Any Tektronix magnetic shield for a CRT? A strange problem...


 

Has anyone a recommendation for a mumetal Tektronix shield that could fit an old CRT of 4 inch face plate, 2 inch neck and 13 inch length?

And... why hasn't Tektronix ever built a round face 4" CRT? It jumped from 3" to 5".....darn!

I experienced a strange problem with a small oscilloscope that I built 50 years ago and I am recalibrating now, and which has this odd 4" round non-flat CRT (the only cheap one I could find at that time, poor kid! )
At that time, I wrapped a metal sheet around the tube neck to magnetically shield it. I have no idea what metal this was. This "shield" was held with some old string, and to make it look more pretty I removed the string and replaced it with tape.

Then I started to experience a weird problem. The brightness of the trace was much lower at the start of the sweep. I broke my head analyzing the HV supply, the unblanking circuit, etc. etc. Maybe a leakage in the HV ceramic capacitors? Maybe the unblanking signal not uniform? I moved things around, and then the sweep was normal at the beginning and vanished at the end. Moving more things around, and then changing the horizontal position the trace disappeared at the left of the tube and was perfectly normal towards the right! The trace could always be adjusted for good focus and astigmatism.

Until I realized that I had been rotating this in-house "shield" cylinder. Fortunately....I found a position where the trace was back to normal. It must be that the unknown metal is magnetized! Had it been like this for 50 years?
I have no idea how the magnetization affected the CRT beam, if it hit some internal electrodes while preserving good geometry, but I know little about that CRT, not even its part number.

This is why I am looking for a CRT shield, if possible a Tektronix one, that has not been banged around. If it is not available, I might try to build one with some permalloy sheet, or forget about it and play with something else.

Thank you for your help, and hopefully you never experience such CRT beam problems (no attaching of big speakers to the side of your oscilloscope)

Ernesto

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