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The Memotech MTX Series

Memotech Hardware Repairs

 

FDX Disassembly / Reassembly

Caution !

With the FDX open, and the rear panel swung away from the end plates, the FDX power supply and the additional transformer used to supply power to the MTX (if present) are revealed. These items have exposed terminals which carry mains voltages - take care when working inside the FDX with the power on, also be aware that the PSU capacitors can hold charge for a few seconds after the power is removed from the system.

The FDX is a nice looking unit, but was not made to be "maintenance friendly" and does not have the modular construction of a modern day PC. There are few, if any, internal parts that can be tested or replaced without an almost complete disassembly of the FDX. The notes here are based on disassembly of my own FDX and photographs of others. Given Memotech's penchant for "customisation", you may find that the internals of your FDX are different from what you see here, but they should be very similar.

Depending on the reason for opening up the FDX, you may, or may not, want to test the system while it is in a state of partial or even complete disassembly. It is more than likely that you will though and you may want to connect and disconnect the MTX keyboard at various times during the process. As you will see later, once the FDX is even partially disassembled, it can be difficult to get access to the interface connector on the bottom of the case to connect or disconnect the ribbon cable. I suggest that you leave the cable connected at the FDX and have the MTX case open to expose the RS232/Bus Interface card and make any connection or disconnection of the ribbon cable at the MTX end as required. It is possible to connect the interface cable onto the RS232/IF card with just the MTX right side end plate off.

 

Disassembling the FDX

Power Off the FDX, and if using a separate PSU for it, the MTX too.

You can always apply power later if required, but with the exposed mains terminals and bits of bare metalwork likely to be moving around, not to mention your hands & fingers, this is an obvious and essential safety step.

The FDX is held together is a similar way to the MTX itself, with a number of fixings, in this case "Phillips" head screws, rather than hex-head used in the MTX, and interlocking aluminium panels.

The case is made from two "U" shaped panels which form the front and back, these are linked by flat plates, forming the top and bottom and the case is completed and held together by two flat end plates secured with screws.

The front and back pieces have profiled edges, one edge is a flat groove, the other, a round channel like the front of the MTX case. The flat edge can be pulled away from the edge of its mating plate but the channel edge must be slid all the away along its edge to separate it from its mate, like the MTX keyboard.

As shown in this end on sketch, the front panel has the round channel at the bottom and the rear panel has it at the top.

(not to scale)

There are a multitude of fixings in the base, I thought it might be useful to do a sketch showing the position of the major components and their fixings. It is useful to know where things are positioned before you make a start on opening the case. This is one of my FDXs, but you may find that yours is different, there seems to be a bit of variation on how some of the components were fixed to the case.
With the FDX facing forwards, remove the six Phillips screws from the left hand end plate. The top panel can then be slid out to the left, exposing the top of the 6" card frame and the disk drive(s).

In this single drive model, you can see the FDX PSU below and behind the silver plate where the second floppy would sit in a twin disk system. If the second disk was installed, you would not be able to see the PSU at this point.

Removal of the two Phillips screws at the rear of the right hand side of the case with allow the rear panel to be swung away from the top of the case and lie flat on the fan cover. Removal of the input and output power leads will allow the panel to sit flat on the bench. The edge of the rear panel fits into the slot in the base panel, this is not profiled, so can be pulled completely away from the base if required.

At this point, the case body and rear panel are linked by the power and earth wiring, care is required to make sure that you don't stress or break any of the interconnecting wires as there is not much "slack" in the wiring.

There is a small, 2-pin connector for the AC input and an earth wire push-on connector that connect the supply from the rear of the case to the FDX PSU.

In the twin drive FDX, with an MTX power transformer, the transformer will need to be unbolted from the base to allow the parts to be completely separated. Unless you are replacing the transformer, this step should not be required.

With care, it is now possible to access the 6" card cage and remove the PCBs as required. and access the drive electronics. However, further disassembly is required before the floppy disks or FDX PSU can be removed.

In the unlikely event that you want to remove the 6" card frame, with the PCBs removed, you can get access to the fastenings which secure the card frame. On my FDX, this is done using four bolts that pass through the base plate and are secured with nylock nuts on the bottom edges of the card frame..

A recurring theme on my FDX pages seems to be the maintenance "unfriendly" nature of the construction - the floppy drive mounting arrangement is no different. Access to the drives to allow them to be removed is a real pain, the case needs to be almost completely dismantled before the drives can be released.

The floppy drives sit on an aluminium plinth and are secured from underneath, there is no real way of removing a drive from the plinth while it is still in the case.

