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Atari 800XL modifications

My late-model (1985) 800XL system

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It was brand new out of the box. Naturally, it needed improving!

I bought an 800XL machine with 1050 disk drive in 1985, when they were being sold off cheap. At last I had a machine I could save software on reliably, and keep me amused between graduating and getting a job. My friend had lots of software for it.

I had fun porting a triangle-filling algorithm onto it. Then I got my first job and soon after, my first PC (Amstrad 8088). My 800XL is still looks in mint condition, never being exposed to smoke or yellowing agents.

The "FREDDIE" chip is the only part in a socket. All other parts are soldered directly to the PCB, making it cheaper to produce and much more reliable. Contrast this with the BBC micro where everything was initially socketed, and needed regular dropping onto a hard surface after 1 or 2 years service.

THE QUARTER-MEG ATARI 800XL in html form.zip

The Byte Article in html form. I can't do it on my machine, but it should work on many others.

Zip Archive - 1.56 MB - 01/03/2022 at 17:37

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Clarence Dyson Supervideo articles.zip

Clarence Dyson Supervideo articles. I used version 2.1a for my machine.

Zip Archive - 488.89 kB - 01/03/2022 at 17:35

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  • Atari 1050 disk drive modifications

    Keith01/03/2022 at 22:06 0 comments

    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.

  • RAM expansion

    Keith01/02/2022 at 04:10 0 comments

    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.

  • Video correction

    Keith01/02/2022 at 03:40 0 comments

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

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