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Graham Farish Class 37 Diesel DCC Sound (using CT decoder)

The N Gauge Graham Farish Class 37 diesel... detailed installation guide for DCC sound... see also additional notes here on using a Zimo MX648 decoder instead.

Ok, so here you have it - the definitive monster guide to installing DCC Sound in a Farish 37... you'll still need to be brave, comptetant, have good eye-sight, and a steady hand. It is a long process, but step-by-step you can see how it is done...

A number of modern DCC-ready Farish diesels can be converted in a similar manner to DCC sound, but here we use a Class 37 as an example.

Pop the body shell off, and dismantle the entire loco, carefully storing the small screws and other parts as you go along, noting how it all fitted together. A total disassembly is required as we'll be grinding the chassis to accomodate the speaker and stay-alive capacitor.

Our first picture below shows the chassis already milled with a recess at one end for the CT sugar-cube speaker, and a smaller, but still deep, recess at the other end for a capacitor. You can use a proper milling machine if you have access to one, but a simple hand-held Dremel-type tool with a tungsten carbide disc will do a good job too - it will just take longer.

Note how thin we have made the metal at the left-end of the chassis, which is where the speaker will be placed. It is as thin as we can practically make it without comprompising the structure - the more depth we have available for the speaker, the easier it will be to fit in the body, and the deeper we'll be able to leave the sugar-cube's enclosure, which will maintain good sound volume and depth.

Once you are happy that you've taken enough chassis away, and it is neat and smooth, thoroughly clean the metal blocks in whatever way you feel comfortable. Milling/grinding produces a lot of metal dust, which you MUST remove, or it will get into the moving parts or short the electrics later! I prefer to use a soft wide brush to work the dust out of all the corners, like the one shown here.

Next, tin the motor terminals with a little solder - best to do this now, as it will be much easier to solder the decoder's motor wires in place later on. You could even add some orange/grey wires at this point if you like, but it is not necessary if you have a long thin tip for your soldering iron.

Carefully reassemble the chassis with motor, bogies, drive shafts, plastic spacers etc. all in correct position. It is worth adding some thick grease to the ends of the shafts and around the motor worms while you are here too... but only a small amount - we don't want excess grease flying about! I use some high quality bicycle grease, but anything thick will do.

Prepare the chassis surfaces where electronic components will sit with firstly some thin electrical tape (the black tape shown here) and then some very thin double-sided tape so that the components will stay in place. Use as thin a tape as you can as height is a premium here. You need to protect the areas where the speaker (left), decoder (centre) and capacitor (right) will sit.

We've added an extra wire to the SL76 decoder for the stay-alive, which is the one closest to us in the picture below. This is the -ve pad for the capacitor.

As a rule, I place the decoder with the motor/pickup wires at the bottom, and the function output wires at the top, as I find it makes a neater job, with less wires criss-crossing each other later on. The SL76 is a perfect shape to fit into the recess directly on top of the motor.

Trim the orange and grey wires so that they still reach the motor terminals, tin their ends, and use a thin pointed soldering iron to solder the wires to the terminals. Orange will normally be on the left-side of the loco, and grey on the right. Be careful with this soldering to ensure that there is no contact between the motor terminals and the chassis, or this will instantly fry the decoder when you apply power!

Trim the black and red pickup wires to put a comfortable length to reach the 2 screw points where the original factory-fit circuit board was previoulsy mounted. We shall make use of these screw mounts as a convenient and reliable place to attach our pickups. Which is red and which is black doesn't matter too much, although by convention you would normally put red on the right and black on the left side of the chassis.

Ok, this is the point of no return for the original factory circuit board! We will chop up parts of it and reuse them in our new version of the installation.

Most importantly, we need to cut out the screw mounts, which we will reuse for our pickup connectors. Also, since we want to retain use of the factory head/tail lights, we also cut out some of the resistors to reuse for that purpose. Many Farish models have a small bank of 2, 3 or 4 resistors together, which we can take advantage of. I'm not going to explain how this is done here, as that isn't the purpose of this guide, but you may be able to figure it out from the pictures anyway.

Basically, snip off the lighting boards, as we'll need those intact for later, cut out the screw holes, and neatly snip the resistor bank.

In the case of this Class 37, Farish have only put 2 resistors in our resistor bank, as there were only 2 types of LEDs used. We shall be adding independent cab lights too, which will need a resistor each, so we'll steal a couple of additional resistors off some other circuit board (I think this one - the green one - was from a Dapol loco of some sort)! Direction of resistors is not important - just tin and bridge the gap to make a nice new bank of 4. Of course, if your soldering skills are not up to this, either skip cab lights, or use normal style resistors instead, which are much easier to work with, but obviously take more space.

Without going into too much detail here, the following photos show how I tested the resistor bank to find out which side was common and which side should be used for output to the LEDs. Essentially I use 2 differently rated LEDs (a red and a super-bright white), and test the various directions. First test one side by connecting a 9-12v DC supply and trying to run both LEDs at the same time off the other side of the resistors. If you got the common side wrong, then the higher-rated LED will suck all the juice and it will be the only one that lights up. If you get the correct side for the common, both LEDs will light up together, which is what we want - giving independent power via a resistor. I'm not going to explain it beyond that, as it isn't the topic of this guide, but the pictures may help...

