MTX PSU Replacement

Background

The Memotech manuals do not give any details on the MTX external power supply, the only "published" information can be found on the label on the top of the PSU, most PSUs that have been seen are marked "Output 22.5 VAC 1A Tapped at 18V and 9V". The PSU with Keith Clatworthy's low serial number MTX512 has additional information - "Output 22.5 V ~ 1A. 18V ~ 0.82A. 9V ~ 0.28A.

Inspection of the PSU internals confirms that the "PSU" is in fact only a multi-tapped transformer, all voltage regulation & smoothing is done on the MTX computer board, for a complete description of the MTX power supply and voltage regulation circuits, see my MTX PSU page.

Design

Since there are no "off-the-shelf" replacements for the MTX PSU (transformer) available, until recently, a lost or failed PSU would render an otherwise working MTX computer inoperable. Mark Kinsey (1024MAK) has designed a modern replacement for the MTX PSU, the design was first publised on the Memotech Forum. With Mark's permission and with contributions from David Kimberlin Wyer - who has assembled a 110VAC version, design and construction of the replacement PSU is described below.

The secondary side of the transformer outputs 2 x 7.5VDC feeds to diodes D1 and D2, used to supply the MTX +5VDC regulator and the -5VDC zener diode.

A +16V DC supply is generated by an LM2577 based step up DC to DC converter, which in turn is supplied by two diodes and a 2200uF 16V smoothing capacitor, which gets it's supply from the two 7.5V AC feeds. This is fed to the MTX +12VDC regulator.

Mark has produced a parts list / Bill of Materials that is mainly UK focused but provides enough information that would allow alternative parts to be sourced in other locations if required. David has produced a version of Mark's spreadsheet that lists equivalent components and suppliers for readers in the US. I have combined both lists into a single Excel spreadsheet which is available for download in the Zip archive at the bottom of this page.

Note : If building the 110VAC version, fuse F1 should be replaced with a 2A time delay (slow-blow) fuse and F3 should also be a time delay fuse (1A).

Construction

Mark's Original
Mark designed the PSU to fit into this multi-purpose ABS box, distributed by Bradford Consultants Limited, available in the UK from Maplin.  This enclosure is 177x120x83mm, it is an ideal size but any similarly sized ABS or metal box could be used. Some customisation is required to mount the power inlet, outlet, switch and indicator.
Preparing the plastic case. Tools [Mark] used: fixed metal rule, tool with a sharp point (for marking), battery drill, metal HSS drill bits or wood drill bits (they have to be sharp), needle files, various shape and sizes, somewhere to work where you don't get told off for making a hell of a mess. As with all these, measure twice! Mark once. Very carefully line up the drill (set to a slow speed), double check that you are drilling INSIDE the marked line, and carefully drill each hole
Cutting the hole for the IEC mains connector Once you have an outline, angle the drill slightly so that you "join the dots" Then carefully file the excess plastic away until you have a neat hole that is just large enough for the connector.
Of course the last picture here is a test fit!
The mains switch is done the same way, but you have to be a bit more careful, as this is a snap in type and too big a hole would be a disaster! Note : the drilling of all holes in the case should be completed before the mains input and switch are fitted permanently.
Don't forget all the other holes.  Go carefully as it is very easy to cut too much, or too little and use a slow speed. Here I show how to do the holes for the transformer fixing machine screws. Once you have drilled the holes for the machine screws, use a countersink drill bit (or similar). Every so often try putting the machine screw in the hole to check if it fits nicely. The idea being that the head of the screw is flush, or almost flush with the case.
With most fuseholders, the hole they need is NOT round. Instead it is round-ish with a flat side. If you just drill a round hole that it will fit in, it will turn in the hole . So drill an undersize hole, then get to love filing, while trying the fuse holder every so often until it fits through...
Before fitting items to the case, it is far easier to connect some of the wiring first. Strip about 5 to 6mm of the insulation, gently twist the wire strands together using your fingers, then using round nose pliers form a "U" loop. This will give the soldered connection added mechanical strength. Pass the stripped wire through the hole on the terminal of the item and if needed, flatten the "U" loop with smooth flat nosed pliers. Cut off any excess wire strands, then solder. Once the joint is cool, inspect it. If happy, cut a suitable length of a suitable size heatshrink tubing. Slide over the wire and terminal and using a hot air gun, (I use a gas soldering iron with the hot air "tip") shrink it down. When shrinking it, keep slowly moving the heat source, as if you try to just do one section at a time, the heatshrink will wrinkle and/or burn. I also use heatshrink tubing to keep the wires together in tidy bundles.
Soldering the mains switch
Soldering the IEC connector
IEC connector and switch done
Don't forget to pass the wires through the fuseholder insulation boot and the plastic nut before connecting both ends of the wires. Note: The terminal on the top of the fuse-holder (as viewed in the photo) actually has three black wires and one red wire connected to it. As you can see from the photo, at first glance, it looks like only two black wires. If you look a little closer, the red wire is hiding behind one of the black wires as highlighted in the touched up insert.
You can see the various stages of making the LED and the associated resistor connections. Using small round nose pliers, form the leads into a small coil. Note the resistor leads are far easier to make into a coil. This makes the connection more robust and easier to solder. Cut off any lead that sticks out the end of the coil, then over with heatshrink sleeving.
Building the DC board Components       Diodes Two 1N4001 or 1N4002 or a higher voltage rating     Capacitors 100nF ceramic multilayer type, 5mm spacing. (the small blue bead)   2200uF 16V electrolytic (the large blue can)     Fuse PCB fuse clips, 5mm pins, suitable for a 20mm type fuse. 1A "Fast" (F) or "Quick Blow" (QB) 20mm x 5mm glass fuse
Step up DC to DC converter LM2577 DC to DC Adjustable Converter Step-up Circuit Board Module [Mark] used a Chenyao19870517 LM2577 DC to DC adjustable converter step-up circuit board module 4-35V output" from eBay UK at £1.95. Set the output voltage to 16V before fitting into the case.
Assembly photos at various stages of the build
Wiring the transformer earth connection
Internal view of the 5A fuseholder and LED fixed to the case
External view of the finished switch, LED and fuseholder.
External view of the finished IEC power input connector

