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
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.