Computers Overview
Commodore PET
Sinclair ZX80
Sinclair ZX81
BBC Micro
Sinclair Spectrum
Memotech MTX
      FDX Power
      FDX Systems
      MTX Capacitors
      MTX Keyboard
      MTX Power
      MTX RAM
      MTX ROM
      MTX Speculator
      MTX VRAM
      Spare Parts
      Video Wall DDFS
    User Groups
    Video Wall
Memotech CP/M
Atari ST
DEC 3000 AXP
Raspberry Pi



The Memotech MTX Series

Video Wall DDFS

Power Supply Replacement


The Video Wall DDFS

The heart of the Memotech Video Wall system is the Distributed Digital Frame Store (DDFS), containing a number (up to 10) of video frame store boards and their associated controllers (2 per DDFS).  The electronics were powered by an Astec "SA130-3400", producing, 5V@15A, +12V@5A,-5V@0.7A and -12V@0.7A.

Unfortunately, the DDFS in the Video Wall system that I bought in June 2013 was DOA - the power supply was totally dead, although mains voltage was reaching the PSU and the internal fuse was intact - there were no DC outputs from the PSU. The original plan was to repair the PSU, but the job turned into sourcing and installing a compatible replacement.

The Original DDFS Power Supply (failed)

The PSU is mounted on the reverse side of the right hand end-plate.

Looking into the DDFS with the right hand end-plate and PSU removed.

The PSU is an Astec SA130-3400 with 4 outputs :-


     V1 +12VDC 5A

     V2 +5VD 15A

     V3 -12VDC 0.8A

     V4 - -5VDC 0.8A
As indicated on the data-plate, the maximum continuous power output is limited by the available cooling. Using only natural convection, the power output is limited to 85W, with forced cooling of 15CFM (cubic feet/min), the continuous output rises to 130W - hence the "130" designation.

In the DDFS, a cooling fan is fitted to the outside of the case and supplied by the PSU's +12VC power output.

The cover and fan removed, the PSU frame is mounted on the DDFS end plate using 4 x 3/4" metal stand offs and the PCB is secured to the base plate by 2 crosshead screws.

The PCB is also secured to the side of the frame by two screws through the front into the large aluminium heat sink to dissipate heat from the power transistors.

The PCB removed from the frame showing the solder side.

The 3 black plastic lugs and 12 (4 + 8) solder connections secure the control module (the black plastic cartridge visible in the photo above) to the PCB.

With no obvious damage to the components on the main PCB, I decided to remove the control module for inspection.

Unsoldering the 12 connections through the PCB and springing the plastic lugs allowed the control module to be removed as shown.

View from the other side
The control board removed from the cartridge, again, with no obvious damage to any of the components.

The module has an Astec part number on it, 851-00201060, but I have not been able to find any reference to that part number on the internet.

Solder side of the control board PCB

Unsurprisingly, the SA130-3400 is obsolete and I was unable to find any technical or service data on the internet that would allow me to make an attempt at repairing the PSU. (Astec are now a division of Emerson, I did try to get some technical information from the Emerson Embedded Power Systems technical support group, who did try to help, but due to the age of the PSU, were only able to locate a product data sheet, but no service details.) A quick "Google" showed that there were a number of companies in the UK who could potentially repair the PSU but the costs were likely to be of the order of 50 or more and I was given one quote for 150-200!

Given that I was unsure of the status of the rest of the hardware, I couldn't justify this cost, so I looked around for a compatible supply that could be fitted inside the DDFS case.

The obvious potential replacement PSU was a micro ATX power supply. As well as the power on signal, standby and control voltages, the original ATX specification included +12V, +5V, +3.3V, -5V and -12V. Later revisions of the specification removed the -5V supply, so newer models of ATX PSU do not have a -5V output. However, I was able to find a used micro ATX PSU which supplied the required voltages, albeit with lower current capacity on the negative voltages, 0.5A at -12V and 0.3A at -5V. Closer investigation of the DDFS power distribution revealed that the -12V supply was not actually used and, having looked at the details of the PCBs in the DDFS, Tony Brewer was able to confirm that the actual -5V load would be trivial - certainly much less than 0.3A.


Replacement Power Supply
The replacement micro ATX PSU
Output Voltage Min Load+ Max Load
+5V 2.0A 20A
+12V 0.1A 10A
-12V 0.05A 0.5A
-5V 0.05A 0.3A
+3.3V 0.2A 10A
+5Vsb 0.0A 1.5A
Dimensions : 150mm x 86mm x 43mm (L x W x H)


As well as the maximum load values, although they tend not to be shown on the data-plate, switched mode power supply specifications typically quote values for the minimum load required on one or more of the voltage outputs. Operation of the PSU below the minimum load values can result in unpredictable operation, including poor voltage regulation and PSU shutdown, although PC/ATX PSUs seem to be pretty forgiving and actual damage to the PSU should not result. (SL Power Electronics have a very brief explanation of the issue in this Application Note)


For initial testing of the replacement PSU, I was not too worried about the minimum load values, the PSU would either work or not and I would have addressed the load issue if the PSU did not run reliably. With the DDFS connected, the current draw should be sufficient to provide the minimum loads for +5V, +12V and -5V, I didn't think that the -12V was too much of a problem, but, depending on the stability of the PSU, I may add a dummy load to the +3.3V line at some point. (A R15 resistor fitted across the 3.3V rail would provide a load of 0.22A equivalent to ~0.7W.)


Original power distribution wiring for the DDFS.

The two card cages have separate +5V and common feeds from the PSU and daisy-chained +12V and -5V lines. The case fans are powered by a separate +12V line.

A close up of the rear of one of the card cages, the lighter gauge wires are the +12V supply to the case ventilation fans. The wires to the card cages follow the same colour code as the FDX :-
+5V Red
-5V Blue
+12V Pink
Common Black
Rather than doing a similar "professional" upgrade as I did when I replaced the PSU in my FDX, I decided to minimise the modifications required to the DDFS, at least, until I proved that it worked.

I removed the PS_ON  wire (green) and a common (black) wire from the 20-way connector and spliced them together so that the PSU would turn on when power was applied to the IEC mains connector.

I removed the eyelet connectors from the DDFS power harness, retaining the existing multi-way plug & socket and made connections to the PSU +5V, +12V, -5V and common lines.

For initial testing, I connected an IEC mains lead directly to the PSU, bypassing the DDFS power switch and applied the power. The DDFS case fans both started as soon as power was applied and it was immediately obvious that in the long term I would need to look at installing low noise fans!

With the fans working and the +5V, -5V and +12V voltages checked, I temporarily "mounted" the PSU in the DDFS and refitted the end-plate and covers, ready for testing as a Video Wall !

mailto: Webmaster

 Terms & Conditions