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Upgrading my Amiga A500 to 1MB Chip RAM (1MB Chip RAM Mod)

1MB Chip RAM Mod


My A500 motherboard is a Revision 6A which means that it is quite a straightforward modification to get a 512K trapdoor expansion to be seen as additional Chip RAM. The 6A comes with the Enhanced Chip Set (ECS) variant of the Agnus chip called ‘Fat Agnus’ which is able to support up to 1MB total Chip RAM. This 1MB Chip RAM Mod will combine the 512k onboard RAM with the 512k Trapdoor RAM to give 1MB total Chip RAM, just like you get in the later A500 Plus models.

Chip RAM is needed for stuff like graphics as it is the only memory that the custom Amiga chips are able to access and 512k goes nowhere! Opening Workbench windows, increasing the screen resolution, adding a wallpaper all gobble up precious Chip RAM. The popularity of WHDLoad as a means to run old Amiga floppy games off a hard drive is also a problem for 512k machines as most of the games require 1MB of Chip RAM to function.

Checking the Amiga board revision

You can check what revision your Amiga is by opening it up and removing the floppy disk drive. The revision number will be etched into a small area underneath along with other interesting information such as the serial number and when it was made. It looks like my board was designed in 1988 (30 years old this year!) but that it might have been assembled in 1989/90 going by the serial number sticker?


1MB Chip RAM Mod

Revision 6A motherboard confirmed


1MB Chip RAM Mod

Here’s a screenshot from SysInfo confirming the presence of an ECS Agnus chip – needed for this mod.

Under the knife!

The first thing to do is cut the small trace between the lower two pads (2 & 3). I used a small craft knife but any similarly sharp knife should do the job. Be very careful not to slip with the knife as there are several other tracks on the motherboard that you do not want to damage! To check that you have successfully cut the track you can use a multimeter – there should be no continuity between the two pads if the track has been cut properly.


1MB Chip RAM Mod

JP2 – Track cut between the lower 2 of the 3 solder pads with a sharp craft knife.

Bridging the gap

After successfully cutting the track between pads 2 and 3 the next task is to solder the top two pads (1 and 2) together. I found the easiest way to do this was just to keep adding blobs of solder to one of the pads until there was enough to drag across to the other pad and bridge the gap.


1MB Chip RAM Mod

JP2 with pads 1 and 2 soldered together.

The shortcut

Traditionally the next part of this modification required cutting a track on JP7A but just by chance I stumbled across an advert for a 512K Trapdoor expansion on eBay that bypasses this requirement completely. This particular RAM expansion has a series of jumpers on it that you can use to enable/disable a number of features including using it to expand Chip RAM. It also features a clock with battery backup so it’s a very complete little package. Anyway, in order to have the 512k of RAM added to the pool of Chip memory you need to remove the blue jumper – this basically has the same effect as cutting the track on JP7A.


512K RAM Expansion

512K RAM Expansion with blue jumper disabled to enable it to be seen as Chip RAM.


The final step (after putting everything back together of course) is to turn on the Amiga and check that it now has a full 1 megabyte of Chip RAM.  Easiest way to check for sure is to load up good old SysInfo and go to the Memory Information screen and you should see 1.0MB Chip RAM displayed. Job done!


SysInfo 1MB Chip RAM

SysInfo screen confirming 1MB of Chip RAM 🙂

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Gamer, gadget lover, retro Commodore computer fan and general all round geek.

15 CommentsLeave a comment

  • I have been reading that you can change a PAL to NTSC by switching to Fat Agnus chip if so were would I get one and what the real chip number

    • This isn’t something I’ve ever done myself but I have read about it. You need to make sure your Amiga has at least a Revision 6 motherboard and the Agnus you want is an 8372A which you will probably be able to pick up off eBay. You’ll also need to be capable of doing a spot of soldering to get it working. Here’s a good guide for you: linky

  • Hi, I have cut between pads 2/3, but don’t have a working multimeter, is there any other way to test if I have made the cut correctly, if I have made the cut properly what would happen after I turn it on etc, is there a way to test that way. Thank you.

    • Hi,

      Sorry I’m afraid I don’t know of any way to test it without a multimeter. You can pick a cheap one up on Amazon for 5 just quid though.

  • Thank you for your reply, I managed to borrow one and good job as first of all I didn’t cut the trace enough, then didn’t solder enough. Tested continuity and all ok, problem is I can’t now boot. I have the Terriblefire 530, could this be the reason or something else, thank you.

    • I’ve got no experience of the TF530 in an A500 but it’s possible. It’s also possible you might have inadvertently overshot with the knife and cut through another track. Inspect the area with a magnifying glass and check the tracks are in tact.

      When you say it won’t boot do you mean you just get a black (or coloured) screen or that it comes on but won’t boot off a floppy or CF card? Normal troubleshooting stuff would be to remove all expansions and unnecessary devices and see if it works then. Also check every chip is properly seated in its socket. Remove the TF card and put the 68000 back in, remove trapdoor RAM, any extra drives, joysticks etc and just have the A500, power and video connected – you should at least be getting the ‘insert disk’ screen. Good luck!

  • I am an idiot at times, I had the riser on Gary still with it’s own jumper settings, which was used for the previous Trapdoor ram. Took this off and working brilliantly. Thank you for your help.

      • So I’m onto step “cut JP7A” jumper pad trace on my rev6A- as my card jumpers are mysterious despite much written on the open design 2mb trapdoor men expansion card on abime .net! It has a jumper Ras 0, ras U2, cas 0 (1st 2 are closed), and I tried combos without luck so have to cut jp7- but I assume cutting the lower 2 pads and *not* soldering the upper 2?

        It’s been awhile since 1992 when I did this on my former A500!

        • Yeah assuming you can’t find the correct combination of jumpers on your RAM expansion (and you don’t want to buy a new one that is guaranteed to work) then you separate pads 2 and 3 on JP7(A) by cutting the track between them and then you leave it there – don’t solder pads 1 and 2 together.

          EDIT: There’s a guide on Aminet if you want some further reassurance. Here:

  • I have a question.

    I’m about to do this modification myself. I have a pal rev5 board with an 8371 upgrading to a 8372a and want it to remain pal, do I have to isolate pin 41 on the Agnus chip to do this.

    I have read elsewhere that this is the case with European versions.

  • thank you so much for clear simple instructions. I was able to upgrade to 1MB chip RAM without any issues. As I was on a roll, I also modified my GVP A500 HD drive to use Amiga power supply rather than it’s own external power supply following another blog. Very happy Amiga User today.

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