I'm having trouble finding high frequency signals to test the 200Mhz end of the scale with.

Here are some screen snaps of the scope showing a column address strobe (CAS) line from a Mk2a. Note that my ground clip was simply on the empeg chassis -- nowhere near the RAM chips.

The screen is much better than it looks in the photographs below, which suffer from some kind of moire interaction with my camera's bayer matrix sensor.

Lovely little "square waves", with a fair amount of ringing: