MTX PSU Replacement

Background

The Power Supply for Memotech MTX computers is nothing more than 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.

Replacement Options

Compatible PSU (AC)

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 published 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 also described on my page here.

Mark's replacement PSU is the easiest way to replace a failed MTX power supply, it is a direct replacement and plugs into the MTX power connector without needing the MTX to be modified in any way. For people who don't have the skills to make changes to the MTX themselves or who want the convenience of just buying a replacement PSU, then it is the best option. 

Alternative PSU (DC)

For anyone with some basic soldering skills, there is an alternative option, albeit one that requires some modification to the MTX itself. This page shows one possible method of providing DC only power to a Memotech MTX computer.

As described on my MTX PSU page, the MTX computer board derives +12VDC, +5VDC, -5VDC and -12VDC from the low voltage AC tappings on the PSU (transformer). With some minor modifications, it is possible to use a DC only power supply and feed the required DC voltages directly to the MTX computer board. Memotech did exactly that when they used a MTX computer board and a "Skynet" SNP-3031 PSU, to build the Reflex Controller in my Video Wall system.

The internals of my Reflex controller, as you can see, all of the power components, including the 5VDC fuse, have been removed. Power is supplied to the board through the wires soldered directly to the pads on the PCB intended for the DIN socket, +12VDC (pink), +5VDC (red), 0V (black) and -12VDC (blue).
Memotech used a PSU manufactured by "Skynet", Model Number SNP-3031 :      Input specifications: 120V-1A, 240V-0.5A, 50/60Hz.    Output specifications; V1: +5V/2A, V2: +12V/1.5A, V3: -5V/0.3A. (An open frame PSU such as this would not fit inside the MTX computer case and would not be suitable for replacing the standard PSU)

MTX Power Regulation

An LTspice representation of the MTX power regulation schematic is shown below : :

Although the four voltage levels are required to fully support the MTX and its peripherals, it would be possible to reduce the required voltages if some changes to the computer board were made or compromises to functionality could be accepted, the only essential voltages are +5VDC and +12VDC.

The 4116 Video RAMs require +12VDC, +5VDC and -5VDC to operate, with -5V being derived from the -12V supply. The VRAMs are a common cause of problems in the MTX, and indeed, other machines that use 4116s, such as the Sinclair Spectrum and Commodore PET.

The reader may like to consider replacing the VRAMs with 4164s as described by Martin Allcorn: that would remove the requirement for -5VDC on the MTX motherboard. If the cassette port was not needed and the RS232 board was not being used, then the -12VDC supply would not be needed either and +5VDC and +12VDC supplies would be adequate.

However, the solution proposed here assumes that all four voltage levels are required and it is necessary to supply +12VDC, +5VDC and -12VDC to the computer board, to accomplish this, some modifications to the power circuit is required as shown below : 

The 6 x regulation diodes, D14 to D18, must be removed, and D14, D15 and D18 must be replaced by shorting links (shown as resistors in the LTspice schematic above).

The 12V regulator (REG1) is removed and a shorting link soldered between the input and output connections on the computer board. The 5V regulator (REG2) is removed, the power transistor (Q4) is removed and a shorting link soldered between the emitter and collector connections on the computer board.

It is assumed that suitably stabilised DC power supplies will be used to power the +12V, +5V and -12V rails, so the capacitors originally fitted should be removed (C38 to C40 and C51 to C56). R60, C49 and ZD3 are retained to generate -5VDC from the -12VDC supply. The simplified schematic is shown below :

Power requirements

The MTX computer board was fitted with a 3.15mA fuse for the +5VDC rail, so, without making any attempt to work out the actual 5V power requirements, I have assumed that 3A will be adequate for the +5VDC supply.

The only consumer of -5VDC are the 4116 VRAMs, the ITT 4116 data sheet gives the maximum -5VDC power requirement for a single chip as 200uA, giving a total demand from the MTX of 1.6mA from the 8 VRAMs. (This power is derived from the -12VDC supply.)

The main consumer of +12VDC on the MTX computer board are the 4116 VRAMs, the ITT 4116 data sheet gives the maximum +12VDC power requirement for a single chip as 35mA, giving a total demand from the MTX as 280mA from the 8 VRAMs.

The cassette port also requires +12VDC and -12VDC and other potential consumers for these voltages would be the RS232 board, if fitted, and/or the motor (+12VDC) of an SDX floppy drive if fitted and powered off the MTX +12VDC rail.

After a bit of "Googling" for ready made PSUs, I came across a Cisco ADP-30RB (34-0874-01), this supply is intended for use with various Cisco routers such as models 506/1700/1720/1721. These supplies are widely available for good prices from eBay and provide +5VDC at 3A, +12VDC at 2A and -12VDC at 0.2A (maximum 30W total).

