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Graham Farish N gauge Class 37 diesel with MX660 drop-in sound decoder

Kit: Zimo MX660 with YouChoos SugarCurve6 - also with stay-alive on SACC16 with 1x 330uF Tantalums, cab lights and crew

The launch of Zimo's MX660 drop-in sound board for N gauge opens up new possibilities to make fitting sound to many Farish (and possibly Dapol) models much easier than before. The MX660 was designed for Kato models, but the design is excellent and suits UK-outline models well too, with careful thought on where to put PCB-edge solder points for the various wires required. Unlike all other Zimo decoders, the MX660 provides all of its' AUX function outputs as 3V (instead of 12V) so there is no need for any resistor components to be added for lighting - brilliant piece of thinking!

This is quite exciting, and we choose to try the board on a Farish Class 37 diesel, the aim being to get good sound out of it without any milling of the chassis, and without modifying any plastic parts, so seems like quite a challenge...

The MX660 is a very thin single-sided PCB, meaning that we only need a tiny clearance on top of the chassis. We've also developed a reduced-height SugarCurve speaker (6mm) for the project, as this needs to sit on top of the chassis too, and the SugarCurve7 is marginally too high. Of course volume and bass will be reduced a little, but performance is still plenty for N gauge.

The MX660 will normally be supplied with wires already soldered by YouChoos, since they need to be added in order for us to have loaded the sounds! You might need to solder one or two extra though, if you intend to use more of the AUX functions for lighting - we'll do that as we go along with the install here...

The model's existing PCB is unscrewed and lifted out, including lighting connection boards:

Loosely placing the decoder and speaker into their approximate eventual position shows that the MX660 will have to sit to one side, as the pickup screw points are not far enough apart for the decoder to fit between them. No problem though - the decoder is narrow enough, and thin enough that this is easy.

Protect the chassis with Kapton tape, and also a layer of thin double-sided tape to keep the decoder in place.

In order to connect the pickups to the chassis, we reuse the screw holes from the original factory PCB. Snip them carefully out, tin them, and solder the red and black wires from the decoder onto them.

We use a slab of Black Tack underneath where the speaker will go, wrap the speaker in Kaptop to prevent shorts, and hold it in position with another piece of thin double-sided tape.

Now for what is probably the trickiest bit of the installation... solder the motor wires directly onto the motor terminals. This requires a long pointed solder tip. If you don't have this, then you'll need to disassemble the chassis to do the soldering at the start of the whole process - not hard, but nice to avoid that step if you have a suitable iron.

At this point you can test that the loco runs OK and the sound works.

Onto the lighting... the factory connection boards come with short wires which might not be quite long enough to reach the MX660 pads directly, and it is nicer to follow the DCC colouring standards anyway, so we choose to unsolder those wires and add our own wires instead. Once unsoldered, it is useful to drill through carefully in order to insert the new tinned wires.

On our Class 37 the positive is in the centre (new blue wire), 'A' is for the red lights (new yellow wire) and 'C' is for the white lights (new white wire). Check your model first though, as they are not all the same! Pop them back into place, and tape them around the side the same way that they were originally. Tidy the wiring up with strips of electrical tape.

Ensure the lighting connection tabs are suitably bent and pop the body back on to test it all. YouChoos can pre-configure the decoder for independent front and rear lights if you wish - just ask when you order!

Ok, so we could just stop there, and we'd have a good sound-fitted loco, with very little effort... however, we don't like to do things by half, so we look to add a little stay-alive too. The loco does run fairly well without one, but just a little back-up power will make a marked difference to the number of stalls it suffers. We use the Zimo SACC16 connection PCB, which joins to the decoder's common positive pad, and the ground pad. We choose to use a single 330uF Tantalum capacitor (flat version, rather than narrow/tall).

We want to keep the stay-alive assembly as thin as possible, so we'll solder the Tantalum onto a top-set of pads rather than the ones on the underside of the SACC16. Snip the final row of pads off, as we don't need them, tin the pads of the SACC16 and also the capacitor, and then carefully solder them down as flat, flush and square as you can.

This shows roughly where we'll place the kit, above the MX660. You can see that it sits lower than the top of the speaker, so there will be no problems getting this to fit under the roof.

Add wires, cover in Kapton, and secure in place.

Check the body again, and test run. The stay-alive won't give a visible length of back-up power, but you will notice the difference at slow speed as it just creeps along, and is much less likely to cut-out.

Well, why stop there when you're having fun?! Let's add cab lights and crew figures too... Ping out the cab interiors (the tab on the top often breaks when doing this, but no worry - it can easily be re-glued afterwards. We use YouChoos mini cab lights.

Drill holes in the cab-backs to feed the LED wires through and glue them into place. We prefer to point the LED upwards to avoid bright points of light in the cab. Position about 1mm below the roof line so that the light reflects back down into the whole of the cab. Also glue in the driver figures.

Once the glue is dry (and not before, otherwise you'll mist up the cab glazing), stick the cabs back into place in the body and connect up the LEDs to the MX660. This decoder has 6 AUX function outputs, so we can control them independently.

Test that the cab lights work OK, and tidy the wiring up with tape. You'll find that there should be plenty of room above the decoder for wires to lay.

Back together, and now with sound, small stay-alive, cab lights and crew!

Of course, there are always other ways to tackle a project, and below is another arrangement of the kit, this time squeezing in 3x 330uF tall/narrow tantalum capacitors for the stay-alive! The SACC16 board is below one end of the MX660, and the tantalums are joined in parallel at another point along the chassis.

Some examples of finished models using this approach...

Disclaimer

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.


A bit of YouChoos poster art for your enjoyment!