Classic UK Minitrix Models  -  Class 27
Lower Board
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 body.

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, Baden-Württemberg

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