The standard cable on the Cisco PSU uses a Molex Mini-Fit Jr connector with the pin-out as shown. The voltage and 0V (RTN) pins are self explanatory, but the ROF pin is worth a mention. The acronym stands for Remote On/Off, similar to the way that an ATX PSU is controlled, the ROF pin must be grounded to turn on the PSU. (This pin could be permanently grounded to have the PSU always on, or used to add a power switch to the MTX.)

Connection Considerations

If you only have a single MTX computer, then connecting DC supplies via the existing connector on the MTX computer board is not likely to cause any problems. However, as I have a number of MTX machines with standard AC PSUs, I wanted to guard against the possibility of connecting the DC supply to my unmodified MTXs, and probably more importantly, prevent me from accidentally connecting an AC PSU to the modified MTX.

The MTX power input uses a DIN connector.
DIN connectors come in various configurations, with 3 to 8 pins, the angle between the first and last pin are commonly, 180o, 240o and 270o. The MTX connector is a 6-pin, 240o socket, the numbers shown on the left hand image show the pin numbering, looking into the plug on the original PSU lead.
The table shows the nominal AC voltages fed from the original MTX PSU to the relevant pin in the socket and the DC voltages required to replace the AC PSU. Pin 6 is not connected	Pin AC Voltage DC Voltage 1 13.5 ~ +12 2 9 ~ +5 3 0 0 4 0 0 5 9 ~ -12 6 (n/c) (n/c)
For convenience, here is an image of the view into the plug along with a mirror image which shows the position of the pins when looking into the power socket from the rear of the MTX.	Front of plug Rear of socket
One option to provide some protection against plugging the wrong type of PSU into a modified, or unmodified, MTX would be to use a 5 pin 180o DIN connector for the DC version. With pin 6 missing, an original MTX AC PSU could not  be plugged into the DC version and the pin angles would prevent the DC PSU being plugged into an unmodified MTX. This is probably the easiest solution but I chose not to go that way.
NB: The colours shown above are those used in the original MTX PSU cable and are NOT the same as those used by Cisco. The colours used in one particular Cisco PSU are shown opposite, these are not guaranteed to be the same for all, if you are going to replace the Molex connector with a DIN plug, ensure that you know which voltage is on each core.	RTN ROF RTN Black White Black       +5 +12 -12 Red Orange Green

My DC Power Supply Solution

Manfred Flume donated a couple of MTX512s to me, including one that he had previously converted to use a DC only PSU. Since most of the power regulation components had already been removed from this machine, it made a good test-bed for a DC only power supply project.
Rather than convert the Cisco PSU to a 5-pin DIN connector, I decided to replace the MTX power socket with a mating connector for the Cisco PSU. In this photo, you can see the 5-pin DIN socket has been removed and the holes cleaned up in preparation for the new socket.
Close up of the socket PCB connections, annotated to show where DC voltages would need to be connected if using the standard DIN PSU socket. You could, of course, just remove the DIN socket and solder wires from an external DC PSU directly to the computer board, but this would mean that the PSU was permanently connected and you would also need to consider including some strain relief for the cable - though this could be as simple as putting a tie-wrap though the DIN connector fixing holes.
The replacement socket for the Cisco PSU plug. Obviously, the pins are not going to fit the PCB without some work !
Solder side of the PCB, there is enough free free space on the PCB to drill a couple of holes for the socket locating lugs, but, as might be expected, the Cisco socket pins do not fit the tracks on the PCB. Modifying the PCB to allow the Cisco power socket to be mounted requires that a couple of the existing tracks are "butchered".
Minor modifications made to the PCB :     2 x 3mm holes made for the Cisco socket mounting lugs     Existing through hole barrel expanded for the +12v pin     Additional hole drilled through the PCB for the -12v pin     Existing through hole barrel expanded for the ROF/0v pin
The +12v and -12v pins penetrate the 0v trace on the solder side of the PCB. I cut the trace to isolate the +/-12V signals, leaving a small amount of copper adjacent to the new pins to give a good connection between the pin and new patch wires that were required. The small track right in the middle of the action is the RELCPMH signal, which just happens to be routed under the PSU connector !
The combined ROF/0v pin is connected to the 0v traces on either side of the cut trace (black wire and wire link). The +12v (yellow wire) and -12v (wire link) lines are patched as shown
Replacement socket fitted It is not clear from the photo, but the left hand RTN pin and ROF pin are linked together at the rear of the socket, the ROF pin is used to connect to the 0V trace on the PCB. The RTN pins are not connected to the PCB. The 5V connection is hidden below the upper row of pins, but connects to the 5V trace on the PCB
The round hole that the MTX power connector would normally pass through needs to be opened out to allow the Cisco plug to pass though it. This creates a slightly weaker plastic filler panel, but once it is refitted to the MTX, it is fine.