
My machine is one of these early 520ST models and it's been in my possession since new back in the 80s.
I just couldn't bring myself to chuck it away so I'm using it now as an excuse to do some electronics and programming projects.
What's been done to it so far:
TOS: upgraded from stock 6Chip TOS 1.00 to 2chip eeproms that can run either 192Kb or 256Kb.
Currently switchable between EmuTOS and TOS 2.06.
Blitter: Self made Blitter expansion for the CPU socket, with 256KB ROM decoder and IDE interface.
(This IDE interface is no longer in use)
RAM: Exxos 4MB ram upgrade.
CPU: TF534 accelerator with 68030@50Mhz. The card pretty much worked out of the box with just very minor Atari tweaks.
Most of the effort was writing the necessary Atari software to make the machine take advantage of the card properly.
Here's an early one with Blitter, IDE interface, 4MB and a pretty crude TOS ROM conversion using sockets and wires.
I was very happy about this

The Blitter is still there, hiding under the the TF.
I have a bunch of more things in mind for the machine and I'm not really sure when this is going to end - presumably when the computer finally gives up and explodes

These are the more realistic things I have in mind next:
- Add realtime clock to blitter board (same as Mega to keep it simple)
- Make a PCB for where the RF modulator (and monitor socket) used to sit.
This will host UBE switch circuitry, VGA + Audio connectors + stuff yet to be fully decided.. the board can be quite large so there is plenty of rooms for more stuff. Thinking of one of those small midi modules. Perhaps internal speaker.
The old holes in the case needs to be filled and new ones made ofc.