Classic UK Minitrix Models  -  Class 27
Fitting the Chip
The Minitrix® Class 27 Diesel
Fitting a DCC Chip - The Lower Circuit Board

The lower circuit board has 3 copper tabs on one side, and one or two coils and a capacitor on the other.  Note that the green wire is soldered to one leg of the capacitor, and to one end of a coil.  It looks as though it is soldered to the board, but isn't.  No need to de-solder it then.    The red wire is soldered to the centre strip.  Leave that in place, we can use it later.  The other points are soldered onto the end of the copper tabs which are clasped onto the edge of the circuit board.  These clasps must be left in place.  Look at the photos, look at your board, and understand what you are de-soldering before you start.
In this photo, in spite of my warning above, the chassis tab came off the circuit board.  It turned out to be a very loose fit.  So here is what that looks like in close up.
Here is the lower circuit board with the two motor tabs on either side of the board, and the single negative tab which presses against the inside of the body.
And here is the dislodged tab fitted back onto the circuit board.  It was held in place by the capacitor before we removed it.  Keep an eye on it, it may slide off again.  When we come to connect wires, careful routing of the negative wire before soldering may help to secure it again.
Here is the circuit board cleaned with everything removed except for the red wire.  I believe that I may be able to re-use this, so I have left it in place.  I may change my mind later.

Note the cleaned up tracks at each end of the circuit board.  This is where the positive contact from each bogie sweeps across and provides the positive power to the loco.

You may notice that the inside of the diesel body has some black paint sprayed on it.  I assume that this is stray paint from spraying the underside of the chassis.  I don't know if it is conductive, but I would guess not.  The tab on the lower circuit board makes contact with this painted surface, and provides the only connection that the diesel has with the negative power.
My suggestion is that you clean this surface, to ensure that the connection is a good one.  The photo shows the lower circuit board in place with the side cleaned of spray paint in the important contact area.  Note how close the contacts are to each other, and when the motor is inserted, how close the side contact is to the motor body. 
Note that the motor has TWO (red) insulating tubes on the brushes.  Normally, this type of motor has one, with the negative power provided through the motor body and spring to the brush.  Had this been the case, we would have to isolate the brush from the motor body.  Given that not all locos are not manufactured in the same way, just check that your model has both ends of the spring insulated.  If not, make it so.  Once they are both insulated, the proximity of the side tab in the previous photo, is not an issue.
Make sure that the contacts on the bogies underneath the lower circuit board are clean and that they press upwards onto the 'sweep' track on the underside of the circuit board.  (You did clean that track, didn't you ?)

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

Website ©2002-2018