5560 ATX PSU Mod

The Problem
Possible Solution
Implementation

Warning: The power supply unit contains dangerous voltages, even when unplugged! Improper manipulation may lead to an injury, death, fire, and/or other damage. The author is not responsible for your actions and their consequences. Proceed at your own risk!

Content by Tomáš "Major Tom" Slavotínek.


The Problem

The PSU in my 5560-W1 had a problem - it would rarely power on, and even if it did, the output voltages were unstable, and the Power Good signal never became active. The problem was accompanied by a "coil whine" in the standby transformer - this was always present, even with the PSU shut down. I've narrowed the problem down to the input stage - more specifically the part that likely deals with the 100-125/200-240 V auto-ranging (based around 2 triacs and 1 phototriac). I've discovered a few suspicious caps and a whole bunch of crummy solder joints. Addressing these problems made the PSU more stable (incl. the Power Good signal), but sadly it wasn't stable every time and usually not for long either. The input stage is controlled by an unknown circuitry on a ceramic board. I say "unknown" because the entire thing is potted by some compound, making further analysis and potential repair difficult and time-consuming.

After a few more experiments, I've decided to put the original PSU guts aside and replace them with something slightly more modern, and more importantly - functional.


Possible Solution

It's Time to Choose...

The most obvious choice is to replace the unit with a regular ATX power supply. But things are not that simple - we can't use just any ATX PSU - we have to respect the original current ratings. Especially the 5 V rail can be problematic as the original unit can supply up to 28 A @ 5 V - this is what powers all the logic (planar, processor complex, adapters, disk logic...).

For this reason, we want to choose a supply that conforms either to the original ATX or the ATX12V 1.x standard. Most PSUs from that era should be capable of supplying enough power @ 5 V - the rail is typically rated at 25 - 30 A for most 300 - 350 W units. The later ATX12V 2.0 standard redefined the power distribution, and the PSUs supply most of the power through the 12 V rail - instead of the 3.3 V / 5 V pair. The 5 V rail on these "new" units is typically rated at 20 A or less, which isn't ideal for our use case. (One could brute-force it and use something like a 750 W supply with a high enough current rating @ 5 V, but that's not great either - the PSU won't be balanced properly, plus these "larger" units aren't exactly cheap either).

The ATX12V 1.x units are probably the best choice - they are newer than the original ATX bricks and tend to be more robust. But you can use whatever you have available, as long as it fits the current requirements.

Form Factor

Luckily for us, the standard ATX units are somewhat smaller compared to the original 5560 unit:

5560: Height: 86 mm front (96 mm back), width: 178 mm, depth: 196 mm
ATX: Height: 85 mm, width: 150 mm, depth: 156 mm

This means that in most cases the ATX guts should fit to the original 5560 PSU with no problem. We should even have some extra room for the wiring and whatnot.

Wiring

Wiring is what we need to discuss next. Since we have decided to go with the ATX or ATX12V 1.x PSU, we are stuck with the older 20-pin ATX motherboard connector:

The availability of the other connectors and their count may differ from one unit to another. Generally, there should be a combination of the following plugs: the 12 V motherboard (CPU) supply, possibly a 3.3 V / 5 V AUX connector, and then at least a few "Molex" disk drive connectors.

When we compare the pinout of the main 20-pin connector to the 5560 PSU pinout, we can see that all required voltages and signals are available to us on this one plug. We will need two additional 5 V conductors and one 12 V line, but that's not an issue, we can borrow these from one of the other connectors. Alternatively, we can splice the original "20-pin" harness to add these lines (this is ok to do if we don't push it too far - the connectors used on the ATX PSUs are typically rated for 7-10 A per pin, the 5560 planar plugs for 4 - 5 A).

There are many ways of wiring the adapter cable - the picture below illustrates the method I've decided to use:

Notes:
Pins marked with a red cross are unused.
The used colors match the original 5560 scheme rather than the "standard" ATX one.

As you can see, I've derived one 5 V and one 12 V line from an additional "Molex" connector, and one 5 V line on the 20-pin connector was spliced - a compromise between cabling complexity and robustness of the solution.

You may be curious about the -PS_ON (-ON/OFF) and PWR_OK (Power Good) signals and whether they need any special treatment. The answer is no. The way these signals work is the same for both the ATX and the 5560 PSU design - we can wire them to the appropriate pins, and that's it, no additional circuitry is needed.


Implementation

Now that we have a fairly good idea about what it is that we are trying to achieve, it's time to realize it.

Planar Adapter

Let's start with the cable adapter. For this, we will need a cable crimping tool and the appropriate terminals and connectors.

