"The suggested D16NE FET is obsolete. Mouser suggests a
512-HUF75307D3ST, which they then say it is obsolet"

Thanks, at least I have somewhere to start. I can just look for the spec sheets on those. I finally did start getting some results on Digikey's selector guide. I guess on soem things it's better to just aplly one filter at a time.

"These are high current devices, Are deflection plates
low impedance? I thought they would be high impedance high
voltage devices. "

As far as I know deflection plates are very high impedance like a control grid, though they migh pick up a chargee due to their placement in the envelope. They do have capacitance though, so at 275 Mhz (the rated -3Db point of this scope) they could concievably impose some load. As far as DC is concerned, the load resistors are IIRC like 160 ohms coing out of 53 volts which is only about 34 mA. I believe the engineers probably chose tthe 160 ohm value becaause it would make the interelectrode capacitance negligible at the highest operating frequency. This makes the compensation less of a PITA. there is no other reason to use such a low resistance.

I also notice that on this unit, the current from the load rresistors is passed through two terminals for the deflection plates. In fact it shows it actually going through the plates, lke it is not just two pins, it is the two ends of each plate. Not sure exactly what they're getting out of doing that but it doesn't bother me. Of course it must make some difference at 275 Mhz.

This is probably one of the faster non storage CROs out there. In digital they can just digitize the waveform and display it at a rate a cheaper deflection system and amps can handle. Not in this baby, what you see is really what you got. Really, once you get through all the hurdles to get flat bandwidth up to that point, I think some RAM and a processor would be cheaper, but of course not when that thing was built.

Thanks. Later today I am off to look at a few datasheets.