Follow these simple instructions to replace the nasty 1200XL OS with
the more favorable Atari 400/800 OS.
1. Pull the Rev.B ROMs out of the motherboard of a 400, or from the
10K ROM board of an 800. Make sure that the chips function properly!
2. If you can't find a set of ROMs, order them from:
B&C Computervisions: Atari Sales
3. At this point you should have three chips, marked C012399B,
C012499B and C014599B, respectively.
4. Place the C014599B piggyback on the C012499B chip with the
notched ends facing the same direction. Solder pin 1 to pin 1, pin 2 to
pin 2, etc. for all 24 pairs of pins.
5. Open the 1200XL using a small Phillips-head driver on the screws,
and a needlenose pliers to pull the pop rivets that hold the RF shield
together. This step is completely non-destructive: there is nothing to
cut, unsolder or mutilate.
6. Locate the two 24-pin ROM chips on the 1200XL board near the
cartridge slot. Notice that they are marked U12 and U13, respectively,
in white letters on the board. Notice also which way the notched ends
of the chips are facing.
7. Remove these ROM chips and give them to someone you don't like.
8. Pop the chip marked C012399B into the slot marked U12, with the
notch facing in the proper direction. Place the piggyback chips in slot
U13, with the notches properly oriented.
9. Test the board before putting the 1200XL together again.
You now have an 800 with a 1200 keyboard, the best of both worlds.