The length of the ATX PSU makes for a tight connection when using a standard IEC-C13 connector so I used a right angled connector rather than a straight one.

Although somewhat "industrial", I found that a really effective way of fixing the PSU was to use perforated steel banding like that shown. This could be formed around the PSU and fixed with self tapping screws into the channels in the FDX base.

This means that the front panel can no longer be removed until the front PSU fixings have been removed. This could be problematic if you plan on reusing 5.25" drives, but as I was going to upgrade the drives to 3.5", this was not a problem for me, however, for completeness, I did check, and while it is very tight, with care, it is possible to refit the plinth with 5.25" drives on it.

If the new PSU was only ever going to supply the original FDX devices, the wires would have been adequate, but since part of the reason for replacing the PSU was to be able to provide power for some potential FDX upgrades, then due consideration should be given to the size of the existing conductors and rating of the mains switch.

The full load current of the selected ATX PSU is 2A at 240VAC.However, the largest current capacity of the PSU is for the supply of 3.3VDC (at 16.7A ~55W) which is required for ATX motherboards, but not used in the FDX. Similarly, the PSU supplies 5VDC stand-by power (at 1.5A ~7.5W) for an ATX which is again not used by the FDX. (These numbers do not take account of the additional load drawn by the transformer that supplies power to the MTX.)

Initially, since I was not going to increase the power consumption of the FDX, I was planning to re-use the original wiring and switch and leave the input fuse as 1A. As the "design" developed though, replacing the internal AC wiring did not require much additional effort so I upgraded the internal wiring to 1.0mm² - this is fine for up to 10A, well in excess of anything that I am likely to need but consistent with the mains IEC mains connectors that were being used. To support the capacity of the PSU, I increased the FDX input fuse to 3.15A.

ATX 20 Pin Connector Pin-out

As the table shows, in order to use this power supply as a replacement for the one in the FDX, the green wire (Pin 14) must be shorted to ground to turn on the PSU, this is normally what the power switch on the PC does.

The Molex peripheral connectors were left unmodified for connection to the FDX disk drives.

This fan is obsolete, but the RS Electronics website suggests a number of replacement tube axial fans, with flow rates in a range between 26-31 cubic feet per minute with a noise level of 28-36 dBA. Based on this data, I planned to fit the quietest DC fan the I could find with a similar air moving capacity .

The JF0825S1M-00 is rated at 32.5 CFM and 27.8 dBA but I went for low noise at the expense of some air moving ability.

This would also allow me to dispense with the existing FDX internal AC wiring, but I also think that it would probably be better to retain a main power switch as is typically used on larger PC PSUs and replace the IEC male socket on the rear of the FDX with one with an integral switch such as this. This is a "future enhancement" and has not been done at this time, mainly because I'd need to enlarge the mains inlet cut-out in the rear panel .

Proposed Low Voltage Switching Schematic

This was obviously undesirable, so I decided to install a relay which would only pass power to the MTX transformer when the ATX PSU was switched on.

This relay from Rapid On-Line can be switched by 9 to 21VDC and will be fed from the 12VDC output from the ATX PSU, the relay will then only allow 240VAC to pass to the MTX transformer when the ATX PSU is on. I have found that the same transformer used in the MTX PSU can get very hot during extended periods of use and I didn't want to risk accidentally leaving the FDX transformer power on for extended periods if I hadn't unplugged the FDX.

Of course other, smaller, relays would have been suitable, but I am happy with this one. Mark suggested that it would be a good idea to fuse the DC outputs from the ATX PSU - particularly as this PSU was capable of delivering far more power than the standard FDX supply, which seemed sensible.

My "simple" power supply replacement project seems to be growing beyond all recognition - probably a function of having too much time on my hands and enjoying "playing"!

The easiest way of fusing the DC power would have just been to add some in-line fuse holders, but since the relay for the MTX transformer was going to be DIN rail mounted and I had spare rail, as well as DIN rail terminals and fuse holders, I decided to go overboard and do the whole lot this way.

The end result is that the inside of my FDX now looks more like an industrial control panel than a vintage computer!

Main AC and DC Voltage Distribution

Note: If you are just replacing the FDX PSU functionality, then the additional wiring shown above is totally unnecessary, but it does make it easier to support future additional power requirements. It was only as I got towards the end of doing the wiring that I realised that to make replacement of the ATX PSU easier, should it fail in future, I should have kept the standard ATX power plug unmodified and made my connections to a suitable mating connector. Doing it as I have, will require remaking the PSU terminations individually should the PSU fail. If I replace the PSU in my other FDX, or this PSU fails in the future, I will make this change.

Now is a good time to check all the wiring, terminations and power supply outputs, since most of the DC distribution is not accessible after the drive plinth has been installed. When doing my checks, I was surprised to find that I had a low resistance between the +12V and 0V lines, this had me more than a little worried until I remembered that this FDX has a 39 ohm resistor between +12V and ground - probably to generate a minimum current draw for the Astec PSU. The Astec specification document lists the minimum 12V load as 0.3A, this is consistent with the resistor value: 12V / 39R = 0.30A.

(This photo shows the plinth with two 3.5" floppy drive adapters installed as I replaced the 5.25" disks at the same time as the PSU)

Reconnect the floppy drive cable to the disk controller and connect the data cable and power cables to the floppy drives.

Reconnect the video output cable on the 80 Column board

Refit the rear panel

Slide the top cover back in place and reinstall the second end-plate.

<The end>

You can see the new style power switch and the replacement 3.5" disk drives though - and even something happening on drive "C"! (It's "PIP"ing the contents of a CP/M System disk from Drive "B").

At this point, the FDX is working fine with the new PSU and, having replaced the 5.25" drives with 3.5", is actually using less power than before. I just need to find some suitable upgrades to take advantage of the extra power now available!