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The Memotech MTX Series

Memotech Hardware Repairs


FDX Power Supply Replacement

A word of caution: I am not an electrical engineer, the information on this page is provided for "information only" in the hope that it may be useful - however, it should not be construed as a authoritative document on the FDX PSU or its replacement. You use it at your own risk and if you are not competent to work with mains supply voltages, which can kill you, leave it to someone who is and put your equipment into an electrical repair shop. [This probably applies to me too, but, anyway...]

My FDX PSU appeared to be working - the DC voltages were slightly low, but should have been adequate to power the FDX. The system booted and the disk drives appeared to work normally, but there was a problem with the 80 Column card output, the display "shimmered" quite badly. Swapping the 80 Column card had no effect on the symptoms and further investigation revealed that the PSU's output voltages had large variations.

Given the age of the PSU, rather than attempt further repairs, (I had already replaced a blown filter capacitor,) I decided that it would be better to replace it with a more powerful modern PSU. Peter Kretzschmar had already identified a possible replacement that would fit inside the FD case - a HEC-200SR ATX PSU.

To access the FDX PSU, you need to almost completely dismantle the FDX, you might want to refer to my FDX disassembly page before you start. On this page, the starting point is assumed to be that you have removed the existing PSU and are ready to install a replacement.


Original FDX Power Supply

The FDX used an Astec AC8151-01 to provide power to the 6" PCB card frame and the floppy disk drive(s).

There is a copy of the Astec Specification and Repair Manuals on the Manuals page

Specification Summary

+5 VDC

2.5 A

Input Voltage 115 to 230 VAC

+12 VDC

2.0 A

Input Current 0.85A (rms)

-12 VDC

0.1 A


Original FDX AC Power Distribution

The power distribution wiring inside the FDX is rather "interesting", so I needed to unscramble it to see how best to install the replacement PSU.

The illuminated DPST rocker switch on the FDX front panel supplies power to the input of an EMI/RFI suppression filter and the connections to the 240VAC case fan are tapped off the input connections to this filter.

The output from the filter is split to feed the FDX Astec AC8151 PSU and the transformer which supplies MTX power.

Initially, I was going to keep the existing fan and filter, and only replace the PSU, wiring it to the input side of the filter.

However, since the ATX PSU had extra capacity available, I decided to replace the fan with a quieter DC fan and decided that I could remove the suppression filter. This had the benefit of making the replacement wiring simpler too.


Original FDX Power Distribution with more clarity (hopefully!)


Proposed FDX Power Distribution Using ATX PSU with Mains Switching


HEC-200SR ATX Power Supply
Specification Summary
Input 115/230Vac ~ 4/2A 47-63Hz
Output +3.3V 16.7A
  +5V 12A
  +12V 10A
  -12V 0.3A
  +5Vsb 1.5A

 ATX Power 20Pin, 3 Peripheral 4Pin, 1 Floppy 4Pin, 1 ATX Pin

Dimensions 190L x 81.4W x 42H mm (7.5" x 3.2" x 1.7")


There are a number of options for mounting this ATX PSU inside the FDX, including fitting it inside the rear curved panel section, but this would probably require removal of the case fan and the internal PSU fan would likely not provide adequate cooling for the FDX card cage. I chose to mount the PSU in pretty much the same place as the original FDX PSU, although it is somewhat longer than the original, the HEC-200SR can be fitted below the floppy disk plinth, mounted lengthwise, with the IEC-C13 power input at the rear, the DC output wires at the front and the PSU fan exhaust facing upwards.

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.

Not wanting to butcher the FDX case too much, I looked for a way to secure the PSU with minimal damage, i.e., without drilling extra holes in the case etc.

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.

The ATX PSU fixed to the base of the case, at the rear, the securing strap is fixed to the rearmost preformed channel on the base. To allow more clearance for the power input connector at the rear, the front of the PSU is fixed to a channel in the bottom part of the front cover panel.

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.

The output of the original PSU was rated at only 40W, requiring in an AC input current of up to 0.85A (rms) and the internal mains wiring was sized appropriately for that current draw, i.e., the wires were small, I would estimate them to be about 0.5mm2. The fuse in the FDX power receptacle was a 1A "anti-surge".

