Putting DCC sound into N gauge models is never going to be a ‘walk in the park’, but it isn’t as difficult as you might think. Sometimes all you need is a photographed example to give you the confidence to take it to the workbench. In this article I'm taking the recently re-tooled Graham Farish Class 25 diesel with the aim of adding a Zimo MX648 DCC sound decoder, a loud 'sugarcube' style speaker and cab lights that will illuminate in the direction of travel.
What makes this model a good donor compared to other Farish diesels is that most of the factory circuit board can be retained and therefore it avoids rewiring which could be a little daunting for anyone with limited electronics knowledge.
John Gymer is the proprietor of YouChoos,
one of the UK’s leading DCC sound specialists
and has been fitting sound and other enhancements
to models for almost a decade.
❚ Graham Farish Class 25
❚ YouChoos Class 25 sounds on Zimo MX648F: £99 See Youchoos.co.uk
❚ YouChoos ‘SugarCube7ʻ speaker: £7
❚ YouChoos micro cab lights: £3 each (resistor and heat shrink included)
❚ Graham Farish driver
TOOLS FOR THE JOB
❚ Quality soldering iron with fine tip
❚ Multi-tool with tungsten carbide cutting disc
❚ Hand files
❚ Small crosshead screwdriver
❚ Fine cutters
❚ Wire cutters/strippers
❚ Electrical tape
❚ Black paint
Pop off the body by gently prising the sides outwards. It’s immediately obvious that space is at a premium inside the bodyshell, but don’t let that put you off.
Using a small flathead screwdriver, pop out the bogies, disassemble all the clip-able parts and unscrew the circuit board (three screws).
Dismantle the chassis, removing all plastic parts. A YouChoos’ 'SugarCube7' speaker, ready to measure up for fitment, is also shown here. It will go in the fuel tank.
Cut a neat opening in the fuel tank underneath where the speaker will fit. I’ve removed move than is strictly necessary, but it makes adjustment of the speaker position, and positioning of wires easier later. You won’t see any of this later. Be careful to cut away the end from the chassis retaining screw.
A tungsten carbide disc is used to grind away the metal of the fuel tank almost up to the retaining screw aperture. A milling machine will do a neater (and faster) job, but most modellers will only have access to a Dremel-like tool. Shown left here is before cutting while the right half shows the difference after the cut.
Neaten the cuts with a hand file. Measure for size, checking that the speaker will easily fit flush and not sit lower than the bottom of the fuel tank. Note the groove in the side
of the chassis where the speaker wire will sit – a groove on each side is required.
Reassemble the chassis, and add the plastic frame. Ping out the speaker’s electrical tabs, bend them up (so that the speaker will sit fl atter), and solder on wires (Zimo
uses purple for speakers). Feed these through the grooves made earlier. Insulate the metal side of the speaker with electrical tape and add a couple of dabs of superglue
on each side so that the bond is permanent – we don’t want the speaker dropping onto the track! Don’t get any glue near the motor or into the speaker.
When you are happy with the fit, press the speaker into place, pulling the wires through to the top and refit the factory circuit board. Note that the lighting wires are not attached; the wire is very brittle, so often comes adrift, but it should be easy enough to reattach to the circuit board later (blue is the positive one).
As a test, join the speaker wires to those on the MX648 decoder and plug in the six-pin decoder (pin 1 is orange). We’ll remove the plug and socket in a moment, but it is
important to test that everything so far has been done correctly. Sound, lights and motor should all function on the track.
Unfortunately, the MX648 is slightly too big, but we only need a tiny bit more space, so chop off the six-pin socket from the factory circuit board, shorten the motor, pick-up and directional lighting wires, and tin them neatly with a little solder.
You’ll need a reasonable soldering iron and a steady hand to solder the wires onto the six pins where the socket used to be, but they are all quite accessible. Test everything again. You may need to run a craft knife between each pin to ensure that there are no short circuits because they are very close together.
Fold the decoder down into the opening and check that the body can be placed back on flush. Ideally the decoder should face down with the larger components (yellow capacitors) into the aperture of the circuit board, for the most efficient use of space. Secure with strips of electrical tape.
The YouChoos ‘SugarCube7’ speaker comes with a green enclosure, which is very cool, but we don’t want it to be visible in our fuel tank, so apply a little black paint to disguise it. Stop here if you wish - the cab lights and drivers in the remaining steps are more a nice-to-have, but do give the model more realism.
Pop out the cab mouldings, being careful not to break the clear plastic light ducts. Glue in a driver. Drill small holes for the cab light wires, ensuring that they are sufficiently to
the sides so that the overhead lighting block will still fit. I used YouChoos micro cab lights here, which give a pleasant white/yellow glow. Glue it in place and be patient waiting
for the glue to dry completely before putting the cab back into the model, or you’ll end up with frosted windows.
Wiring of the cab lights is very simple; blue goes via a resistor to the decoder’s blue. Grey wire goes to decoder’s green or brown (green for front light and brown for rear light). Neatly cover the resistors and your joins in heat shrink, and test again on the track to make sure it all works, and in the right direction.
Finally, reassemble, taping up the wires inside and taking care not to snag anything as you squeeze it back on. It is a tight fit, but done neatly, the bodyshell will go back on flush. That’s it - you’re done, and you have a fantastically loud Class 25 in N gauge.
CAN I DO THIS TO OTHER GRAHAM FARISH DIESELS?
The Class 25 is unusual in that you can leave the factory circuit board
largely untouched, and the only feasible place to locate such a large
speaker is in the fuel tank. Other diesels tend to require total removal
of the circuit board, therefore requiring re-wiring of motor, pick-ups and
lighting. This isn’t necessarily complex, but can be fiddly. Speakers are more
commonly fitted on top of the chassis, which would have to be filed down to
create extra space. YouChoos’ ‘SugarCurve7’ speaker is ideal in those cases
as the shape will fit the roof of many diesels perfectly.