Graham Farish Digihat Conversion Method

Mistakes may damage your locomotive, controller or other accessories. Use a multimeter to make absolutely sure you have completely isolated the motor from the track before testing. Always use the Programming Track for testing. Our guides are provided for reference only, and you proceed entirely at your own risk. Unsure about something - try our DCC Conversion Service instead.

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This is a general guide to converting many Graham Farish locos where one or more motor connections need isolating from the live chassis. This is done by replacing the factory-fitted brass sleeve with a plastic one.

The general layout is:

• One wheel contact comes up through the chassis on a long screw, which has a wire trapped under a bolt that provides power to the top motor brush, and also bridges to the other wheel contact via a capacitor.
• The other wheel contact is essentially the entire lower section of the chassis, including the other end of the capacitor. It provides power to the lower motor brush by virtue of that entire area being live.
• The motor brush is inside a brass sleeve, held in by a small spring and a clip over the top. The clip is in contact with the metal chassis and the spring, which effectively connects the track pickup to the motor. To convert to DCC, this link needs breaking so we can insert the decoder between the track pickups and the motor.

There are differing views on how the conversion should be done, but the procedure is basically the same.

Step 1 - Remove Body

On most diesels, the bodies come off easily by pulling gently upwards. You may need to put your fingernail between the body and the chassis to free it. If in doubt, consult the manufacturer's instructions and visit the loco's specific conversion page. On steam locomotives, there will be one or more screws in the bottom to remove, after which the chassis will pull forwards and down to separate.

Step 2 - Replace Brass Bush With Digihat

Locate the brass clip on the underside of the chassis, and remove it using tweezers or a small screwdriver. Once removed, it will reveal brass bush, with a small spring inside that is pushing a carbon brush onto the motor armature. Carefully remove the brass bush, spring and carbon brush. Replace the brass bush with the plastic 'Digihat' version.

On some chassis versions (e.g. Class 91), intead of a brass clip, there is a brass arm on a pivot that holds the spring and brush in place. Simply move it out of the way for now, but take care not to bend it upwards as you do so.

Step 3 - Attach Wire To The Spring

Solder a grey wire to the very top of the spring that was removed in step 2. Be sparing with the solder, as you do not want the spring to become stiff. Put the carbon brush back inside the Digihat, followed by the spring so that the wire is sticking out at the top. Take care to match the curvature of the carbon brush with the armature, otherwise the motor will need running in again and may run poorly initially until a new groove is made.

Step 4 - Refit The Brass Clip

Cut off a small piece of insulating sleeve and place it so that it covers the middle of the brass clip. Apply heat from the soldering iron or a cigarette lighter to shrink it in place. The insulation should be such that the brass clip cannot make an electrical contact with the spring.

On chassis types with the brass arm instead of a clip, apply the insulating sleeve to the whole of the arm in a similar way. The arm is moved back over to hold the spring in place, but now it no longer makes an electrical connection between the chassis and the motor brush.

Alternative to steps 3 & 4: The other way to do this, is to solder the grey wire to the groove on the brass clip instead of the spring. Rather than applying insulating sleeve to the middle of the clip, the sides are insulated instead. This means that the clip will no longer make an electrical connection to the chassis, but still does with the spring and carbon brush. It's personal preference which way you do this, the choice usually depends on which chassis type you have and how much clearance there is between the clip and the wheels.

Step 5 - Isolate Top Part

Snip off the capacitor that bridges the back of the chassis to the upper brass clip. Leave a few millimetres of the capacitor leg where it meets the chassis, and solder a red wire to it.

Remove the upper brass clip, and remove the remaining capacitor leg and the black wire. You will need to loosen or remove the nut to get the black wire completely off. The capacitor is not needed, so can be discarded. Solder an orange wire to the edge of the brass clip, and refit it. Make sure the spring and carbon brush did not pop out - if they did, put them back in.

Step 6 - Connect Second Track Pickup

Using your black wire, strip off about 5mm of insulation and form it into a loop. Put the loop over the screw that comes up through the chassis, and screw the nut back on. It is sometimes helpful to run some solder into the wire loop to ensure a good contact when the nut is tightened up.

Step 7 - Test

At this point, you should have four wires: red, orange, black and grey. Using a multimeter, verify that the track (red and black) wires have continuity to one side of pickup wheels (only). You can place the chassis onto a piece of isolated track if you prefer, and check that the red and black wires only have continuity with the left or right rails.

The red and black wires should not have continuity to each other, i.e. infinite resistance.

Check that the grey and orange (motor) wires do not have continuity to either the red/black wires, or each other. Once satisfied that there are no shorts, and everything is good, you can optionally attach a 9V battery to the grey and orange motor wires, and the motor should run.

If you are using a harness, you should be able to plug in a blanker plate and test the loco still runs on a DC track (or apply power to the wheels using a 9V battery).

Step 8 - Connect Decoder

Decide on a suitable decoder location, and route your four wires there as appropriate. Solder the wires on as follows:

Step 9 - Test

On your programming track, try reading the decoder ID. If all is well, the motor will judder a couple of times, and it should come back with the default ID of three.

At this point, refit the body and you're done!

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