Amiga 500 – mods

… or Amigaaaaaaaahhhh !

Yup, above photo should be enough for a whole post but I decided that I’ll add just a bit more 😉

Mods

I am a happy owner of quite a few A500s in pretty crappy shape. I’ve finally managed to start some serious work with modding ’em.
I won’t post single machine mods but rather a several mods in a few Amigas that I’ve pimped.

As usual, I’ve started with simple things 🙂

Disassembly and sorting.




Keyboards cleaning and whitening.




Cases whitening in progress.


1MB CHIP RAM Upgrade

Next on my list (I’ve skipped standard repairs) was to upgrade CHIP RAM on motherboards and make it switchable – trap door expansion enabled/disabled. However, It turned out that I have quite a few different mobo revisions in disassembled Amigas – 3, 5, 6A and 8A.
I’ve decided that revisions 3 and 5 will be repaired, refurbished, cleaned and left in their original form.
6A and 8A revisions are far better for modding, plus has unpopulated vias waiting for RAM chips 🙂


I’ve decided that I’ll go with a standard 1MB upgrade. Yes, I know that 2 megs are easy to do on rev. 8A or 6A motherboards with Super Fat Agnus(8375) but I just don’t have that many spare 8375s.

Like I wrote, this upgrade has to be switchable. I wanted to achieve two options. The first, where I would have 1 MB of CHIP RAM available but with trapdoor expansion port disabled and a second option, 512 KiB of CHIP RAM available and trapdoor expansion ready to accept another 512 KiB of RAM. This is necessary because some stubborn programs (hardcoded memory regions) simply won’t run without expansion port RAM.
In order to achieve such mod in a 6A mobo, two jumper pads (JP2 and JP7A) has to be connected to a dual 3-way-switch. First jumper pad (JP2) is located between Agnus PLCC socket and CPU socket. It enables/disables more than 512KiB of onboard CHIP RAM.



Second jumper pad (JP7A) is located near the expansion port and it simply responsible for turning on and off expansion port.



Both jumper pads have bridges by default and both bridges have to be cut/desoldered.
Jumper JP2 has three pads and all of ’em has to be connected to a 3-way-switch.
JP7A only requires two wires instead of three and these go to the second section of 3-way-switch.

In a rev. 8A motherboard, this is a bit more complicated.
There are three jumper pads involved – JP2, JP7A, and JP3.
JP2 and JP7A are configured the same as in rev. 6A.


The easiest way to describe the changes needed is:
JP3 has four pads that are by default connected like this -> “||”. This has to be changed to following config -> “=”
Pics made prior to cleaning of course 😉






I began testing this mod and started to get various, random GURU MEDITATIONS. Internet says that it is just a matter of re-seating a FAT AGNUS chip.


It is the only chip in a PLCC socket.
After several tests, I figured that it is simply not enough.
Five out of six times cleaning was needed as well but that still didn’t help much and I was still getting random crashes.




The simple measurement revealed that chip is 3.3 mm wide (including pins)




I’ve tried to squeeze pins with a clamp but that yielded poor results – 3.35 mm




I’ve ended up moving each pin individually with a small screwdriver.
Mid-work photo so you can see a difference.




Later, I’ve noticed that FAT AGNUS comes from various chip factories – the Philippines and US.



The one from the Philippines is a bit easier to work with because of small pockets right underpins.

Although 4.7 mm was much better it still wasn’t enough to make proper contact with a socket and resulted in much less crashes but still, a system wasn’t stable enough.
After some work with a PLCC socket and decent cleaning, it finally stopped crashing.




Internal modulator – S-Video and Composite

Next on a list was a built-in modulator.
I’ve described it in one of my previous posts



This module is available at retrohax.net online store and requires minimal soldering skills. Only six wires are needed – RED, GREEN, BLUE, SYNC, GND, +5V

I had to run some tests on both (6A and 8A) motherboards to see if there are no unexpected pitfalls or surprises 😀
I’ve soldered signal and power supply wires to soldering pads near RGB socket.
Here is how it looks on both mobo revisions.






Tadaaaa! It worked like a charm, plus s-video is very crisp and the composite is way better than through original A520 modulator.

