DCC conversion and rear
light switching update of the Hornby Class 50 locomotive.
The smaller pictures can be enlarged by clicking on the image
This page provides a summary of the process adopted to incorporate a DCC decoder and DCC switchable rear lights in the BR liveried Hornby Class 50 Locomotive "Ark Royal" (OO gauge).
Class 50 with a train of BR mark 2 coaches plus a mark 1 buffet car in the centre.
|Purpose of the exercise:
To replace a previously fitted TCS T1 decoder with a T4X decoder type, in order to be able to switch the rear lights on or off via the DCC controller. When pulling a train, the locomotive rear lights are switched off. For contemporary operation, e.g. on a rail tour, a flashing rear light on the rear coach, provides the rear warning indication.
Lighting arrangements: The Hornby model includes directional headlight, upper marker lights and rear lights, using LED and light-pipe technology. The lighting is fine for rail tour simulations except for the permanently "on" rear lights.
DCC Conversion Approach:
The plan is to use a 4 function and back emf motor controller TCS T4X decoder. (To provide the extra switching line to enable/disable the rear lights).
Hornby Circuit Configuration:
What would otherwise have been a straightforward modification, is complicated by Hornby's lighting PCB circuit, which uses a common negative supply for the three sets of lights powered via each PCB. In order to simplify the final circuit, the negative lead from the red rear light LED on each lighting PCB was cut, so that an independent negative wire could be provided for each rear light LED. A single PNP transistor can then be used to enable or disable rear light operation. The surface mounted parts on the Hornby PCB are just not needed for DCC operation. Indeed they get in the way of dimming functionality in the TCS decoders. So I plan to remove all of the (now redundant) surface mount components from the loco's main PCB.
The Class 50 Chassis
The PNP transistor enables or disables the positive voltage to the rear light LEDs under control of the green decoder wire. The white wire is programmed to go negative when the locomotive moves forward and the yellow wire is programmed to go negative when the locomotives moves in the reverse direction. Both white and yellow wires are enabled via DCC controller button zero. The resistors in series with each LED control the current flowing through that LED. This determines the intensity of the light generated by the LED. The above values were established by experiment. Some adjustment may be required if different lighting levels are preferred. The motor and track pick-up connections to the decoder make use of the 8 pin NEM652 connector fitted to the model's main PCB.
Pin Out for BC556,557 & 558
|Number 1 and number 2 Cab
The number 1 end cab is adjacent to the large side ventilators and the roof fan. The opposite end of the locomotive is referred to as the number 2 end.
Achieving Access to the loco:
First the upper body shell must be separated from the wheeled chassis unit. REMEMBER to first disconnect the miniature orange dummy cable plugs from the cab front. Then the loco sides can be carefully pulled outwards to enable some thin pieces of stiff cardboard to be inserted between the outer and inner body clips. (See the photos below for the location of these clips.) The chassis unit can then be carefully extracted from the upper body shell.
Upper body shell and lower chassis, showing the clip locations
|Modifying the Hornby
The upper leg of the red LED (located to one side of the board) is cut and a new wire is connected to the now isolated LED lead. The wire is fed up and over the cab door aperture, running parallel to the existing wiring on that side of the cab.
|Removing the Hornby main
Apart from the 8 pin socket and the cable attachment clips, all other parts are removed using a pair of miniature soldering irons. A piece of plasticard is then cut to size and glued to the top of the PCB to insulate the new circuitry from the original PCB tracks.
|Fitting the new Circuit:
The new circuit is constructed on a double sided adhesive foam pad placed on top of the plasti-card insulator and connections are made to the wires from the lighting PCBs. The T4X decoder is also mounted using a double sided adhesive foam pad. It is connected to the motor and wheel contacts via the 8 pin NEM652 connector on the PCB. The lighting control wires are individually soldered to the relevant points on the new circuit (as indicated in the circuit diagram above and layout diagram below). The decoder is programmed using the cv values indicated in the table below.
Circuit diagram redrawn to show the approximate layout of the wires and components on the locomotive.
New circuit and T4X decoder assembled and held in place by double sided adhesive foam pads
|Programming the CVs:
Using the DCC controller (mine was a Bachmann Dynamis) the CV values were programmed into the TCS T4X decoder.
Headlight and marker lights on via button zero
(Forward end only)
Rear lights enabled at the back via button 9
(Switch off button 9 and they switch off)
The finished Class 50 (with additional hoses dangling) at the head of the train
An alternative approach without the transistor:
|With a 4 function decoder,
another approach is to control forward and reverse lights
independently via the decoder, at each end of the loco:
The negative end of the red rear lights LEDs need to be isolated from the negative ground board track as in the solution above. This time however, all the LEDs connect to the decoder with no transistor in sight!
Having removed the unwanted Hornby main board circuit as before, an approximate layout would look something like this:
Programming the CVs:
The CV values for the TCS T4X decoder are a little different.
Press button 0 for number one end directional lights.
Press Button 4 for number 2 end directional lights.
So if there is a train on the hook only use button 0. If you are running "light engine" use buttons 0 and 4 to activate all the running lights.
If the dimming programming options shown in red above have been used: The loco forward lights should dim whenever the loco comes to a halt. The forward lights should revert to full brilliance as soon as the DCC controller is used to move the loco. The dimming level can be adjusted via CV64 between 1 (minimum brightness) and 30 (max brightness). To keep the forward lights dimmed while moving, press button 5.
|Supplier website links:
The photo of the real Class 50 was taken on the MNR at Dereham in Feb 2010. The model photos were taken using a tripod, on the kitchen worktop, usually with fill-in flash.