A non-functional Tektronix 465 oscilloscope with a dented case and several serious faults was recently donated to my local HackSpace. Since they didn’t want another broken scope, I took it to see what was wrong. Eventually, after learning about CRT theory, with the help of very knowledgable people on the TekScopes forum, and gingerly measuring waveforms at -2450V, I came to the conclusion the CRT’s grid was faulty. Reluctantly removing the CRT, I found that it could not be repaired. Since parts of the CRT are rather beautiful, I decided to salvage those parts for display. This note outlines the original tube, the fault, and the process of turning it into something fit for a display cabinet.
Examining the CRT’s neck showed the grid’s pin had become detached from the grid itself. I was expecting to find a subtle fault, but the gap is >1mm. The connection pins are soft and flexible, so I am surprised that there was sufficient internal tension to cause the pin to spring so far away from the grid. I would have guessed that any jolt sufficient to separate the two would also have broken the glass envelope.
I decided to try to keep the metalwork sculpture, for display in my dining room along with my other curios. Since the CRT is too large and the black paint is unappealing, some surgery was required. Unfortunately I don’t know anything about working with glass, especially a glass envelope that will implode when provoked.
My first thought was simply to smash the glass by putting it in a deep dustbin, and then thwacking it with a nail on the end of a pole. Crude, but the flying shards probably wouldn’t injure me. Then a laboratory maintenance manager told me that a heated nicrome wire could be used to make or propagate a clean cut. I was going to try that, but then Richard Sewell introduced me to the concept of portable glasscutting bandsaws, and offered to use his.
It looked like it would be sensible to cut the black glass section and use that as a base, probably painted a less obtrusive colour. But that didn’t work out since it is actually a tough ceramic. In the end he cut the glass just above the black ceramic, revealing the metalwork protruded further into the ceramic than could be seen.
What emerged from the base is a fragile, very fine, convex wire mesh lens about 3″ from the faceplate – not to be confused with the mesh in a colour CRT. I’m still trying to find the mesh’s dimensions, but it can barely be seen with a *20 hand lens and is transparent to photons and electrons. In the picture below, the colour fringes are real, not an artefact of the camera. Even puffing gently at the mesh is sufficient to buckle it into a partially convex shape; rolling a drop of water around inside it is sufficient to restore its shape.
The mesh is at about -150V compared to the cathode at -2450V and the anode at +15kV, and the electrons pass through the mesh on the way to the phosphor. Without a mesh the deflection voltages would have to be several hundred volts, which is no problem with valve amplifiers. With a mesh, the deflection voltages are a few 10s of volts, which is compatible with semiconductor amplifiers. In addition the mesh improves the geometry and focussing of the beam, as described in detail in 1962 issue of the HP Journal.
Finally, after fixing it to a base, the CRT can be displayed in a cabinet.