David's US (110VAC) Version
On Mark's recommendation, David used a Triad Magnetics VPS16-2700 (Digi-Key Part Number 237-1261-ND). Mark commented : "Wire the primary 115V windings in parallel (see the datasheet from the Digi-Key web site) and the secondary windings in series, but use one of the connections used to join the two secondary windings as the centre tap."   Mark also provided comprehensive instructions for wiring the transformer for the US. "Before wiring up the transformer, measure the resistance (Ohms, 200 ohm range) of each of the primary windings. So the first winding is between the connections labelled 1 and 2. The second winding is between the connections labeled 5 and 6. The resistance should be about the same."
Mark also created this helpful sketch to show how to connect the transformer for the US and provided the specific instructions below :
Note, connect the wiring to the tag provided (this not shown in the picture for clarity) by gently twisting the stripped copper strands, threading through the hole, and then bending the wire back to form a "U" or hook shape. This improves the mechanical strength of the connection. Once soldered, cover with heatshrink The resistor connects to the casing of the transformer at one of the mounting holes by using a "solder tag". Also not shown, at the same point where the resistor connects to the transformer case, a green (or green/yellow) earth wire should be connected, again by using a "solder tag". Ensure that the transformer case where these "solder tags" are connected is clean and free of any varnish or other non-conductive protective coating by scrapping at the metal with a screwdriver. We need a really good connection. Put the machine screw through the case, put the transformer over it, then put the two solder tags (with the resistor and the green wire already attached) on the screw, then add a washer and finally the nut. Once you have the mains wiring finished and the fuse fitted in the IEC connector, measure the resistance between the Live/Line (L) and the Neutral(N) connections at the mains plug, or the IEC connections. It should be about half of the value you got for one winding that you took earlier.
David's case with IEC connector and mains switch installed.
Internal view of the IEC connectors and mains switch
Transformer, fuseholder and power indicator installed
All done !  - Ready for power on and testing
Close up of David's fully assembled unit . . . . . Just missing a nice label :-)  - One possible label design is shown below - other suggestions welcome.
Testing completed . . . .       Connected to an MTX computer . . . . . . . .             It actually works !

Testing

Parts list, Datasheets, etc.
Replacement MTX Power Supply Unit Parts list (V1-02 UK and V1-01 US) Click on the image to download the Zip file containing Mark's original parts list, along with David's list of parts used to build a US (110VAC) version. (The speadsheet includes hyperlinks to sources for the components on the 'net)
Datasheet for Mark's transformer - a Vigortronix: VTX-126-050-2075 (Rapid Electronics Order Code 88-3928) Specifications : 230V 50VA 7.5V+7.5V Update 27/03/23: The transformer that Mark used has been discontinued, but other similarly rated units are suitable. A slightly lower power, but acceptable option is an Indel TS 40/020 available from tme.eu
Datasheet for David's transformer - a Triad Magnetics VPS16-2700 (Digi-Key Part Number 237-1261-ND). Specifications : 115V/230V 43VA 8V+8V
Suggested label designs for the 240V and 115V versions. Alternative suggestions are welcomed, possible changes include swapping the positions of the "1024MAK" and "MEMOTECH" texts - comments?