This is a pretty straight forward modification guide that shows you how to double your Amiga 500's chip ram from 512kb to 1Mb!

Retrobrad has been created and maintained by me - Brad!
This page has been visited times since August 2007

by retrobrad 11 August 2007
  Right now thats out of the way we can get on with the mod. You will notice at the bottom of the baord there are four ram chips - there are also four spare spaces for extra ram chips (this is where our new ones will go! - surprise surprise...)

The spare spaces are filled with solder so you will need to desolder them all before you can put then new chips in.
  First things first - This mod is for revision 6a Mainboards. So you need to see if you have that revision, if you don't (i.e you have a revision 5 board etc..) then you will have to wait untill i put up a mod guide on how to upgrade those mainboards.

So now that you know your board is a revision 6a, you need to see if you have the 'fatter agnus' chip. Look on the mainboard for a square chip just like in this picture. Check the number on it, if it says 8372A - then you're in luck (this is the 'fatter agnus' chip which will allow 1Mb of chip ram. If it is an earlier version (i.e 8371) then you won't be able to do this mod unless you track down a 'fatter agnus' chip.
  Okay now they're all soldered in we need to TELL the fat Agnus chip that we actually have 1Mb ram installed. To do this we need to locate JP2 (on my mainboard it was located near the Crystal oscillator and the kickstart chip - see pic)

Now JP2 has three pads, the middle is the COMMON pad, the left is the 1Mb pad and the right is the 512k pad (since this is a new mod, your JP2 will be connected from the middle pad to the right pad - telling it that there is 512k installed).
  Now we need to actually find some chips to put in our board right? I have LOADS of trapdoor 512k ram expansions lying around so i just stole the chips from one of them. Most 512k trapdoor chips will work but not all of them - if they have chips that look like the one in this pic, then you should be right to go.

Now we need four 128k ram chips, don't be confused by whats written on the chips though they will always have 256 written on them in the part number which leads you to believe they are 256k bytes per chip WHICH IS WRONG! - they are 256k nibbles! (which is half a byte) so therefor they work out to be 128k bytes!

(hope i didnt confuse you...) Anyway just desolder these chips - AND each ones filter capacitor (the round thing above each chip) and then solder them into the mainboard! - Simple huh?
  So we need to get our scalpel and cut this track. Now you can do one of two things - either hard wire JP2 to the 1mb position (middle pad connected to the left pad - like i've done in this pic) OR you can connect a switch so you can switch between 512k mode and 1Mb mode.

The switch is there incase you run across a program that won't work in 1Mb mode (but i've never had this problem so i left it hard wired)

But if you do want a switch, just connect your switch terminals to the three pads (left pad to left contact, middle pad to middle contact and right pad to right contact)
  Okay here's the completed mainboard, all ive done is desoldered each 128k ram chip and it's filter capacitor from the 512k trap door expansion and have soldered them into the spare spaces on my Amiga 500 motherboard.

Now there's just a couple more steps...
  Nearly there...

The last thing to do is to cut the CNX line comming from the trapdoor expansion, this is pin number 32.

(see pic) just cut it wherever is easiest for you along that track)
  Soldering Iron
  Solder Sucker (or someway of desoldering chips)  
  Four 128kB ram chips (read below for more details)  
  Four Filter capacitors (0.1uF will do the job nicely)  
  One SPDT switch (optional)  
  lengths of insulated wire (if you want a switch)  
  Scalpel (or some sort of sharp knife)  
  Time for testing!

Turn your Amiga on and boot into workbench, check up in the top info bar and it should now say that you have somewhere in the order of 800k free mem! (whereas you used to have around 300k free)

and thats it, good job!