512K RAM CARD ALTERNATIVE FOR THE A500 

                         by Les Ayling 

   Ed: As usual, all disclaimers apply! If you aren't sure of your
   abilities to do this mod yourself, get someone who does to do it.

   Author's note : This project is DEFINITELY not for the unexperienced
   experimenter. A mistake could DESTROY your machine, and it is probably
   fair to say that your friendly COMMODORE service centre won't want to
   see your machine after you have fitted the MOD. If you can persuade
   them to fix it they may take the mod out as a matter of course!

   This article will allow you to add the extra 512k to bring your
   standard AMIGA 500 up to 1meg.

   You will need the following :
   1. 16 X 41256 memory chips (or 4 X 44256 for the more adventurous
   amongst you).
   2. 2 feet of hookup wire (the dick smith coloured ribbon type cable
   is of a suitable diameter).
   3. low wattage soldering iron.
   4. Long nose pliers.

   First of all unplug everything from the machine and turn it over.
   Unscrew the 6 retaining screws from the underside of the case,then turn
   the machine back over.  Remove the lid. Then unplug the keyboard
   taking careful note of the orientation of the plug.  Remove the 4
   screws and from the RF shield and it should come free.

   At the very front of the board you will see 16 41256 memory chips
   labelled U16-U31. We need to piggyback our new chips onto the existing
   memory ICs.


   After taking the appropriate static precautions (if you really believe
   all those rumours spread by 3M to sell their anti-static equipment)
   carefully bend pin 4 (RAS*) of each of the new memory chips with the
   pliers so that they stick out horizontally.

   If you are unsure which is pin 4 refer to the data pages of either ROD
   IRVING'S or DICK SMITH's catalogues.  Next piggyback the new chips onto
   the backs of the existing chips (TAKING CAREFUL NOTE TO GET THE
   ORIENTATION CORRECT!! ) and solder the remaining 15 legs of each to its
   corresponding leg on the existing chips below.  Be careful of solder
   blobs at this stage as even the tiniest blob can cause frustrating
   hours to be saved later on.

   Next locate the connector CNX/CNX1 which is used for the memory card.
   Using your schematic from the rear of the "introduction to the A500"
   page F-4, and the numbers printed on the circuit locate pin 38 of the
   connector.  Take the hookup wire and connect pin 38 to EACH and EVERY
   pin 4 of the new memory chips.

   Then locate pins 32 and 34 of CNX and short them together. Pin 32 is a
   signal called EXRAM* (* meaning ACTIVE_LOW) when grounded (pin 34)
   instructs the GARY chip that an A501 board is connected. Or in this
   case our equivalent.  Alternatively fit the memory switch as detailed
   in previously to allow enable/disable of the extra memory.

   Put your Amiga back together, power up and you should be pleasantly
   surprised.  If you don't really need the clock this mod represents a
   cheap alternative form of memory expansion to 1meg, with minimal
   additional circuitry to your main PCB. All up the memory chips should
   cost about $80 (about $5 for the 41256) which keeps my wallet happy!


   I mentioned at the start that the more adventurous could use 4 x 44256
   chips. These chips are the equivalent of 4 of the 41256s and are
   currently availabe for about $16 in SYDNEY. It is not practical to
   piggybach them to the 41256's as the signals are not pin for pin
   compatible. However when developing my 2meg internal cheapskate's
   memory board I used a 20pin dip 4-chip array board, with short wire
   jumpers for the appropriate signals and data lines with great
   success. Keep in mind that 16 X 44256 chips = 2megs, whereas you would
   need 64 X 41256 chips! Doesn't sound viable does it!  Stay tuned to
   MEGADISC for the 2meg inboard memory expansion!

   I hope that this is of some use.