Classic UK Minitrix Models  -  Britannia
Electrical
A common problem when reassembling the engine is that of getting the polarity wrong. The left hand side of the engine and tender are electrically isolated from the right. The Mallard has a total of 10 axles, and all 10 are responsible for providing an electrical connection with one or both of the rails.

It is easy to think of the right hand wheels as being connected to the positive rail, and the left hand side being connected to the negative rail. The chassis is connected (mainly) to the left hand rail.

From the front. Both front bogie axles have a pickup which is connected to the chassis and to the left hand side wheels. The right hand wheels are isolated.

The 3 drive wheels have a cog on one side, and this side is connected electrically to the axle. The right hand side of the axle is seated in a plastic bush to isolate it from the right hand wheel. On the right hand side, the front and rear of the 3 drive wheels have a brass electrical pickup which rubs on the inside of the wheel as the train moves along. This is held in place by the natural springiness of the thin brass strip. If this has been bent away from the wheel, it is difficult to perform an effective fix without taking the wheels out. A hand lens helps to inspect whether or not contact is being made, and it also helps to locate the muck and fluff that the pickups have collected. A strip of paper or card inserted between the pickup and the wheel can help to polish the connections. Traction tyres are fitted to the left side front and rear drive wheels. This reduces their effectiveness in making a connection to the left rail unless the flange makes contact. However, the centre wheel will make this connection.

The rear bogie has its left wheel connected to the chassis.

The 8 wheel tender has an interesting arrangement. The axle is connected electrically to one wheel and is isolated by a plastic hub on the other side. All 4 axles have an electrical pickup, but the rear pair have the conducting wheel on the right (positive) side, the front pair have their conduction axle on the left (negative, chassis) side.

If testing the isolation between the left and right side, it is important to remember that the motor will conduct electricity between the left and right side. On my multimeter, a reading of 1ohm is typical for a direct connection, and 8 ohms for a connection through the motor.

Click the thumbnails for a more detailed picture.

Main chassis with no valve gear to get in the way. The wiring has been replaced. I used the centre core from some screened multicore hi-fi cable that I had. Its quite thin, but still thicker than the original. Notice the brass pickup for the right hand side on the upper half of the front and rear main wheels. Note laso the black insulation on the brass strip.

From below. The base plate is homemade (described elsewhere). From this view you can see the brass contacts on the front and rear drive wheel and the plastic, isolating hub. Even after a clean, you can still see bits of fluff and muck.

Main chassis from the left. Note the red insulating blocks and the black sleeve insultaing the joins in the blue wire to the lamp. The capacitor which is supposed to connect the blue wire to the central pillar, has not been replaced.

Main chassis from above. It is important to get the routing of the blue wire correct, and properly insulated. If it is necessary to straighten the pickups, then the strip can be removed by unscrewing the screw in front of the central pillar. The pickup is soldered to the blue wire. Later models may not have the front lamp.

The front bogie is located in position by a plastic pin which forms part of the baseplate moulding. As well as acting as a spring to keep the bogie wheels on the track, the brass strip provides the electrical connection between the chassis and the left hand side wheels. It does this by making a sweeping contact with the raised metal moulding at at the front of the chassis underside. On this model (a Flying Scotsman which has the identical chassis to the Mallard), the raised contact was painted over at manufacture, effectively removing one of the negative side pickups.

Detail of the insulating blocks. The central block holds the tang on the upper part of the valve gear. The moulding of the red block keeps the valve gear on one side electricaly isolated from that on the other side.

The weight is a prime suspect for shorting out the left side and the right. Note the slot provided to sit over the blue wire, and the gap for the insulation for the copper strip. After much use, it is possible that the blue wire could wear through its insulation.

Two views of the underside of the tender. The copper strip provides an electrical connection between the axle and the wheels on one side.

The bottom photo shows the plastic hub on the left hand side of the photo and the direct metal on metal contact between axle and hub on the right hand side of the photo. So the tender has two wheel picking up the positive rail and two wheels picking up the negative.

The 3 wires from the loco to the tender.

The bottom photo shows the soldered joints - here filed down so that no sharp edges can prick through the insulation layer (not shown) which sits between the circuit board and the metal on the tender chassis. Note the spring to the rear of the tender which provides the electrical contact for the rear (positive) axles. The coil is a choke which helps to prevent electrical interference with radios and TVs.

Wiring to the motor. The green wire is the positive connection from the tender. The black wire is the negative (chassis) side. Electrical connection to the chassis itself is made via the brass strip which sits under the motor and which is soldered to the bottom connection of the motor. The blue wire takes the connection from the electrical pickups on the driving wheels to the circuit board in the tender, and is routed on the right side of the cab. I have cut the wires too long, but this helps to work with the loco and the tender when they are uncoupled.

Maintaining Classic UK Minitrix Locos
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