Classic UK Minitrix Models  -  Class 27
Upper Board
 

First of all, knowing how the Class 27 Diesel's electrics are configured, straight out of the box is a great help.

Power is drawn from the rails.  When the loco is going forwards, the right hand rail is connected to the positive 12 volts DC supply, the left hand rail is connected to the 12 volt negative supply.  To go backwards, the positive and negative supplies are reversed by the controller.

In the D27, all 4 right hand wheels have pickups which supply power to the motor.  The left hand wheels have an electrical connection to the axle, which conducts the negative side of the 12v power to the chassis.  The upper circuit board has tags which contact with the chassis.  The lower circuit board has tags which contact with both motor brushes.

The D27 has lamps - the lamps light according to the direction in which the diesel is travelling, something that is normally achieved by a silicon diode, but in the case of a D27, the effect is achieved using selenium plates. 

The diode symbol can be thought of as an arrow.  Positive power can flow in the direction of the arrow.  It cannot flow in the opposite direction.  The diode blocks it.  In the diagram, if the centre connection is positive, the power cannot flow to the right.  It has to flow to the left, in the direction of the left arrow (diode).  The only way the right hand lamp can illuminate is if the right hand side of the diagram has positive power.  (Eg from the left rail if the loco is in reverse).

Putting it another way, if the right hand rail is positive, as it will be when the loco is going forwards, power can pass through the diode to the forward lamp.  When going backwards, the diode blocks this path, but the positive power is picked up from the left rail, passes through the bulb and diode to the right hand rail, which is now negative.

Here, for sake of completeness, are all of the relevant bits removed and laid out.  Note the labelling on the paper of 'F' and 'R'.  I've also labelled the loco and the boards. The green bit across the middle of the body is a bit of insulating tape that is put in during manufacture.  I sliced  it on one side to remove the motor and later, I sliced the other side.  Later photos just show the stub.  I may need to put something in its place, depending what potential contact with the motor body could occur from the upper circuit board.
Information:

The photo shows the upper circuit board, connected with two wires to the lower circuit board.  In the middle, the yellow covered square shape is the selenium diode, which is soldered to the circuit board in 3 places.  The selenium diode controls the flow of power to the appropriate bulb, as described above.

Laws demand that electrical items such as model trains are fitted with devices to prevent interference with radio, TV and other similar electronic equipment.  The model trains are fitted with a capacitor which bridges the positive and negative lines, and choke coils which are fitted in series with the motor, which filter out the high frequencies caused by sparking which create the interference. The photo shows the yellow capacitor and the two tiny coils of wire which are mounted on the lower circuit board.   However, most DCC chips have their own methods for blocking interference built into their circuits. 
Here is another example of the lower circuit board.  Same model loco, but a different age.  I do not know which one is older, but this one has a black capacitor and only one choke coil.  Whichever version you have, all of the components need to be removed (except the red wire if this is in good condition).
 
Loco Orientation - Which Way is Front ?
There are a number of clues that will help.  Maybe.
To be absolutely correct, the circular ventilation fan moulded onto the roof, should be at the front.  But of course, that depends on who put the roof there, as it will fit either way around.
By turning the motor over and looking on the inside of the wheels, it is possible to see the copper electrical contacts.  Shown here with the bogie partially removed for clarity.  The contact strips are on the right side of the loco.
The wires from the lower circuit board should rise up the right hand side of the loco - there are two notches in the edge of the board to allow this.
The lower circuit board should have an earth contact with the body on the right hand side of the body.
The motor should have the removable bracket (with the slot) at the front.  Note the small circular location pin on this bracket, and the matching hole in the circuit board.
Upper circuit board with free motor bracket held underneath, locating pin in place.
The screws for the worm gear covering plate are on one side of the loco only, and can be used to remind you which way is which.  Not reliable though becasue the chassis can be turned round 180 degrees and be still assembled correctly.  This photo is from a brand new, previously unopened Class 27.  The model I used for fitting the DCC chip had these screws on the opposite side.

Maintaining Classic UK Minitrix Locos
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