Classic UK Minitrix Models - Mallard
Ok, now pay attention. The A4s (Mallard and Sir Nigel Gresley)
and the A3s (both versions of Flying Scotsman) are the same design and
construction, but no other Minitrix tender is built like this one.
Launch into it in a ham-fisted fashion thinking that you know what you
are doing and something will break and/or your locos will not run
smoothly. Or your tender wheels will fall off.
The loco normally has 3 very fine wires running from the tender to the loco.
Why 3 ? Well these locos were built in the 1980s and the Minitrix version
of DCC was coming onto the market. With the 3 wires it was possible to
open up the tender, replace the circuit board and the loco was ready to run with
the new electronic control system. In short, the power from the right hand
wheel pickups in the loco was transferred to the tender via the blue wire.
The smoothed or controlled power was sent back to the motor via the green and
First, some information about the Tender Wheels. Power is picked
up from the axle, rather than from the wheel. The rear pair of
axles collect +12v from the right hand rail. The right hand
wheel therefore needs to be insulated from the axle on the left.
You can see the circle of black insulation in the centre of the
wheel in the photo.
The forward pair of axles need to have their
insulation on the right - because these axles collect the negative
feed from the left hand rail. I'll say more about those winged
pickups later, but for now the rear bogie is +ve feed, right hand
rail, the front bogie pair are -ve feed, left hand rail.
Front bogie pair - insulated axle on the right. Rear bogie
pair - insulated axle on the left.
First the components that make up the tender - from left to right, the
parts are arranged in 'columns'.
1st column - Tender body,
tender weight, securing screw and spring washer. No need to
worry about these at present.
2nd column - Circuit board showing the underside; Thin
insulating layer. Note how the circuit board fits over the top
of the insulation. I've positioned it so that it appears to
need to fold over on its lower edge.
3rd column - everso tiny spring and red plastic insulating
4th column - metal chassis moulding and plastic tender base.
Note the threaded screw hole at the top of the metal moulding.
This secures the front bogies and the draw bar for the loco.
(Front is towards the top of the photo)
5th column - One bogie pair, showing the underneath.
Also shown a single bogie retaining screw.
I'm going to describe how to put the tender together, focusing
particularly on the rear pair of bogie wheels. Once you know
how it goes together, you will be particularly careful about taking
it apart again - or even making a decision not to do so.
plastic insulating 'nut' is exactly that. A nut. It has
a thread inside and it is this that secure the screw for the rear
bogey. It is very tiny - the outer diameter is about 3mm and
the threaded hole is a tad less than 2mm - so there is little more
than 0.5mm of plastic wall for the screw thread. The screw
only screws into the cylindrical part of the red plastic.
So what normally happens with these things is that the repairer
often tightens the bogie screw as they would any other screw, and it
shears. Look at the enlarged image (click the thumbnail) - you
can see where this piece has sheared, and every A4 or A3 that I have
seen has had the same problem. The broken example in the photo
has been 'repaired' - carefully aligning the two parts - with
superglue. Clamped and left to set overnight. Don't
bother - it doesn't work. It broke as soon as I started
turning the screw into it.
Here is the broken plastic nut and the tiny spring that fits into the
hole in the top. It sits on the top of the top end of the
bogie retaining screw in order to make electrical contact.
This is a shot of the rear half of the tender base. The nearest
hole is the central one that is for the screw which holds the tender
body onto the base. The small far hole isn't used on this model.
The middle hole in the photo is the one for the rear bogie retaining
screw. Note that it has a lip part way down. The
cylindrical end of the red plastic nut will rest in on this,
preventing the tightening of the nut from pulling the plastic apart.
That's the theory - but it means that the break is caused by the
twisting force, not the tightening force.
And here are the 5 key components ready to put into place. The
chassis moulding fits on top of the black plastic tender base which
is not shown in this photo and the bogie needs to be assembled with
the screw in place. The red plastic nut fits into the
rectangular recess (note the provision for a rear tender bulb fitted
on similar models just below it).
The insulation fits over the red
plastic nut - note the tab which slides into a slot just behind the
red plastic nut.
The spring is carefully fitted into the hole to rest on top of
the bogie securing screw. I fins a sewing pin handy for sliding the
screw vertically into position. There's then no risk of it
springing off to be lost forever.
The circuit board fits on top of the spring so that the circular
solder track near my thumb at the bottom left of the photo presses
down on the top of the spring, making a good electrical contact.
To achieve this, the tag on the end of the circuit board has to be
slotted in with the other end raised. Once in place, you can check
the location of the spring from the side, and gently press down the
other end of the board, securing it into the threaded hole with the
dumpy cheese head screw.
In this photo I am showing the circuit board from the side so that it
easier to see how the soldered side matches up with the holes and
spring. You cannot fit it like this as the tab at the right
hand side needs to be fitted first. Plenty of opportunity to
dislodge that spring.
Note that three wires are soldered in
various places. Take care when soldering as any large blobs
will prevent the circuit board from resting. The raised screw
hole at the left of this photo should come into contact with the
circuit board slightly obscured by my thumb - otherwise the left
hand rail pickup will not be transferred to the loco. So any
soldering needs to have a smaller profile than that raised screw
hole ! If it isn't, either get the soldering iron, melt the
solder and draw the iron away, rather than lifting it off - or get
the dremel out and grind the solder down - but be careful you are
not grinding away the wire.
It is when removing the circuit board that you are most likely to
forget about the spring. Get a felt pen and write on the upper
side of the board 'SPRING'. It might save you hours on your
hands and knees.
