1980's Cathode Ray Oscilloscope -- The EIectronics Inside 34

Thanks for that. I've got a 2215, which is in the same series but has the addition of a B-sweep and B-trigger to the horizontal timebase, so it was interesting for me to see what's inside.

Although everyone is saying the tube shield is mu-metal, in this case it's just steel. At least, that's what the service manual for mine says, and I'd imagine Tektronix knew what they made it out of.

This service manual [so not necessarily the same as your 2213] says the delay down the delay line is 100ns. It doesn't seem to mention the differential characteristic impedance, but the series termination at the driver end is 73.2R and the parallel termination at the receiving end is 78.7R, so that gives us some idea [this is all so critical that it looks like those have been modified to take into account the additional effects of the transistors at each end].

This is the symbol for the delay line. [The drawings were done by hand and are a work of art.]

At the receiving end, the circuit is like this. Look at all the compensation to get the response level.

Tek is pretty well know for using mu-metal in their scopes. The problem with a steel shield would be that it can retain a magnetic field and mess things up by itself. And generally steel is not a great magnetic shield. Its not like the part was something that was ever going to be replaced.

Like you, I would have assumed it was mu-metal. I'm just reporting here what the service manual says. Perhaps I'm misunderstanding it or it was wrong.

I think I've read somewhere that once you get up to about 100kHz, mu-metal is no more efficient at magnetic shielding than steel, so perhaps it's down to having a SMPS rather than the mains transformer. We might also indirectly be explaining why they had the mechanical linkage for the mains switch, rather than just running a mains cable to the front. Another possible factor is that this is very much a portable, service instrument, rather than one that would sit on a bench or ride around on a scope trolly, so degradation of the mu-metal from banging it around might have been a potential problem and could have spurred the development of an instrument that didn't need that level of shielding. As you say, though, that still leaves the problem of the steel magnetizing.

Like you, I would have assumed it was mu-metal. I'm just reporting here what the service manual says. Perhaps I'm misunderstanding it or it was wrong.

I think I've read somewhere that once you get up to about 100kHz, mu-metal is no more efficient at magnetic shielding than steel, so perhaps it's down to having a SMPS rather than the mains transformer. We might also indirectly be explaining why they had the mechanical linkage for the mains switch, rather than just running a mains cable to the front. Another possible factor is that this is very much a portable, service instrument, rather than one that would sit on a bench or ride around on a scope trolly, so degradation of the mu-metal from banging it around might have been a potential problem and could have spurred the development of an instrument that didn't need that level of shielding. As you say, though, that still leaves the problem of the steel magnetizing.