Amiga 500 Power Supply Build

The power supply of my Amiga 500 is a bit unreliable. I’ve had some issues with the machine where the PSU could be the culprit, so I thought that it would be better to get a new power supply.

There are used Amiga 500 power supplies occasionally available on online auctions, and there are also unused (but probably quite old) power supplies available on some online retailers. The issue with these 20-30 year old power supplies is that the capacitors are starting to dry. This can be a fire hazard, as old capacitors may even explode (this has happened to the PSU of my old IBM XT, it was not a pleasant experience). So in order to get safe and reliable operation from an old PSU, the capacitors should be replaced.

Rather than taking that route, I decided to use a modern PSU. Ian Stedman from the UK has designed an adapter that allows an ATX PSU to be used for powering the Amiga. He sells this adapter for reasonable price of ₤10.

However, the tricky part is to get the PSU connector. Amiga 500 uses a strange rectangular 5-pin DIN connector. This connector is extremely difficult to source. All net searches I did just resulted in discussions on various Amiga forums about how these can’t be found anymore from anywhere, and the last possible places where these have been sold lead to just dead domains. There were even rumours that Ray Carlsen – who builds replacement power supplies for Commodore computers – has made these connectors himself after his stash of connectors has run out. I even found some part numbers for the connector, such as PDI DP-215 and Kobiconn 171-4405 from Mouser, but these were obsolete and no longer available anywhere.

I had “Amiga” in the search terms I used, but while searching I learned that Commodore 128 has the same connector. Then, when trying with “Commodore 128” in the search query, I got a hit from an eBay auction of these connectors. Unfortunately the auction had just closed, but the seller informed me that there was more coming, so after some wait time I was able to get the crucial connector. At the time of writing this, the connectors are still available at Soigeneris.com.

Ian Stedman recommends genuine PicoPSUs for his adapter, but I obtained a clone from a local retailer, as I couldn’t find anyone who sells genuine PicoPSUs in Finland, and didn’t want to pay extra for the international mail.

Then I obtained the rest of the parts – case, cable etc. – from a local retailer. Below is the complete parts list with links:

And the same stuff in pictures:

The first step in the build process was to drill the holes for the cable, connector and switch. For the cable and switch I used an adjustable drill bit that makes drilling large holes easier. I also filed a small notch to the hole of the switch, as the switch has a ridge that prevents it from rotating when it gets locked in the notch.

The next step was the DIN connector. The cable is quite similar to the cable of my original Amiga PSU – there are four wires and conductive shielding. The shielding is connected to the outer metal shield of the connector, and to the ground at the PSU end. The four cables are 0V, +5V, +12V and -12V. The fifth pin in the connector (labeled as “shield” in some diagrams) is not connected to anything in my original PSU.

The first thing to do was stripping the insulation from the cable and wires inside, and tinning of the ends of the wires. The pins of the connector have hollow ends, and tinned wires can be easily inserted to the pins and soldered in place. Then the cover of the DIN connector had to be persuaded over the cable – this was quite difficult, as the cable was slightly too thick. I ended up stretching the gland of the connector with pens and other small tubular things to make it fit over the cable.

The next step was to solder the wires to the connector. The metal shielding of the connector can be just bent to fit tightly over the cable, but I also soldered the copper net to the shield of the connector to make sure the contact is  good.

For the order of wires I used the instructions from Ian Stedman’s page.

After soldering the wires to the connector, the bottom part of the metal shielding had to be inserted and the cover of the connector pulled over it. It snaps into place (and is rather difficult to open again).

I could have put some hot glue or other insulation inside the connector after soldering to prevent accidental contacts, but the solder joins were so tidy and well separated that I thought that it wouldn’t be necessary.

Next step was to prepare the other end of the cable. The Amiga ATX adapter has screw terminals, and the ends of the wires have to be tinned for good contact and durability.

Also, in the original Amiga power supply, there are five contacts: ground, 0V, +5V, +12V and -12V. In the ATX adaptor, ground and 0 are the same – as the power cord of the mini ATX supply has a Schuko plug, it was easy to check this with a multimeter from the ground contacts of the plug.

The shielding of the cable provides ground contact for the shielding inside the Amiga, so in order to get the Amiga properly grounded, it is important to connect the shielding. The zero pin alone doesn’t provide grounding for the shielding inside the Amiga, it’s just the zero voltage level for the electronics.

The shielding of the cable was twisted together and insulated with a heat shrink tube to form a fifth wire. It was then soldered together with the 0V wire – if the cable would be used with the original Amiga power supply this fifth wire should connect to the ground pad in the PSU.

The next step was to make the cable for the power switch. This was just some left-over cable, one end soldered to the switch – with appropriate heat shrink insulation for protecting against accidental shorts – and with a 2×1 pin header connector at the other end.

With the switch done, the final step was to assemble everything. The power cable was routed through the metal gland, and wires attached with screws to the Amiga ATX adapter. This was connected to the ATX DC-DC converter, and the PSU power cord was screwed to its hole in the box. The switch was just pressed in place, and the connector fitted to a header on the Amiga ATX converter.

For attaching the PCB to the case I used just some PCB holders with double-sided tape. There are just two holes in the Amiga ATX adapter and none in the ATX DC-DC converter, so only two of the holders are installed properly through the holes. The other two are shortened and attached to the bottom of the PCB to provide some extra support. The construction is however surprisingly stiff, because in addition to the PCB holders the power cable keeps the PCB in place.

Two dip switches on the Amiga ATX adapter should be set to “on” position. This is because the connector is of latching type. For a momentary connector the dip switches should be in “off” position.

The last step was to attach some small rubber pads to the bottom of the enclosure.

The final PSU consists of two boxes, the one I just built, and the power adapter that provides the DC current for the ATX DC-DC converter. It’s still smaller and lighter than the original Amiga 500 PSU. Based on the initial testing it also works fine with the Amiga, but future will tell if I can get rid of the issues I’ve been experiencing.

finished_psu

Advertisements

17 thoughts on “Amiga 500 Power Supply Build”

  1. Nice work! I like the way you bring old things back to life. I have an old Amiga 500 with 512 kb expansion that I put together again for a testplay. It worked fine for a while…then, surprise, surprise the power supply stopped working. I would gladly donate the whole setup or parts of it to you if interested. I happen to live in Inkoo too so there should not be any logistic problem. Och samma på svenska, ja sama suomeksi 😉

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s