Subject:	7800 EPROM conversion
From:	btomlin@crl.com (Bruce Tomlin)
Date:	18 May 1996 14:07:11 -0700

Parts needed:
   74LS04 chip (No other 74x04 variants!  I tried a 74F04 and it didn't
work.)
   28 pin IC socket
   EPROM chip, either 2732, 2764, or 27128
   About 5 inches of wire wrapping wire, and a razor blade to help strip it
   Soldering iron, solder, desoldering iron, etc.


1) Desolder the old ROM chip.  This is the one marked "CO24922".  If you
   haven't desoldered chips before, get some practice or buy a pizza and
   split it with a friend who has.  (See end of this file for tips on how
   to desolder an IC chip.)  Also suck the solder out of the four extra
   holes.

2) Strip one end of the wire wrap wire and put it into the pin 22 hole.
   This is the one to the "right" of the letters "DBM".

3) Insert the 28 pin socket into the holes and solder it down, along with
   the wire wrap wire.  Test your work by putting the ROM chip back in and
   plugging in a 7800 game.  If the "ATARI" screen comes up and then the
   game starts, everything is okay.

4) If you are going to be using a 28 pin EPROM chip, suck the solder out
   of the holes marked "W1" and desolder the resistor-like thing from the
   adjacent holes (which are marked "W2").  It may look like a resistor,
   but if it is, it's zero ohms.  Solder the resistor-thing into the W1
   holes.

5) Take the 74LS04 chip and bend up all but the four "corner" pins (1, 7,
   8, and 14).  Piggyback it over the 74AHCT08 or 74LS08 chip which is
   just above the biggest chip on the board.  Solder down pins 1, 7, and
   14.  Now bend up pin 8.

6) Strip off the other end of the wire wrap wire and wrap the end around
   pin 2 of the 74LS04.  Before putting it on the 74LS04, thread it under
   some resistors and capacitors to keep things tidy.  Solder the wire to
   the pin.

7) Find the trace on the bottom of the circuit board that goes between pin
   1 of the 7408 and the ROM pin that you stuck the wire wrap wire into.
   There is a place next to the 7408 where you will have enough room to
   cut the trace safely.  Cut it.

8) Program a 2732, 2764, or 27128 EPROM (or a 28xxx EEPROM if you prefer)
   with a copy of the 7800 ROM in the last 4K.

9) Put the ROM into the socket and test everything by plugging in a 7800
   game.  Now you're done!


Hints for desoldering:

* I use a Radio Shack desoldering iron.  This has a red rubber bulb on it
  and a hollow tip.  It's cheap and works well.
* Use a fresh tip for important projects!  It's not worth trying to save
  two bucks only to ruin a circuit board or a chip.  Tinning your tip is
  a good idea, too.
* Wait for the joint to completely melt before sucking out the solder.  If
  you suck too soon, you may not be able to melt the joint properly any
  more.  Wait about four seconds, or five for the four "corner" pins.
* When you have sucked out the solder from all of the holes, push all the
  pins to "crack" them off of the remaining solder.
* If you have done everything right, the chip should practically fall out
  of the board.


7800 ROM dump:

BEGIN 644 BIOS7800.BIN
M2&ST /______________________________________________________
M____________________________________________________________
M____________________________________________________________
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M0CGJ________________________________________________________
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END