Dealing with a Broken Red Insulation Nut
Fitting the Tender Pickups.
Its seems to be illogical that
these copper contact 'wings' fit on top of the screw head, rather
than being held in place underneath it. But if you try to put
it underneath, you find that tightening the screw will clamp the
bogie carrier in place, preventing it from turning to go round the
No, they are supposed to fit over the top as shown in the top
photo on the left.
Part of the issue is that most electrical pickups are spring
against the wheel or the axle. These don't. They are
designed to use gravity to simply fall and rest on top of the axle.
The large screw head is a nice snug fit inside the hole of the
pickup and the two are always in contact.
So how do you get it in there without bending it ?
you look closely next to the axles, you will see two little pegs
sticking up. These are spaced to allow one of the wings to fit
between, but they can also be used to help get the pickup into
Place the copper pickup upright, between the axle and the pegs.
It will stand up by itself, as shown, and the wing that is under the
axle will spring into that position. It isn't a permanent
bend. The copper has a fair amount of spring in it.
Note that I am resting the bogie on top of the white metal
chassis moulding so that the other end of the screw has somewhere to
go. It might be easier to use a piece of scrap foam and push
the end of the screw into that - that will also hold the bogie in
place as your work.
With a small-ish screwdriver in one hand and another screwdriver
through the hole of the copper pickup, use the first screwdriver
blade to gently press the upper wing towards the other axle.
The idea is to get the copper pickup to form a smooth curve as it
bends over. Push closer to the square part of the pickup at
first and move toward the tip as it bends further. The
screwdriver through the hole will help to keep the copper pickup
from moving as you bend it over. Eventually, the tip of the wing
will just clear the axle and you will be able to press it down far
enough to be level with the underneath. The copper pickup
should arc, not crease.
Keeping the screwdriver there to hold it in place, focus on the
driver through the hole and using the screw head as a levering
point, gently ease the hole over the screw head - keeping an eye on
the wing that should now be easing under the axle.
Make sure that the copper pickup is seated properly all around
the screw head and particularly in between the two little pegs.
If you have a clean break like the one shown on the left, and the
small circular piece has enough thread for the screw, then the
solution can be quite straight forward, but you have to do some
Check the fit - by placing the small circle of red plastic inside the
corresponding hole in the tender base. Its the middle hole in
this photo which is quite close to the rear of the tender. It
is the larger hole with a lip a couple of mm down. Try fitting
the bogie and the bogie screw.
You will see the problem - the red
plastic bit turns, but the fix is simply to glue it into place using
either superglue (cyanoacrylate) or epoxy resin. I'd prefer to
use thicker cyanoacrylate so that the glue fills the small gaps to
the side. Whatever you do, do not allow glue to flow into the
thread. As a precaution, grease the screw threads and insert
the screw until the glue has set.
The problem now will be that the bogie screw is not securing the tail
end of the tender. This can be sorted by gluing the white
metal chassis moulding to the black plastic tender base. It
doesn't need much - the front end will be held with the front bogie
screw, so a dab either side of the recess for the red plastic
rectangular nut would suffice. I cannot see any
circumstances when it would be essential to have these two parts
The rectangle end of the red plastic insulating nut can
fit loosely on top, and the spring inside that. The insulating
layer and circuit board will keep it in place.
If the red plastic piece is shattered, then there is another option (and
this is still possible if you have previously followed the
suggestion above). You need the hole to be clean and roughened
on the inside. What you are going to do is to create a
permanent plastic piece with its own thread, using epoxy glue.
I suspect the longer setting epoxy glues are better for this - the fast
grab glues may be a bit brittle. If you have never done this
procedure before, then test it out with a different screw and a
thick bit of plastic.
I can't go into detail - its a long time
since I did this, but the idea is to fill the hole with epoxy, coat
the screw threads with grease, and wipe off the excess so that the
surface of the threads has a light grease coat. Insert the
scew into the epoxy, and twist it around to make sure that the epoxy
flows into the thread grooves. Let it
set, with the screw held securely in position. Its a lot
easier if you can find a longer screw with the same thread so that
it pokes out at both ends, and the use of masking tape to prevent
epoxy getting onto the model. Let it set hard. Note that
epoxy will go 'cheesy' in texture just before it goes hard.
This is a good time to cut off any excess with a sharp knife.
When it is properly hard, unscrew the screw from its new thread.
This should be easy if you covered all of the surfaces.
As I said - if you have never done this before, it is worth
trying out first so that you are aware of the pitfalls - like too
much epoxy; epoxy flowing out of the hole before it sets; too much
grease on the thread; not enough grease on the thread; not being
able to get a tool onto the screw to remove it.
If all else fails then it should be possible to use a nut and bolt all
the way through the assembly, suitably insulated using heat shrink
tubing. You would probably need to clamp the wing shaped
pickups and have the wings pressing upwards on the underside of the
axles. You would need to address the issue of the bogie being
prevented from turning, and you would need to make alternative
arrangements for getting the power from the top end of the bolt onto
the circuit board (instead of the spring).
Another possibility is to use the front pair of bogie axles for the
positive right rail pickup. The retaining screw screws
directly into the white metal chassis moulding so this would become
live. The circuit board presses onto it, so the wiring on the
circuit board would need to be different. The loco/tender draw
bar may make contact and would need to be insulated. But I
reckon that this would be possible, although I have never had to do
it. Every wheel on the left side of the loco provides pickup
through the axles from the left side of the track, so loosing
another two axles from the tender should not be a problem.