Classic UK Minitrix Models - Class 27
The upper circuit board is attached to the lower circuit board with
two wires. The lower circuit board is underneath the motor.
It is not necessary to lift out the worm gears, but if you do, watch
out for the spacers and washers at the exposed end of the worm gear.
They will fall off when you least expect them to. Best remove
them and keep them safe. Note that the washer is there as a
spacer. On my two Class 27s, one side has a washer, the other
doesn't. Take note of which side has one if yours is the same.
See the section on the motor for photos on how
to do this.
Here is the loco body with the motor removed and the two circuit
boards lifted out. Note that nothing holds them in place
except friction, and only the red and green wires join them
together. The friction is provided by the copper tabs - in the
case of the upper board, the tabs slip behind the bulbs for the
positive electrical contact. For the lower board, the copper
strip for the negative pickup presses against the side of the loco
Use a soldering iron to unsolder the three contact points from
the selenium diode. The diode is held firmly with three stiff,
soldered contacts. You may have to release it in small steps.
Use a small screwdriver under the diode to ease the square plates away a
fraction as you melt the solder on one leg. Then do the same with
another leg, and then the third. The diode will probably
remain soldered, but it will have been raised a fraction, making the
next pass much easier. Keep doing this, raising each leg a
little at a time until it is free.
Do not leave the soldering iron on the joint after the solder has
melted. It is possible to overheat the board, and the copper
conductive strip will become unstuck.
Remove the two wires which connect the lower (smaller) circuit board
to the upper. On mine, these are red and green. Clean
the soldering iron and pass the tip over the blobs of solder to
remove surplus and to clean it up a little. Clean up the
contact strips with a very fine wet and dry emery paper.
The circuit board now looks something like this at present. No
wires, no components. Solder points have been 'refreshed' with
a hot soldering iron, and circuit board tracks gently cleaned with
fine wet and dry emery paper. (They don't really need this,
but there's no point in it looking grubby).
On the diagram,
the white track is for the front light, which contacts the bulb end
via a copper tab on the underside of the board. The yellow
track is for the rear light. The black link has no purpose on
the Class 27, it is there for the positive model 2931 which uses the
same chassis and has overhead power pickup. The purple
sections of the circuit board are for the selenium diode
contacts which control the directional lighting.
Maintaining Classic UK Minitrix Locos
The Minitrix trademark is currently owned by Märklin Inc.
Gebr. Märklin & Cie. GmbH, Stuttgarter Straβe 55-57, D-73033 Göppingen,