Composite video


S-Video

Now it was just a matter of finding a nice spot to drill holes for sockets.
I must admit that drilling a plastic case with a 12mm drill us extremely tricky.
After a couple of failures, I’ve ended up buying a tool that did the job nicely.


A bit of work with the file later …

Below you can see an already mounted modulator.

Here is how it looks from the outside.

Dual KickStart – 1.3 / 3.1

I also wanted to swap original ROM for a bigger chip that would host two kickstarts, switchable of course.
That was achieved by burning two kickstarts on a 29F800 that was soldered to an adapter.


Chip programming was done using my super crappy Willem EZO programmer only because I already have an adapter for such a chip package. …. and yes, it takes ages to program it via Willem …




Running some final tests 🙂

GOTEK – USB Floppy emulator

I’ve bought several GOTEK drives to arm my A500s.

The idea here was to install a GOTEK USB floppy emulator along with an original Amiga floppy in case I wanted to run some software in an old-school way 😉 Of course, it had to be switchable. As usual, there were some tricks involved.
The first tricky bit was to actually flash a GOTEK with new firmware.
When I was playing with it, the only available open source firmware was a Cortex. It is an excellent piece of software. However, not so long ago, FlashFloppy firmware was released which is another awesome alternative, plus it runs on Amiga, Atari ST, Amstrad, etc.
Unfortunately, I wasn’t able to program GOTEK through a usb2serial dongle (Yes, I’ve tried all three of ’em – pl2303, FTDI based and cp2102) and even soldered directly to a chips RX/TX lines …. nothing worked. Fortunately, there is an alternative method of flashing ’em – through a USB with an STM32 in DFU mode.
Jumpers setup for flashing in DFU mode.

Flashing in progress 🙂

Below you can see pics of both firmware in action 🙂



I’ve prepared some 34PIN wires, USB thumb drives and I was ready to run some tests.





After successful tests, I could start working on the installation itself.
The first thing that needed a small mod was a bottom metal shielding.
The small part had to be removed from the one on a picture below

Troll mode on

Amiga is soooo awesome that even its shielding does demo effects lol 😀
0 CPU EFFECT below

/Troll mode

A case had to be prepared as well.
I decided that the best place for a bare GOTEK PCB will be under the original floppy drive. I had to cut holes for tact switches, USB socket, and LCD.
At first, holes were drilled and extended to proper size with a file.
Here is how it looked.






With a GOTEK in place.





…and with a floppy drive on top of it.
Fits perfectly.






Assembled and ready for final testing 😉


That’s all for now. I am currently working (a shitload of projects would be the best description here) on hydrographic projects that will sort out problems with cases that are in very bad shape. Hopefully, I will be able to present ’em soon 😀

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4 thoughts on “Amiga 500 – mods

  1. Like to know if you have a template that I can use to drill the hole of the gotek in the same are that you did the mod.

    1. Actually, I do, but I’ve made it out of a sheet of copper. However, you just gave me a cool idea. I can redo it in software and upload to blog 🙂
      I don’t know when I’ll manage to do it though.

  2. Hi can you help I’ve got an a500 rev6 board and I’m getting I suppose it’s a light green, and green light blinkin, I already tried to reseat angus 8372a several times, exchanged cia’s , reseated rom , and other chips. I felt some of the soldered rams were warm . this machine had an a501 installed. What I tried was to disable all the VCC supply going to all rams asto check if I still got a green screen. I disabled voltage to rams and still green screen. So what could be the issue? . Is there any jumper I need to move to enable the external a501?

    1. Hi Paul. By green light, do you mean green screen after turning on A500?
      If yes, then it indicated a CHIP RAM problem. Most often is a PLCC socket issue where FAT ANUS is. Reseating doesn’t always solve a problem. Remove Agnus and check if the pins of the socket are not corroded.
      Another thing to check is RAM onboard – 4 chips. They can go warm while amiga is powered on but look for a chip that is really hot. If that doesn’t help RAM has to be swapped with new chips.
      If it is about A501 there are usually two jumpers that can cause issues JP2 and JP7A. JP7A disables trapdoor expansion connector. Also, check for a jumper or switch on a A501 itself.
      In general, there may be several causes. Let me know if above helped 🙂

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