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

Pin Name Colour Description FDX Use Comment
1 3.3V Orange    +3.3 VDC N fused spare
2 3.3V Orange    +3.3 VDC N -
3 COM Black    Ground  N spare
4 5V  Red    +5 VDC Y fused
5 COM  Black    Ground Y card cage
6 5V Red    +5 VDC Y power switch
7 COM Black    Ground Y power switch
8 PWR_OK Grey    Power OK (from PSU, +5V=OK) N spare
9 5VSB Purple    +5 VDC Standby Voltage N spare
10 12V Yellow    +12 VDC Y fused
11 3.3V Orange    +3.3 VDC N -
12 -12V Blue    -12 VDC Y fused
13 COM Black    Ground Y 12V return
14 PS_ON Green    PSU ON (Short to ground to power ON) Y power switch
15 COM Black    Ground N spare
16 COM Black    Ground N -
17 COM Black    Ground N -
18 -5V White    -5 VDC (Not available in HEC-200SR) N -
19 5V Red    +5 VDC N -
20 5V Red    +5 VDC N -
Not required for FDX

ATX PSU Power On Sequence

Power applied to the PSU

Pin 9 (standby voltage) goes from 0 to 5V


Pin 14 (PS_ON) goes from 0 to 3.7V

Pin 14 shorted to Ground

PSU is turned on


Pin 9 (Power_OK) goes from 0 to 5V

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.


It is highly unlikely that I would ever have needed to reuse this PSU in a PC, so I decided to use the 20 Pin connector to break out wires for the FDX card frame rather than wiring it into a Molex peripheral connector and insert a shorting link between pins 14 and 15 to have the PSU turn on when AC power is applied. The rest of the terminals in the connector were going to be left in place to make them available should they be required for future expansion.

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


The original case fan was a Toyo fan, model number TF80230RAW, an 80mm, 230VAC unit.

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 .

I selected a JF0825-00 Series fan, the JF0825S1L-00, it is a 12V model with a flow rate of 25.5 CFM and a noise rating of 22.2 dBA.

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.

As shown in the earlier diagram, I had intended to reuse the original FDX power switch to switch mains power to the ATX PSU. I have now decided that it makes more sense to use the PSU in a similar way as in a PC, i.e., to have the facia switch turn on the PSU using the ATX PSU_ON signal.

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

I had some difficulty in sourcing a suitable replacement illuminated facia switch that would operate at low voltage. Mark Kinsey pointed me in the direction of this switch from Rapid On-Line. It is a SPST rocker switch, but crucially, has an internal LED powered by a separate circuit. (To limit the current flow through the LED, resistor needs to be wired in series with the LED.)

As the facia switch would now only control the output from the ATX PSU, this would mean that the transformer supplying low voltage AC to the MTX would remain powered on when the FDX power lead was live and the ATX PSU was on stand-by.

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.


It is possible - just, to install the disk drive plinth with the MTX transformer in place, but it is much easier if the transformer is removed. The most awkward task is mating the holes in the front edge of the drive plinth with the hex headed bolts that are held captive in the channel in the front panel. With the transformer in place, there is much less room to manoeuvre the plinth to mate it with the bolts.

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.


As I was finally fitting the drive plinth, I found that there was not quite enough clearance between the top of the PSU and the drive plinth to allow the plinth to be installed easily. I probably could have "squeezed" the edge of the plinth over the PSU but I didn't want to risk damaging anything - particularly the new wiring. Removing the PSU fan finger guard created enough clearance to make the fitting easier - with the PSU now inside the FDX chassis, the finger guard is not required anyway. (Having removed the fan guard and spacers, I shortened the screw in the bottom left hand corner of the fan to prevent if from fouling the ground wires as they exited the PSU case, the other screws were unaffected.)
The drive plinth is fixed by two machine screws through the base, secured with nylock nuts - as shown here, these are easily accessible, as is the captive bolt in the right hand side of the front panel. The captive bolt in the left hand side of the front panel is a little more problematic. The FDX fan exhaust panel must be removed to be able to install the nylock nut on the captive bolt. This is quite awkward and needs many partial turns with a small spanner to tighten up the nut - I found that I could only rotate the nut by about 1/8th to 1/4 of a turn each time. (Removing the card cage would allow easier access to the nut if you prefer.)

(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)

The MTX transformer and the front grill panel can now be refitted and that is just about it . . . .

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>

A photo of my refurbished FDX - obviously, you can't actually see the new PSU - but it's there - honest :-)

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!


Replacement Wiring

Conductor Size (XSA) Strands Rating
Live, Neutral & Earth 1.0 mm2   10A
+3.3 VDC 0.5 mm2 16 / 0.2 3A
+5 VDC (tri-rated) 0.5 mm2 16 / 0.2 11A
+12 VDC 0.5 mm2 16 / 0.2 3A
-12 VDC 0.5 mm2 16 / 0.2 3A
0V 0.5 mm2 16 / 0.2 3A
0V 0.75 mm2 24 / 0.2 4.5A
Power On 0.5 mm2 16 / 0.2 3A
+5 VDC (stand-by) 0.75 mm2 24 / 0.2 4.5A
PWR_OK 0.5 mm2 16 / 0.2 3A


FDX Mains input 3.15A (F)
ATX PSU supply 2A (F)
MTX Transformer supply 0.5A (F)
+3.3 VDC (spare)
+5 VDC 2A (F)
+12 VDC 0.5A (F)
-12 VDC 0.5A (F)




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