Atari 800XL modifications

My friend had a 'disk doubler' which made his 1050 drive work much faster. Essentially it fits bigger ROM, RAM, and a 40-pin 6502. I felt the finished product was expensive. 

I had the chips, but no PCB. I noticed that most of the memory chip connections could be made by simply mounting piggy-back on the existing memory chip socket, so I did that and made the rest of the connections by flying wires. The crystal had to be made to lie horizontally, to allow space for the new CPU to fit. It worked!

After losing the 9VAC PSU for my disk drive, I looked for an alternative using the more common 5V and 12V DC switch-mode PSUs I had lying around. The 1050 rectifies the 9VAC to 18V for the 12V regulator, and 9V for the 5V regulator. The regulators dissipate a lot of heat. I modified my drive to take regulated 12V in, and this fed the input of the 5V regulator. I could replace the 7805 regulator with a small switch-mode converter, but it wasn't worth it for something I seldom used.

5.25" floppy disks are very rare, so I bought an Atari SIO to PC adaptor. This allows the PC to act as a slave disk drive for the 800XL, or as a master to talk to Atari SIO peripherals.

There are ways to replace the 64K RAM with 256K, but most software does not use it. The 1984 Byte magazine article requires machines with 74LS158 multiplexers for the DRAM address.

It piggy-backs on the DRAM address multiplexer U27.

There are two circuits, depending on the type of U7. 

Figure 1 for the CO21697 

Figure 2 for the CO12296

My machine has the CO21698 and the FREDDIE chip which replaces the DRAM address multiplexers , thus the Byte magazine circuit is not compatible. I would need to redesign the circuit. I don't think it would be very hard to do.

I might piggy-back a 512K RAM chip over the OS ROM chip. Twice as much RAM, plus the original 64K RAM.

After reading the articles by Clarence Dyson, I corrected my machine's video circuitry. This was straightforward. Video output was visibly better.