I have in my possession a couple of Compact Macs.
One of these macs I would like to gut and replace with a fishtank :)
The other one I would like to turn into a totally retro-electropunk arduino project that does nothing but make amazing lights and sounds.
So what is the feasibility of controlling the CRT yoke to produce basic vector graphics (shapes, lines, etc) using Arduino? I assume I need some kind of DAC in order to actually do anything of worth, but what about power levels? What kinds of voltages are required to deflect that electron beam? I guess I should really just start ripping these guys open and probing around but I don't have an o-scope...
CRT deflection voltages are very high, so a) do NOT start probing around (even with the power off, if there are big capacitors in there they could have dangerous charge stored in them), and b) driving from an Arduino is unlikely unless you really know what you're doing.
The MegaRAM shield (http://ruggedcircuits.com/html/megaram.html): add 128 kilobytes of external RAM to your Arduino Mega/Mega2560
I remember converting an old TV into this kind of display when I was a teenager. The voltages used to drive the CRT anodes are indeed high; but you don't need high voltages to drive the deflector coils at low frequencies (up to 1KHz or so). I made a couple of class B amplifiers (similar design to standard audio amplifiers) to drive the deflector coils of my TV, and I used two signal generators to feed them.
One complication is that the TV circuitry may not be isolated from the mains, i.e. "ground" on the TV may be connected to one side of the mains supply. However, as long as the deflector coils are totally disconnected from anything else in the TV and well insulated, you can drive them from external amplifiers with a true ground connection.
As a minimum, you would need 2 DACs and two audio power amplifier ICs and associated components. I would start by disconnecting the deflector coils from everything else and measuring their DC resistance. That will give you some idea of what voltage and current you might need from the amplifiers. The vertical and horizontal ones might be quite different.
You won't get colour with this arrangement, although I guess you could add colour by hooking opto isolators into the RGB output stage circuits and driving them from digital pins, giving you 7 colours plus black.
Do remember that you get very high voltages (25Kv) in a CRT display, so you need to know what you are doing.
Another problem is that if there is no AC signals in the coil, the beam is too strong and will burn out the phosphor at a point.