Amiga 4000: PSU Mod

My power supply died again. It has been very unreliable for some reason (probably bad caps, or overload), so I finally decided to mod an ATX power supply for the 4K to give it more power and reliability. I based my mod on a mix of 2 other ATX mods:

I took the PSU case mod part of this: and the ATX part of the mod from this: which gets you the concoction that I came up below. It’s not too bad I think, other than the not-so-straight cut lines. I put links at the end of this post below so you can find everything that I bought for this mod easily.

A couple of notes if you are going to attempt this mod:

  • You don’t need the ATX conversion cable that is available. It won’t actually do anything more for you than the mod does.
  • Some people say that you can’t use a standard AT switch as a power switch (on/off non-momentary), this is not true. In fact, on an ATX motherboard, the logic behind the momentary switch you press does exactly what a physical switch would do, short the ATX green and ground wires and keep them shorted until the button is pressed again or the motherboard tells it to turn off, which is the reason for the design. This mod does exactly that, short the green and black together to keep the PSU running. In my 15 years of computer repair I have done the “paperclip trick” to do this also, and have never had a problem. This is good news for you, you can save $30 just in that, but at the cost of a little bit more labor.
  • Use a proper grinder to cut out the vents on the back of the PSU. I used a Dremel, but there doesn’t seem to be any way to control it enough to get a good straight line.
  • I am NOT responsible for what you do with this mod. By reading below you agree that I am not responsible for what happens to your Amiga, even if you break it or blow it up.

Ok, on to the meat and potatoes.

First, strip out all of the guts of the existing power supply, but do it carefully. we need to save the switch and the socket that goes to the mains. I cut all of the wires at the psu board, since I was going to re-strip them and solder them into the new power supply. In hindsight I probably would have taken the time to de-solder them instead.
I don’t have any pics of this, but it should be straight forward, just don’t break anything!

Then, dissemble the rest, removing all of the parts, pieces, and screws. It should come apart in many pieces. Now it’s time to make the modifications. Just a side note, I normally don’t ever physically alter my classic computers. In this case, since the original power supply was undersized, it’s worth it.

Amiga4kPSU_Hack1 Amiga4kPSU_Hack2

On the back, you can see where my cut lines are. The female plug to the monitor is never really used anymore (since the invention of the power strip!) so we can sacrifice that connector, and free up the space to move the mains connection up. That leaves us with a nice big spot to cut open with. Those louvers are very restricting and have to go! I also cut out the fan grille on the other side to replace with a standard one that sticks out so there’s lots more room for air to flow. Use your grinder and have at it. I used a Dremel since that’s what I have. It works, but it’s very hard to control.

Now, to fill the giant gap in the back that was created, I had purchased something called “modders mesh”. It’s kind of like little chicken wire. It’s thin and easy to cut. The fan hole will just get a standard fan guard, but since it’s a standard 80mm, you can go wild. Since I don’t have access to a spot welder, I decided to use epoxy to put the mesh on, in hindsight I would have but more effort into finding someone with a spot welder. Thankfully it’s hidden, but it’s very messy, and doesn’t look very professional.


After cutting the mesh to size, I applied the epoxy as carefully as possible. Unfortunately it wasn’t my all time favorite J.B. Weld substance, so it was a bit messier than anticipated. I used a rock to weigh the mesh down while the epoxy set. As you can see there is a happy family there that seems pleased with my mods. I don’t have a picture of the fan side, but it’s just as easy as cutting out the grille part of it in the middle, don’t modify the circle at all.

Next comes the paint. I chose black to hide all the marks that showed up, and because I like black. Another in hindsight thing for this is I would have waited longer and put another coat on, and probably something to seal it with. The paint has already pretty much scraped off any spot where it touches the computer case.

Oooh, shiny!

Now, while it’s getting ample time to dry (and I mean AMPLE), time to work on the insidey parts. It’s time to make modifications to the new power supply. Yes, this will void your warranty. Don’t expect to be able to return it if you mess it up. My advice is to not mess it up. 🙂 Seriously though, take your time and double/triple check things, just so you know it’s right.

First disassemble the new power supply like you did the old one. I used the fan from this power supply too, so keep it connected. In hindsight I would have bought a quiet 80mm fan, since the stock one sure isn’t. Eventually I will get around to changing it out.

