Atari Mega ST - what exactly have I got?

[Gordon JC Pearce]

Hi all,

	Through the wonder of Freecycle, I've just picked up an Atari Mega ST. 
  There are some interesting mods inside it, and I'm not sure what some 
of them are...

Everything seems to be liberally hot-melt glued together.  I don't want 
to start picking it all apart.

The 4M upgrade I sussed easily - even more than 12 years after I last 
did one, I recognise the board with the video IC and it doesn't take 
much to suss the 4 1Mx8 simms.

There is a board with two PALs, some small caps and a small electrolytic 
cap.  It seems to be unmarked.  The PALs may be something like 16AS255 - 
there is paint on them.  One has one stripe of silver paint, the other 
has two.  A white 10-pin connector goes off to some leads.  The ones 
I've figured out are:

Green wire goes to pin 4 of the video socket - mono detect/clock 
according to the manual
Blue wire to /RTS on U15, one of the UARTs
Brown to the cathode of D18 (no idea)
One red and one black are clearly +5 and ground
the other black, along with white, orange and yellow vanish off to below 
the PSU
the remaining red splits into two thin white wires, one going to a pin 
near OSC1 (may not be connected) and one ending up near the memory.

There is also a board marked "Compu Software 1992".  There appear to be 
ROMs soldered to it, it is soldered to the CPU via another socket, and 
stuck in the top is a board marked C103261, with a 74LS74, a small 
capacitor, and a row of pins marked 12 - 18 that plug into pins 12-18 if 
the CPU socket.  A red wire runs off to a jumper pad near U39.

Don't know if anyone recognises any of this, but any insight would be 
great.  I haven't fired it up yet because I don't have a keyboard cable 
- trip to Maplin tomorrow...

Gordon.