Dapol N gauge Class 56 diesel with MX660 PCB sound and speaker in body

Kit: MX660; SugarCurve6; SACC16 + 2x 470uF Flat Tantalums; Drivers; Mini Cab Lights

I've never been a fan of putting speakers in fuel tanks, especially in N gauge diesels where the speaker's magnet ends up so close to the track. There are ways to take precautions, but I would much rather have the speaker safely under the roof, which is not only safer for operations, but improves the sound in N gauge too.

So this guide started out as an experiment to see if it was possible at all, using the Zimo MX660, so called 'drop-in' sound decoder. Below, a BR blue example.

Unlike many of the Farish diesels, the Dapol counterparts tend to have chunkier chassis and motors, which is good for traction and running, but not so good for fitting DCC sound into. With the MX660, the theory is that we can grind a skinned layer off the top of the chassis in the roof to make just enough room for the decoder, which is very thin. We'll aim to place a SugarCurve speaker in the area where the 6pin socket is situated to begin with. Grinding of course means total disassembly, which isn't too hard.

Here is the chassis and the MX660 with a SugarCurve7, which we are hopefuly of using.

We cannot even do a test-fit until some grinding is done, so with all the mechicals and electrics removed, we bind the chassis halves with tape and take to the milling machine for an initial cut.

With a layer skimmed, we clean off the burr and re-assemble all the parts in order to do a first fit with the decoder and speaker roughly in place.

A little over-hopefully, I attached orange and grey motor wires prior to re-assembly, thinking I'd taken plenty off and that would be it...

First try with the speaker toward the centre of the chassis, but that doesn't work yet, partly due to the spring clips on the speaker, and partly because of the roof detail that protrudes into the roof space a little, so we move the speaker along more toward the end and try again. Seems to fit well if the 2nd retaining bracket is left out.

However, it turns out that it is the decoder that won't allow the body to fit back on correctly, so we have to disassemble once again and skim a deeper layer off. We actually have to file down the black plastic brace that holds the motor too, and shift the location expected for the decoder along away from the motor.

A more serious try this time, laying electrical tape, the double-sided tape to keep the deocder from moving, and similar for the speaker.

This is much better, but we find that the roof detailing still affects the correct seating, so we carefully sand it off flush inside, and finally we have a good fit. Clearly not much lea-way, but it does go.

We opt for a SugarCurve7, as that seems OK, bend back the terminals completely and cut them short. This means that the speaker will sit flat and we have convenient tabs to solder wires onto as well.

We begin the process of wiring up - orange and grey to the motor terminals, purples to the speaker pads, and red and black to the track pickups - both ends fo the chassis. At this point, we are happy, and can run the loco without problems, with the body back on (but cabs removed).

With the cabs removed we peel back the insulation pads and replace the wires with finer and longer decoder wire, using standard colours - blue for common positive, white for head lights and yellow for tail lights. Do the same for both ends.

Drill holes to accept our cab light wires, and fix those LEDs and the driver figures into place.

You may have spotted already, but there is a nice little space between the speaker and where the chassis steps up to the motor - perfect for a bit of stay-alive! We cut down the SACC16 PCB and wire up a couple of 470uF Flat Tantalums. The capacitors sit next to each other across the chassis, with the SACC16 PCB on top - it is below the level of the speaker top, so we know we have space for it.

The SACC16 is attached to the decoder's GND and a +VE pad.

At this point the wiring is very tidy and we can do a quick running test to prove the use of the stay-alive - and it does work really well indeed! 2x 470uF is loads for N gauge, and you can certainly tell the reduction in stall risk... in fact, we ran ours for quite some minutes on crawl in both directions without it missing a single beat!

Of course, we have no cabs or lights installed yet, and this is where the serious work starts...

Trim and attach the blues to the various +VE pads around the decoder, and the function outputs at the appropriate ends - we use F0Fwd for front headlights, FA1 for front reds and FA2 for front cab. Then F0Rev for rear headlights, FA3 for rear reds and FA4 for rear cab. This is generally how we supply all MX660 in terms of function key mapping, as it means we can have the shortest length of wires for lighting at the appropriate ends.

With all this extra wiring to pack in, we realise that the SugarCurve7 is no longer going to work, so have to switch that down to a SugarCurve6. Pity, but OK, still a good speaker. Placing of the wiring is crucial here - anything from the end with the speaker must be routed around the speaker and not over it - tiny blobs of Black Tack help achieve that until the roof finally goes on flush again.

Done - finally - and quite an achievement. This is certainly not one for the novice - careful grinding, measuring, re-measuring, and very fine soldering required throughout as well as a lot of patience to get the body back on again at the end. However, it does prove the possibility, and this model actually runs, sounds, and looks fantastic!

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.