Classic UK Minitrix Models  -  Class 27
DCC Intro
This DCC chip or wiring socket has 6 wires

Black - Left Side Power Pickup (-ve)
Red - Right Side Power Pickup (+ve)
Orange - Motor +ve
Grey - Motor -ve
Yellow - Reverse Light
White - Forward Light

There is no common blue wire for the lights.  Instead, the chassis connection is used, which results in the lights being lit for only one half of the AC cycle, so will appear slightly dimmer.  This is fine, since the bulb in the loco is connected to the chassis, and wiring it in any other way would be a pain.

The top circuit board is now used only for providing power to the lights.  There is no diode, and the other circuitry is for the overhead power lines for some models which share the same chassis as the Class 27.  The diagram shows the location of the Yellow (Reverse Light) solder point, and the White (Forward Light) solder point.
The photo shows the underside of the lower circuit board, with front on the left.  The solder points are circled with the colour to match the wires from the dcc chip.  Note the black lead which attaches to the tab which presses against the right side of the chassis.  This helps to get the correct orientation of the board when placing it in the diesel body. 
Here is the Bachmann DCC Decoder number 36-558, which has back EMF built in, and two functions additional to motor control.  The leads and socket are supplied separately in packs of 3.  Note the white spot on the socket and on the chip.
Take care over which lead to solder first, in order to reduce the risk of the tag being soldered conducting heat into a wire that is threaded through the adjacent hole.  I chose to thread each wire through the hole furthest away from the solder point, so that the wire didn't have a bend in it immediately after the soldered joint.

So here's the orange wire ready to be soldered.  Note the wire is already tinned, and the tag is already soldered.

And the completed joint.  The use of eyebrow tweezers (shown in the photo above) is for 3 reasons.

i) It stops the insulation being melted further back
ii) It acts as a heat sink preventing heat getting to the chip (not actually a problem here because the wires are quite long)
iii) It stops me from burning my fingers, and allows me to keep the joint still until the solder has set.

It does mean that the insulation near to the joint is melted flat.  Tough.

Here, soldering the white wire to the tab adjacent to the hole through which the orange wire has been passed.  The strip of card is to prevent the heat from melting the orange wire. 

The pliers help to keep the copper tabs under the circuit board from being squashed, and als help to keep other wires out of the way.

Note the earth contact tag has fallen off the circuit board.  I hadn't noticed it at this point, and spent a while looking for it afterwards.

All 4 wires soldered into position.  It looks a bit of a mess here, partly due to the flash and partly due to the flux from the solder.  Note that the soldered joints are shiny, not dull.  They go dull if the joint is moved as the solder sets.  Not good.  I've also smoothed the blob of solder by removing the soldering iron sideways.  Any spiky bits then point to the side (as can be seen on the orange wire.
Check the location for the bottom board.  Note the copper tags either side of the centre well.  These need to be sprung upwards so that they sweep across the underside of the circuit board as the loco goes round  bends.  The centre well is where the wiring and soldered joints on the lower circuit board sit.  It looks like a thin piece of sponge at the bottom, but it crumbled away when I picked it up, and was soaked in oil - presumably from overenthusiastic lubrication.  Whatever, I took it out, degreased the well and placed a rectangle of double sided sticky pad.  This is to ensure that the lower circuit board stays put when inserting the motor.

Sounds well thought out, doesn't it ?  But it was trial and error.  I couldn't get the circuit board to stay in place as I was inserting the motor.  Sticky pads were the solution.

Here's the lower circuit board.  I was still unhappy about the risk of the right motor contact touching the earth contact when the motor was pressed down onto it.  So a bit of black insulation tape and a scalpel helps to put that right.
Locating the lower circuit board.  Main concern is that the board faces the correct way, and that the sweeping contacts from the bogie cannot be seen at either side.  Move the bogies left to right to ensure that this is the case.  This is where the sticky pad in the well came in handy.  The front of the loco is on the left of the picture.  Note that the chassis contact is on the right hand side of the diesel body.

A couple of photos to show how the bogie contacts sweep across the underside of the lower board.  The silver bit either side of the black chassi that says 'Trix' and 'Western Germany' are the two ends of the lower circuit board.  The contact can be seen touching this silver strip in both photos.  Note I have partially removed the bogie locating pin to allow the bogie to drop a little in order to take the upper photo.

Preparing the motor for insertion in the well.  Note the contacts for the brushes.  Note the copper strips with which they should match.  The lower picture shows the two components out of the loco body.  Get this the wrong way round and the motor will not get any power.

All 4 wires will come up the side of the motor from the rear on the right hand side (furthest away from you in the photo.  Try to keep them clear of where the brush ends are. 

Not that the loose motor bracket (which is not screwed to the motor) is to be placed at the front.

Showing the rear (brush end) of the motor.  Make sure that the oval locating lug on the ned of the axle is pointing straight up and down.  In the photo it is pointing to the 11 o'clock position.  It needs to be moved to 12 o'clock.  The locating lug at the other end may not be in the same position.  Make a mental note of where it is.

While you are at it, align both slots at the end of the worm gears to the 12 o'clock position, ready to accept the motor.

Here showing the top circuit board with yellow/white leads soldered, and the motor with brackets.  Both shown with the front of the loco to the left of the picture.  Note the locating pin on the removable bracket and the corresponding hole in the circuit board.  I have filed out the two slots in the circuit board to allow 4 wires to be routed from the lower circuit board.  Previously, only one wire emerged through each hole.
Here is the motor dropped into place.  This takes a little manoeuvring.  Make sure that the wires are flat against the side and do not cross over each other.  Lower the rear into position first, checking the alignment of the motor lug with the worm gear slot, by viewing from the end.  You may need to turn the motor a little to align the front end.  When everything is right, the motor drops in easily.  If it requires force, something is stopping it.  Don't use force.
  At this point I did some checking with a multimeter.  Take out the chip and use a pin in the appropriate pin sockets.  The grey socket should show no resistance when tested against one of the brushes.  Ditto the orange when checked against the other brush.   This confirms that the motor is making contact with the lower circuit board.
Place the diesel on a track.  The red socket should show no resistance when checked against the wheels on one track.  The black socket should show no resistance when checked against the other track. 
Checked, the top is replaced. Push the bulbs outwards to leave room behind for the tab on the underside of the circuit board.  Locate the tab at one end, then offer up the other end.  You will need a screwdriver to spring the tab behind the second bulb.
I small strip of tape helps keep the board in place.  Note that the yellow and white wires are much longer than the others.  I chopped these shorter and re-soldered the ends at this point.
The strips of tape are covering the holes for the screws which locate the body.  A quick prod with a small pointy thing sorted that out.  The small pointy thing is shown here in the hole as I routed the wires around it  - the bit of white is a sellotape sticky pad.  There is plenty of room in the roof of the diesel to accommodate the wires and the chip.
Finally tidied up (well almost).  The loco runs nicely, and the directional lighting works well.  Function 0 needs to be turned on at the controller for the lights to work - which had me puzzled for quite a while.  If if the connection is intermittent, the state of function 0 is reset on my (Hornby Elite) controller, so the lights never seem to illuminate. 
Given the auto-turn-off feature of Function 0, ensuring a good contact is essential.  I forced a small 'U bend' into the centre of the earth clip by removing the bulb, re-inserting the clip, and pressing down on it into the hollow in which the bulb sits with a small screwdriver shaft.  This provides a much more solid contact with the body of the bulb.

Maintaining Classic UK Minitrix Locos
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