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Graham Farish Class 31 Zimo MX660

Farish Class 31 with Zimo MX660 sound decoder and stay-alive

Kit: Zimo MX660; SugarCurve6; SACC16 + 1x 470uF Flat Tantalum

The Farish 31 is like many of their modern 6pin diesels, with the added complication that the headcode lights are lit by LEDs on the main PCB itself. Therefore in order to use the Zimo MX660 drop-in sound decoder we must do some work to reuse those LEDs.

A quick measure up shows that it will be tight in terms of length, retaining part of the original factory PCB so we can reuse not only the fixing screw eyelets for pickups, but also the headcode LEDs. The MX660 and SugarCurve6 just fit in between these, but we'll have to be very neat indeed.

So we remove the factory PCB and check that the speaker will fit in that location with the body back down flush - it fits quite happily, so no worries in terms of height.

Though we don't have to modify the model's chassis at all, we do have to disassemble it all because the terminals on the motor are not accessible otherwise. We therefore take everything apart and solder orange and grey wires direct to the terminals, then reassemble.

A layer of electrical tape will protect the decoder from short-circuits, and held down with some thin double-sided tape.

Solder the motor wires to the appropriate tabs on the MX660

Change the wires that go to the red tail lights for finer and longer decoder wire. This 31 only has red lights at each end - the whites are the headcodes at the top. Blue will be positive and yellow to control the tail lights, independently each end.

Next some butchery - chop the factory PCB so that we have 2 short lengths from either end for just the headcode LED and the fixing screw eyelet. Needs to be neat, and as short as possible, so file down once cut.

With those cut-off sections in place, we can begin the wiring. Red and Black wires for pickups - may need to have the screw very loose while soldering onto the eyelets to avoid dry-joints and to make it nice and secure once the screw is tightened back up. We add wire directly to the LEDs for the headcodes - blue for positive and grey (in this case) to control the lights.

Connect red and black pickup wires to the MX660, and also the headcode LED wires - we use F0Fwd and F0Rev for these so that the headcodes are directional. Use any +VE connection on the MX660 for each LED, but don't share them - one pad each! We have also connected the blue+yellow wires for the red tailights to the decoder - FA1 at the front and FA3 at the rear, so we can have independent control of the reds directionally. Again, use different +VE pads on the decoder for each tail wire.

A layer of electrical tape, then some double-sided tape on top of that, and the SugarCurve6 speaker sits on top. We have bent the speakers legs out and cut them a little shorter to reduce the height, and reduce the risk of short-circuits too.

Connect the purple wires for the speaker to the MX660 and give it a test - we should get motion, headcode lights and sound - no red tail lights of course, until we pop the body back on.

All good so far, so we tidy the wires up, and secure the speaker better.

Since the MX660 is so thin, we have vertical space to add some stay-alive on top of the decoder. Here we snip a SACC16 PCB and add a single 470uF Flat Tantalum which is enough to noticeably improve running reliability. A slither of Kapton tape between the decoder and stay-alive, position on top, and all done!

Bend the connecting tabs for the tail lights a little to ensure good contact, then place the body back on - wonderful!


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.

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