The Hornby OO M7, with DCC sound using a Zimo MX648 and firebox glow. The M7 is a pretty thing, but very short on space for treats! So we've
got to do away with the DCC socket, to make room for the decoder or speaker. In fact, there isn't going to be enough room to get both the decoder
(Zimo MX648) and speaker in the boiler, so we decide to put the decoder in the bunker. It could just as easily have gone the other way around, but
personally I prefer for the sound to come from the front of the boiler.
Off with all the gubbins, socket, supression capacitor etc.
Grind the socket's screw mounts flat:
The CT sugar-cube has had its enclosure slightly reduced in height, but only slightly. It could sit upright like this, with some electrical
tape between it and the chassis...
...but in fact, we choose to place the speaker the other way up, and stick it under the chimney like below. Just measuring the space at the
moment, and refitting the body to ensure that it will be able to go back on flush when we're all done:
We're happy with the fit, so we add speaker wires, and stick the speaker into its' final position using a thick double-sided pad:
Since we don't have much room, we will totally remove the unwanted wires from the decoder (white, yellow and brown in this case). I have also
replaced the orange, grey, red and black wires with much longer ones, as we need to run these up to the motor and pickups. We keep
the green and blue though, as we want to add a firebox glow light... before and after wire removal (DO NOT do this unless you have good soldering
skills and a decent fine-tipped soldering iron!!):
Cut out holes under the bunker floor for the decoder wires to go through - it is actually easier to remove the coal from the bunker and feed
down from the top - you don't need holes as large as what I've made here! I've also routed trenches for the motor, pickup and speaker wires
to sit in as they go forward, including a channel vertically up behind the smoke box - this will prevent the wires from getting pinched when we put the body back on later.
The firebox glow light is an orange SMD LED, glued onto the smoke box door. We can get away with this simple approach on a Tank engine, because
the LED is not directly visible. Paint the white parts of the LED, and the wires black to help disguise them though, and feed the LED's wires through
the floor, routing back to the bunker.
The firebox LED needs a resistor on the positive wire, before attaching to the decoder's blue wire. The LED's negative wire attaches to the decoder's
green function wire. We've used heat-shrink to totally cover the resistor and electrical joins.
Stuff it all into the bunker, and chop the pivot legs from the coal moulding, as the coal will have to sit slightly higher than it previously did. In fact, once in,
we'll glue on some lump coal to improve the look!
All in, tested, firebox glow on - beautiful!