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The Memotech MTX Series

MTX PSU Disassembly

This page provides basic guidance for opening up your MTX PSU.

The PSU is formed of two ABS plastic half shells which enclose a multi-tap transformer and mains power switch, there are no active components inside. If the PSU has a fault such that the expected voltages cannot be measured on the pins of the MTX power connector and the problem is not caused by a broken connection on either the mains or low voltage sides of the transformer, then it is probably beyond repair, almost certainly a case of "no user serviceable parts inside" !

There are four square pads on the bottom of the case that might fool you into thinking that they are hiding screws beneath them, that's a red herring, the feet are just to protect the surface that the PSU is going to sit on, the two halves of the case are clipped together. In some cases, it has been found that additional glue has been applied to the clips, meaning that splitting of the case halves is not always easy.


Opening the case

This video by a YouTuber called NatureAndTech suggests that getting the case open is pretty easy - but, whilst the principle is right, it is not quite as easy in practice.

Click on the image to open the video on YouTube
This photo, taken directly from above, shows the internals of the PSU. You can just about make out the two clips at the top and bottom of the shell.

However, it is not clear from the top view, how close the transformer and its terminals are to the flat surface of the shell.
Bill Brendling has kindly shared his experience and photos that he took when trying to open the case of his faulty MTX PSU.

Being unable to prise the case open, Bill concluded that there was no way of getting into it without causing damage and decided to carefully slice off the top of the case.
At the left, you can just make out the thin sliver sliced off the top off the case. Unfortunately, the input and output side wiring have both been damaged in the process, fortunately, the transformer itself was not.

With the benefit of hindsight, Bill suggests that anyone attempting something similar cut around at about 15mm above the centreline, being careful to only cut the box, and not in towards the centre.
Profile view with the top removed, showing how the mains cable extends well into the top half of the shell.

You can also see the clips which secure the two halves of the case. There are four plastic lugs with square holes that the the lugs in the top half clip into.
The transformer, with attached mains switch, after removal from the case.

You can see how the mains and low voltage wiring have been damaged.
Photo of the lower part of the shell, with the PSU removed.

In the full size photo, you can just make of the remnants of what looks like "hot" glue on the clips.
Photo of the top part of the shell, with the PSU removed.

Again, you can just about make out the remnants of the glue used on the clips. The lugs that would mate with the square holes have been cut off.
Bottom of the case, with the internals removed, you can see the four rubber shock mounts that the transformer normally sits on.
The corresponding rubber mounts from the top side of the case, cut off during the opening exercise.

The transformer is normally sandwiched between these rubber mounts in the top and bottom halves of the case, the mounts are centered on the holes at the corners of the transformer's laminated steel core.
Typical PSU Internals
With the case open, you can see the minimal internals. This photo is of a UK spec MTX power supply. At the top you can see the mains cable, DPST mains power switch and its connections to the primary side of the transformer.

At the bottom, you can see the connections to the MTX 5-pin DIN power plug. Take a look at my Technical Note on the MTX power supply for full details on how the power is supplied to the MTX, including the pin-out of the DIN connector.  
This photo, courtesy of Steven, shows the internals of an MTX PSU with a European mains power plug. As you can see, there is an internal fuse connected to the right hand terminal on the LV side of the transformer which feeds the -V input line to the MTX.

This fuse has been seen on more than one PSU so it was apparently fitted by Memotech but the reason is unknown.


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