Once identified, we attach a blue wire to the common side. This will eventually be connected to the decoder's blue wire. The resistor bank is stuck to the chassis using some thick double-sided tape.

For comparison, some normal resistors (1k in this case) are pictured, which we could have used instead of the SMD resistors.

A stay-alive capacitor is always a good idea with the SL76, as this decoder can be susceptible to audio crackles unless the pickups are perfect. We are using a tiny Tantalum capacitor rated at 100uF/20V in this case, but you could go a little larger, or even down to 100uF/16V. 16V should be ok for most DCC systems, as the track voltage runs at 13-16V, but if you run the track at a higher voltage, then you'll need to use a capacitor rated at least as high as your track voltage.

Tantalums have a bar at one end to mark the positive terminal (the opposite of electrolytic capacitors, so be careful!). We shall trim our decoder's stay-alive wires and solder them now. The black wire was the extra one we added to the decoder earlier, which is the -ve side - do not confuse this with the black motor wire of the decoder! The blue wire is the SL76's common +ve wire, which serves both purposes.

Stick the capacitor, terminal up, so they are away from the metal chassis completely.

We also trim the trailing blue wire from the resistor bank and solder that to the Tantalum's +ve terminal - it is easier to do this than to attempt to add 2 blue wires directly off the decoder!

Let's reuse those screw terminals that were cut from the factory circuit board now... tin the contacts with a little solder, taking care not to clog the hole itself with solder, and solder the red and black wires to a ring each. Very carefully insert the original tiny screws and screw them onto the chassis using the original screw mounts. Do not screw too tight, as the screws sheer very easily - they're only little!

At this point it is worth testing that the install so far works ok - you should get good motor control from the decoder. So, tie off any lose wires to avoid short circuit and run the chassis on your DCC system. If the engine runs the opposite way around from what you expect, swap the grey and orange wires.

Time to install the speaker! This is a CT sugar-cube (1W, 8ohm), which we will need to modify the enclosure of, by reducing its height and shaping the top to match the shape of the Class 37's body. You can take quite a lot of height off these enclosures before sound quality/volume starts to suffer, so no fears!

For comparison we show an unmodified enclosure and the shaped enclosure. The sugar-cube's terminals can be prised outwards and bent up, which means that it will sit flatter - this will prove useful to gain another millimetre in height.

We shape, reduce, shape, reduce, test-fit, with the 37's body, until we are happy what it all sits flush and the body goes on the way it is supposed to.

When you are totally happy that it fits nicely, use super-glue (sparingly) to seal the edge of the enclosure and pop the speaker itself onto it. Do not get any glue on the speaker's foil, or it will ruin the sound! Trim and solder the decoder's brown speaker wires to the speaker, and test that it sounds ok.

The following series of photos show the installation and wiring for the factory-fit head/tail lights. The wires are a little short, so we have unsoldered them, drilled a neat new hole, and soldered in some new wire which is a bit longer, and whose colours make a bit more sense on DCC!

Blue is the common +ve shared between all LEDs on the lighting board, then we've used a white wire for the white lights, and a yellow wire for the red ligths, which matches forward and reverse motion lighting wire colours in the world of DCC.

The blue from one lighting board will go to one of the resistors on our resistor bank, and the blue from the other end's lighting board will go to a different resistor on the resistor bank.

For forward motion lighting, we want the white wire from the front, and the yellow wire from the rear, joined together onto the decoder's white.

For reverse motion lighting, we want the yellow wire from the front, and the white wire from the rear, joined together onto the decoder's yellow.

Connect, protect with heat-shrink, and test. Testing could be interesting at this point as you'll have to get the 37's body shell in place so that the bendy electrical connectors make a good connection with the lights in the nose and tail. It is a good opportunity to plan where all the additional wires will need to go, avoiding snags, pinches etc.

The final part of our installation is to add drivers and cab lights.

We've used Bachmann/ModelScene figures, chopping off their legs in order to fit them neatly in - don't worry for them though - they feel no pain, and are much happier once seated behind the controls of a Class 37!

For cab lights, we've used some ultra-bright SMD white LEDs, which we will point upwards toward the ceiling of the cab. The ceiling gets a small piece of kitchen foil, super-glued in place, which gives a much more even spread of light around the cab than if it was pointing straight down. The LED therefore must be positioned 1-2mm lower than the ceiling to give the light a chance to escape up and bounce down again. We drill tiny holes in the back of the cab wall to feed wires through and glue the LED to the back wall.

It is important to wait for the glue to dry completely before putting it near the body shell, or you'll get 'glue fog' on the cab's windows, ruining the whole effect, and impossible to clean off afterwards, so be patient!

The positive wire from each LED needs to be joined to one of the spare resistors in our resistor bank, then the front LED's -ve goes to the decoder's green, and the rear LED's -ve goes to the decoder's purple.

Replace the cabs, ensuring that the LED's terminals do not touch the kitchen foil, and test using your DCC controller.

There you have it, wires all packed in neatly, body on flush... the finished article... brilliant!


Please note that these guides are provided as useful resources for you, as-is. YouChoos cannot be held responsible for errors in the information, or for any damage caused to your models or equipment if you choose to follow any of the steps detailed here.