass, without impacting your conscience. (Unless you have hundreds of CRTs, in which case – please use a commercial disposals company!)

4. Poisonous Phosphor

Some kinds of phosphorous materials used in CRTs are quite poisonous, and can be absorbed through the skin. Do not touch the phosphor or attempt to re-use it in mad-scientist experiments without extreme caution – always use gloves and goggles when working near these chemicals.

Reference

I have many superb books describing the glorious guts of cathode-ray tubes, but one of my very favourites, and the source of the extracts above, is The Cathode-Ray Oscilloscope and its Use by G. N. Patchett. This recently saved me literally tens of hours in a frustrating week of failures using an unknown type tube from China, as it was the only book in my library that helped me understand maximum-scan limitations in non-spiral PDA type CRTs.