Actually, there were two bad tantalum caps. Who would've thought that these diode looking parts are tantalums

Let's dial back a bit though and start at the beginning.

Here's my (one confirmed working) 7000 mainframe.

Technology from 1972. 100 Mhz bandwidth with a pretty big and bright tube. Non-storage.

Tektronix was famous for their modular scope designs. The scope itself was just the screen unit and the power supply. Everything else was handled by plugins. The 7603 can take two vertical and one horizontal plugin. If these are input amplifiers or a time base is up to the user

It could also be combined plugins, like this TDR/Sampler (7S12) or spectrum analyzer (7L12)

I showed the spectrum analyzer plugin here before. It's this channel's icon.