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The Memotech MTX Series

Memotech MTX Keyboard Problems

 

This page provides some generic tips for fault finding of MTX keyboard problems as well as a specific example courtesy of Ramon Merchan Sanzano.

MTX Keyboard - Basic faults

For an overview of construction and normal operation of the Memotech MTX keyboard, see my keyboard page.

The system board is fixed to the bottom of the case and the keyboard PCB is fixed to a steel plate mounted on the underside of the top of the case.

The grey ribbon cable connects the keyboard PCB to the main board by plugging into the "J1" connector above the edge connector on the left hand side of the main board.

Photo courtesy of John Hancock

"J1" on the computer is the 20-way SIL header located in the upper left of the board.

A similar SIL header is fitted to the edge of the keyboard PC, and the two are connected by the grey ribbon cable shown below.

As you can see in the photo, the ribbon cable is somewhat unusual - unlike a more typical IDC cable, the ribbon is not supported by the connectors, instead, each core is stripped out from the ribbon 5-10mm from the connector.

There is no strain relief on the cable and as a result, the the most common "keyboard" faults are associated with breakage of one or more of the conductors.

Photo courtesy of John Hancock

Cable / Connector

The photo shows the type of cable damage that can result if the keyboard has been subjected to a number of disconnections and reconnections or the cable has been overextended when opening the case, although it is more usual to see a single broken core.

Photo courtesy of Andy Garton

I have used this type of cable to replace the original cable in one of my MTXs, they are available off ebay UK and cost around 1-2, including shipping from China - search for "DuPont cable".

The connectors are not fixed together into a single plug like a normal IDC connector and the cable is a little stiffer than the original, but neither of these issues caused me any problems.

Martin Allcorn's solution - using an IDE cable, as the MTX only uses 20 pins for the keyboard connector, only one row of the IDE plug terminals and 20 cores in the IDE cable are actually used.

The photo also shows a 4164 DRAM being used to replace a faulty VRAM, the 8 x 4164 DRAM installed in sockets in place of the original 32K of RAM, and a replacement cable for the Video/Audio outputs (they also prone to breaking).

Key-switch Operation

The MTX keyboard is a passive device, consisting of 79 individual SPST key-switches.

It is relatively easy to check the function of the individual key-switches using a multi-meter and checking that the contacts close when the key is pressed.

With the keyboard inverted, it is also easy to see whether there is any mechanical damage to the PCB.

 

Key-switch Repair / Replacement

The key-switches used in the MTX keyboard are the same as those used for the Acorn Electron and BBC Master computers.

The switches can be replaced without completely disassembling the keyboard, but it is much easier if the surrounding keycaps are removed first, in this photo, the keycaps for the faulty "H" key and the keys surrounding it have been removed.

De-solder the faulty key-switch, using a solder sucker to ensure that the pins are completely clear of the surrounding solder pad.

It's a good idea to double check and mark the position of the key that you want to remove before you start !

With the switch de-soldered, it can be lifted off the PCB.

The switches are held in place by three plastic securing lugs located at the sides and bottom. Use a small flat-bladed screwdriver to gently apply some upward pressure to the switch while you release the securing lugs.

If you intend trying to repair, rather than replace, the switch, take care not to break of the plastic lugs.

Some good photos of this step on a BBC Master are shown on the classic acorn site

Switch removed, the top side of the PCB is visible through the resulting hole.

Whether you decide to replace the switch or attempt to repair it will obviously depend on whether you have access to replacement switches. I don't think that you are likely to find new replacements, but keys recovered from the Acorn computers mentioned may be available from ebay.

I wasn't able to get very good photos of the key switches, these excellent close-ups are from the Deskthority wiki.

The key switches were made by Futaba and are the low-profile linear type, the Deskthority wiki has a dedicated page about these switches that gives clear instructions for disassembly/assembly. 

The threaded pins are removed by unscrewing them ~2 turns anti-clockwise.

The MTX keys just "snap fit" together, secured by three retaining tabs, the two halves can be split by gently prising them apart whilst levering the retaining tabs out of the way.

(I believe some Futaba keys of this type may be glued together, so it may not be possible to split them without damaging them.)

With the switch split, the component parts can be seen :
 
   Top half of the switch the plastic plunger (white)
             the leaf contact (silver/gold coloured)
   Bottom half of the switch showing the spring
Top half of the switch completely disassembled, the leaf spring contact is very fragile and unless damaged, it is probably best not to try and remove it, if necessary though, it should push out when the slider is pressed.

If the contact looks undamaged, cleaning the pins may be all that is required to give the key switch a new lease of life.

The image on the right is a not very good picture of the contact from the switch being removed from the MTX above, as you can hopefully see, it is pretty mangled. Although it may have been possible to straighten and refit it, I didn't want to take the chance and bought a "for spares" Acorn Electron off ebay and replaced the faulty switch with a known (tested) good one from the Electron.

This means that I also have quite a few good switches that I can use to replace keys on other faulty MTX keyboards.

 

Keyboard Fault Finding

If the fault is not a mechanical one such as those described above, fault finding is a little more involved, but not too difficult.

Ramon Merchan Sanzano had a fault with his MTX keyboard with a number of keys ("W", "R", "Y, "I", "P") not working.

Ramon managed to fault-find the problem and put a very helpful post on the Memotech forum, he has kindly allowed me to include some of the details here too. Most of the content in this section is Ramon's but I have made a few changes, mainly to aid readability.

As described on my keyboard page, the MTX keyboard is essentially a 10 x 8 matrix of keys, the status of which are read using Z80 IN and OUT instructions to the assigned MTX I/O ports.

Diagram reproduced from "The Source", by Keith Hook, 1987

The computer board schematic in the Operator's Manual shows the keyboard drive and sense lines, but there is not a good description of the keyboard operation in the manual and "The Source" is, or at least was, not commonly available.

Ramon did his own analysis and came up with this keyboard map and schematic diagram of the MTX logic circuits.

It can be seen that all of the affected keys are associated with the "DR2" drive line which is on "J1" Pin 13, connected through diode "D2" to the 74LS273 Octal "D" type flip-flop (board position 3A).

Diode "D2" and the resistors were checked with a multi-meter and found to be OK.

All of the affected keys are read through Input Port 5 using the 74LS244 Octal Buffer/Line Driver (board location 2A) but use different sense lines indicating that the fault was likely to be in the drive line circuit.

Ramon replaced the 74LS273 (3A) which did not cure the problem, indicating that the problem was the data bus input to the "D2" drive line flip-flop.

A 74LS244 Octal Buffer/Line Driver (board location 2E) is used to buffer the data bus lines going to the majority of the physical output ports, including the keyboard drive lines from Output port 5.

Ramon replaced this component which fixed the problem.

This fault would likely also have been evident had any of the other output ports using data buffered by IC 2E been in use at the time.

(The port control logic is performed by the 74LS138 3-to-8 line decoders in board positions 7C (IN) and 7D (OUT), which were operating correctly since most of the OUT(5) and IN(5)/IN(6) were picking up correct key strokes.

 

 

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