It **may** be possible if you have a mirror, have appropriate tools and are a bit of a contortionist, but as you need to work in close proximity to the PSU, there is a good chance that you would damage it, so the best way is to remove the drive plinth from the case.

A profile view of the plinth. The two "feet" shown at the left hand side of the photo straddle the PSU at the rear of the case, each is fixed with a bolt through the case bottom and secured with a nylock nut.

The front of the plinth is supported by a ledge in the front panel and two smaller diameter bolts are held captive in a channel in the front panel, these bolts are also secured with nylock nuts.

This is a view of the underside of the plinth.

Each drive has four "L" shaped brackets bolted to its sides, the bottom of the brackets are tapped to mate with the bolts that you can see on the underside of the plinth.

In this overhead view, between the two drives, you can see two of the bolts on each drive that fix them to the angled brackets.

They were cunningly selected to be just too long to allow them to be removed from above without fouling on the opposite pair of bolts!

Remove the right hand end-plate and disconnect the floppy power and data cables. Remove the two bolts that pass through the base and remove the nylock nuts that hold the plinth to the front panel.

The front panel can be tilted forward slightly, hinged on the round profile mated with the base, to make removing the drives a little easier.

By tilting up the drive plinth it is possible to slide it to the right until it clears the case. Care must be taken not to catch the wiring to the power switch on the front panel , the power wires to the card cage, or the low voltage AC wires from the MTX transformer. You need to be careful not to damage the main PSU or the MTX transformer as you slide out the plinth.

   

Is it probably not necessary to remove the front panel, but if you want to do so, it is fairly easy. The plastic trim is clipped into the cut out on the front of the FDX. Working from the front, slide you fingers behind the panel, in front of the card cage. There are three plastic clips on the top edge, working from the left, gently push out each clip in turn until the top edge clear the cut out. The bottom edge has a groove in it. Lifting it slightly will allow it to be removed. The easiest way to detach the trim from the front panel is to put it back inside the case, this can be done by angling it as required.

The front panel can now be slide to the right until it clears the base plate

 
Front view, showing the card cage, the main FDX PSU and the MTX PSU.

The fan exhaust grill and power switch panel is lying adjacent to the PSU and are still attached to the AC input wiring on the rear of the case.

(If you want to completely separate the base and rear panels, the MTX transformer can be removed from the base.)

There are probably few occasions where you will want to remove the card cage, but it is easily done. On my FDX, four bolts, secured with nylock nuts, fix the card cage to the base of the case.

As you can see from the photo, it is not necessary to remove the card cage in order to remove the PSU.

Rear view of the 6" card cage, showing the shadow of the three PCB connectors fitted and the available slots for additional PCBs - originally intended for the Silicon Disk cards.

When viewed from this side, the connections from the FDX PSU are, from left to right : Common (black), +12VDC (pink), +5VDC (red) and -12VDC (blue).

The rear of the card cage is positioned just inside the front, left hand cover, behind the fan exhaust grill and power switch panel.

Front view of the card cage, showing the three PCB connectors and the power lead to the FDX PSU (unplugged).

When viewed from this side, the connections from the FDX PSU are, from left to right : -12VDC (blue), +5VDC (red), +12VDC (pink) and Common (black).

In the bottom right hand corner, there is a large power resistor between +12VDC and ground, I think this must be to make sure that the PSU 12VDC current is above the minimum shown in the specifications. (Only 1 of my FDXs has this.)

On my FDX, the main power supply is fixed to the base with two self-tapping screws that fit into the metal channel close to the front edge of the base plate (at the top of this photo) and two bolts fixed through the base. Other FDXs I have seen use four bolts that are fixed through the base plate rather then the self-tapping screws.

All fixings have insulating spacers fitted and the bolts are secured with nylock nuts. A protective earth wire is fixed to the corner of the PSU with a push-on connector.

The remaining component fixed to the base plate is the transformer used to supply low voltage AC to the MTX from a double drive FDX - it is not fitted to the single drive version.. This is the same transformer that is used in the MTX desktop PSU, without any protective cover - the primary and secondary winding taps are exposed - you need to be aware of this if working on the FDX under power.

Similarly, the mains input and connections to the fan may also have exposed terminals.

Stripped down to the (almost) bare minimum.

The only thing left, which should not usually need to be disturbed, is the ribbon cable that links the external connector to the SM1 Interface board in the card frame. The black plastic sleeve provides additional insulation and protection between the solder side of the PSU PCB and the ribbon cable.

 

Reassembly

Like all good instruction manuals say . . . . . . Reassembly is just the reverse of the above steps 

(Award yourself extra points if you have some screws left over! )

 

 

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