Subject: UPGRADE: 600XL->64K

From the February 1988 issue of Michigan Atari Magazine by Don Neff.

This memory upgrade requires 3 short lengths of wire and two memory
chips (HM50464P or equivalent 4464) to provide a full 64K.  
          _____________________________________________

MAKING THE MODIFICATION:

Open the case of your 600XL and remove the circuit board.  The 4416 
(4 bit x16K) memory chips, U11 and U12, are located at the front right
corner of the cartridge holder.  Gently remove both of these chips and 
replace them with HM50464P (4 bit x 64K) memory chips (or 4464 equivalent).

Remove U16 (a 74LS51 chip to the left of U11) and bend pin #8 up so it 
sticks straight out from the chip body as shown in Figure 1.  Insert U16
back into its socket.  Unsolder and lift the lead of resistor R36 (as 
indicated at point "Y") in Figure 1.   Solder a jumper wire between this 
resister lead and pin #8 of U16 (point "X").  Insulate the ends of the
jumper so that they will not touch other parts of the circuit.
of the circuit.

Locate U5 and U6 (74LS158) which are directly in front of U11 and U12.  
These chips handle the row and column addressing inside the memory matrix.
We have to add two lines to these chips.  Gently remove U5 and bend 
pin #3 up so it sticks straight out from the chip body as shown in 
Figure 2.  Insert U5 back into its socket.  Now remove U6 and bend 
pin #10 out and replace it back in its socket.  Notice that the two pins 
you bend are nearly touching the other chip.  Cut enough off of each of 
the two pins to prevent any shorts.

Locate U14 which is to the left of U5 and U6.  Notice the circuit traces 
to the left of U14.  We are interested in the two traces indicated in 
Figure 2 as points "C" and "J".  Connect the jumper from the pad at 
point "C" to point "D" (pin #3 of U5).  This jumper sends address bit 
A15 to U5.  Connect jumper from the pad at point "J" to point "K" 
(pin #10 of U6).

Make sure all your IC ships are inserted properly with their #1 pins in
the lower left corner.  Check your jumper wiring agains Figures 1 and 2.
You should have a jumper from point "X" to point "Y".  You should have
a second jumper from point "C" to point "D", and a third jumper between
points "J" and "K".  Make sure the jumper ends are insulated to prevent 
shorts with nearby objects.  Reassemble your computer case and run the
memory self test as a final check.  

If all has gone well, you should now have the same memory capacity as
a stock 800XL.

 










                                  U12--------------   R36    R38
                             ---->   )    4416    |   |       |
             Replace with   |        --------------  |^|     |^|
             HM50464p  (4464)                        |_|     |_|
                            |     U11--------------   |       |
                            ----->   )    4416    |   0(Y)    0
                  (X)                --------------   | \  
                   /----------------------------------|  Unsolder and
  U16 |_|_|_|_|_|_|_  Lift pin here                      lift here
     |            8 |             
      )   74LS51    | 
     |o_____________|
      | | | | | | |
                        (FIGURE 1)



                                          U5 |_|_|_|_|_|_|_|        
               U14                          |               |
                |_|_|_|_|_|_|_|              )   74LS158    |
Board  || |    |               |            |o___3__________|   Lift
Traces || |     )   74LS158    |             | | | | | | | |    pins as
       || |    |o______________|      ___________D        _____ indicated.
      / | |     | | | | | | | |      |                   K     |
      | C |                          |    U6 |_|_|_|_|_|_|_|   |    
    0 0 0 0   __Connect to pad C_____|      |            10 |  |
    | | | J                                  )   74LS158    |  |
   / / / /    __Connect to pad J____        |o______________|  |
  / / / /                           |        | | | | | | | |   |
__|_|_|_|_                          |                          |
          |                         |__________________________|
   U9     |
__________|
                        (FIGURE 2)

Craig Lisowski (clisowsk@mcs.kent.edu)