First order of business is to get rid of that 110v/220v red switch. This switch is actually important, but we don’t have a place to put it, and honestly unless the machine gets shipped to a different country, it would never get used. Now, using a multimeter in continuity mode, you can tell what that switch does. In this case in the closed position was 110v. This means if we get rid of the switch we still need to make the connection somewhere (since I live in the US). Now, in 220v mode, the connection is open, so this can be skipped. Don’t take my word for it though, test it to find out. Seriously, test it first.

atxpsu_mod_115vswitch atxpsu_mod_115vswitch_boardatspsu_mod_115vswitch_board_bridge

Simply put a bridge over the two spots where the wires lead back to. I de-soldered the wires and used a small piece of stripped wire to make a good quality connection without having to worry about a soft solder bridge. You can see where the bridge is made in the lower right corner where there is a space cut out of the PCB. Then de-solder all of the wires going out to all of the internal power connectors, everything, ATX plug, all of it.

Next is to switch out the old power wires from the new psu to use the socket from the original power supply. I used some of the wire from a line of molex connections I knew I wasn’t going to use. Just be really careful here that you don’t mix HOT and NEUTRAL!

atxpsu_mod_board_power atxpsu_mod_powersocket

I used Black for hot as normal, and yellow for neutral. If you have some white wire sitting around, I would recommend that. I de-soldered and soldered one leg at a time so I was sure not to mix them. Now to solder the power switch in. Find where the green “power on” wire was soldered to the PSU (this particular one had everything marked clearly). I used the same green wire from the ATX connector and a black wire from it. Solder the green into the appropriate spot and solder the ground into one of the ground holes. Then solder the two leads to the switch. Order doesn’t matter, but make sure you put them on the same side of the switch, otherwise you will never get a connection.


Now solder all of the other leads in. In this picture you can see I had already done that, which was ok, but a pain to work with since all of that was in already. Below is a couple pinouts so help you along. I added back in all of the Amiga original connections, plus another set of molex connectors with a floppy connector on the end for my CF card adapter.

WARNING: The Amiga uses YELLOW for +5v!!! DO NOT CONNECT AMIGA YELLOW TO +12v!!!! Amiga uses ORANGE for +12v!!! Double check, and then double check again that when you solder the connections on, the Amiga connections are done correctly. You WILL fry something if not.


This was taken right after I soldered on the motherboard connector. The yellow wire is very thick, in hindsight I might have soldered in two short yellow leads and soldered the larger lead to them, using some heat-shrink tubing to cover it. My goal was to not have any slices. I ended up cutting off some of the strands, which isn’t the best way to do it, but I haven’t had any problems yet. Amiga blue is Ground.
Here are some VERY useful pinouts/color charts for the Amiga side: (also seen at the bottom of post in case site goes offline). All connections for the motherboard power connector must be made (correctly). The PSU should also have a power good connection for the brown wire.

WARNING: Amiga uses RED for -12v!!! DO NOT CONNECT AMIGA RED TO +5v!

Double check to make sure the connections are made right. I can’t stress enough how important it is that they are right. Amiga gear is very expensive, and it would be a shame to have something blown up because someone was in a hurry.

Whew, now that’s all done, we can start assembly.

I got a package of generic standoffs to use for the new psu logic board, since I knew it wouldn’t fit on the screw spots on the case. Unfortunately I couldn’t find any that didn’t have threads on the bottom. I really wanted to find some of the sticky backed ones that I know used to exist. The answer though was just to chop off the threads.


Now, we can use hot glue to attach them to the case. Wait until the very end to do that though. For this PSU, you need to hack 4 of them.

Now that the paint has dried, it should look something like this put together:

AmigaPSU-Painted2 AmigaPSU-Painted

Except you know, better than that, but hey, it works.
Now the easy part, re-assemble the power supply in the case. Hot glue the standoffs in place, and viola!


As you can see I ended up splicing together a longer ground wire. I decided it would be a good idea to ground the power supply, so it goes down between the board and one of the standoffs. I didn’t have any heat-shrink on hand.


All put together. Not too bad, except the crooked cuts. Once again I totally ignored the front side when taking pictures, but there is a cheap and simple fan guard on the front. Now all that’s left is to put her in, plug her in, and watch some demos!

Tools List:
Soldering Iron
Hot Glue Gun
Spot Welder / Epoxy (J.B. Weld)
Philips screwdriver
Wire strippers/cutters
Parts List:
Hot Glue
Blue Star 325W 20+4-pin mATX Power Supply w/SATA
Modders Mesh
80mm Fan Grill
There is a side effect to doing this mod. An ATX power supply gives you 3.3v, which is a good thing if you want to use a modern-ish video card on a Mediator board!