Possibly the easiest way to approach this would be to snip the original ATX connectors and replace them with the "new" terminals. However, this approach has one major downside - if the ATX PSU fails or you decide to use a different model, you will have to do all this work again. (or you could desolder all the wires from both units and swap the harness, but that can be hard because the wires are typically bunched together and flooded with large amounts of solder)

A more future-proof approach is to buy one of the 20-pin ATX extension cables or am ATX 20- to 24-pin adapter and modify it instead. You can easily unplug this ATX to 5560 adapter later on and use it with another PSU. That's the approach I've decided for:

The 20- to 24-pin adapter I've used was about 15 - 16 cm (6 inches) long, and that proved to be the perfect length (I've lost perhaps 1 cm by cutting the old pins and crimping the new terminals). The used wires are 16 AWG - the same as the original ones. The black retention bracket is from the 5560 PSU and comes quite handy even when building the adapter, as it holds all the wires in place and in the correct order.

Three additional conductors - 5 V, 12 V, and Ground - go to a standard male "Molex" connector. There were enough ground connections on the 20-pin ATX connector, but it's a good practice to have a return path on every power connector.

When you have the adapter ready, check it for any defects - especially shorts. Then test your work by connecting it to the ATX power supply you intend to use as a replacement, shorting the -PS_ON pin to one of the grounds, and measuring the voltage on all the other pins (PWR_OK/Power Good should be logic high - between 3.8 and 5 V).

If everything checks, you can connect the adapter to the 5560 planar, "flip" the switch, and see if the systems works (you should also double check all the voltages again).

Molex Adapter

The 5560 PSU has one male "Molex" plug for the drive power harness. Once again, you can either crimp a male plug in place of one of the female ones - directly on the original ATX wiring harness or build a male-to-male extension (see the photo above).

Mounting

When we are done with the electrical part we have to figure out how to hide the ATX guts inside the 5560 PSU shroud. The actual method will somewhat differ from PSU to PSU and person from person.

I didn't want to modify the original PSU shroud in any way - in case I wanted to return to the original 5560 PSU electronics later on. In any way, the ATX PSU board is somewhat smaller than the 5560 one, so you will have to get creative with the mounting solution. There is a metal post in the middle of the metal base - make sure it doesn't touch any of the parts or traces (don't rely on the solder mask as your only insulator!) - that could easily lead to the destruction of the power supply! I've decided to reuse the metal base of the ATX PSU - removing only the front and back panel and using the rest as an interposer between the PCB and the 5560 PSU base. That solves not only the mounting problem but also puts the PCB way above the annoying metal peg. I've drilled new holes into the ATX base - some of which mate with the original 5560 mounting tabs other are used for brass standoffs... That proved to be an easy, yet robust solution. I've also used the original base to mount the passive PFC choke (originally mounted on the back panel).

Whatever you do, make sure the new PCB is not shorted anywhere and is secure enough - keep in mind that the board will be upside down once you install the PSU to the 5560 unit, so it can't be just "flapping in the breeze"...

AC Input

We are almost there... The next item that needs our attention is the power socket and the input filter/fuse board. You could potentially reuse the original 5560 solution, but you will have to find some way to mount it because the original mounting post that went through the PCB is now gone. I had trouble finding a good way to mount the rather bulky board, and I wasn't happy about the clearances either. So I've instead decided to remove the 5560 input board together with the large ferrite bead. I've only reused the original AC plug. The ATX PSU had a much smaller input filter inside it, so I've modified the PCB a bit by relocating one of the terminals and drilling a mounting hole in its place. I've then used this hole to secure the board to the ferrite bead post on the backside of the PSU. I had to replace the original wiring with better and longer wires because they wouldn't reach the connector on the base PCB. The other set of wires is soldered directly to the AC socket. The small PCB has some clearance from the other side of the PSU cover, but I've decided to add a plastic sheet on top of it... just in case.

Fan Connector

The last thing is the fan connector. In my case, this was by far the easiest part - both the ATX and the 5560 PSU used the same type of connector, both used a 12 V fan, and even the polarity was the same. "Plug-n-Play"... If you are less lucky, you can either replace one of the plugs or connect the fan directly to one of the rails with the appropriate voltage.

Assembly

Now we just have to spend some time putting everything together - trying to make it as neat and tidy as possible... (I've decided to remove all the unused power connectors and wiring - leaving only the 20-pin motherboard connector and one dual-"Molex" cable in place). Once you are happy with the result, close the PSU and test it outside of the machine before connecting it to your precious PS/55.

From the outside, you can't even tell that it's not the original PSU.

At least until you start peeking through the vent holes...

Content created and/or collected by:
Louis Ohland, Peter Wendt, William Walsh, David Beem, Tatsuo Sunagawa, Jim Shorney, Tim Clarke, Kevin Bowling, Tomáš Slavotínek, and many others.

Ardent Tool of Capitalism - MAD Edition! is maintained by Tomáš Slavotínek.
Last update: 19 Nov 2021 - Changes & Credits | Legal Info & Contact