Rapido / Model Rail OO GWR 16XX

Kit: MX648; IceCube18X13X07-1W; LifeLink + 4x 470uF Tall Tantalums; Orange firebox glow LED

Rapdio / Model Rail's GWR 16XX Pannier in OO looks promising... heavy, good detail, nice packaging. On DC the model runs well - a little grindy perhaps, but presumably that will improve with running in. It comes equipped with a Next18 socket, ready to take an MX658N18 sound decoder, on board speaker, and a firebox light... so on the face of it, very good spec.

Blanking plate out, sound decoder in (MX658N18). SugarCube type speaker vertical in the bunker.

This should theoretically be it, all done, nice and simple... however... 4 problems...

First is that the sound is very poor with the factory speaker. Luckily this is easily resolved as there is space to simply swap this out for an IceCube18X13X07-1W. In fact, an 8mm version will just squeeze in with a little persuasion. The whole of the original bracket is removed, and the IceCube mounted vertically with some BlackTack to hold it up. Worth putting a piece of Kapton tape over the metal parts to be sure it cannot short-circuit on anything too.

Second issue is the firebox LED, which is red... some people will agree with this choice, but it just doesn't look right to us, so we replace that with one of our orange SMD LEDs. Remember to add a protective resistor (1K ohm here) for the firebox LED. The factory PCB is prised out, then our new LED position behind the hole and held in place with a piece of electrical tape.

Third issue is that there doesn't appear to be any connection provided for GROUND on the model's PCB, so it is not possible to add any stay-alive, should we deem it desireable. OK, have to accept that, as there is no way around it if the factory PCB is to be retained.

Final, and biggest problem, is that the factory PCB seems to have some poorly specified suppression components, which confuse the BackEMF of the Zimo sound decoder. It runs fine for a few seconds, then abruptly stops, just to start off again, repeating this cycle. It is a classic behaviour where the decoder cannot understand the feedback from the motor, so doesn't know how to adjust itself properly.

This leaves us with a problem, as it isn't easy to work out which components to remove from the PCB to resolve it, if it is even possible. So, we opt for the brute-force approach and ditch the PCB altogether, in favour of hard-wiring an MX648 decoder. This not only evades the problem of the BackEMF, but also opens up the possibility of adding stay-alive, which is well worthwhile on any 0-6-0... so, though it is disappointing to be forced to do this, it will actually end up with better results!

The decoder will sit above the drive shaft, so to be safe we add spacers either side to raise the height of the decoder clear of the shaft. Note that there are 3 pickup wires for each side, which need to be joined to the decoder's red and black wires.

The easiest stay-alive to add is an array of Tantalums - here we have 4x 470uF Tall Narrow Tantalums joined via a LifeLink to teh MX648. Fits quite comfortably and gives a very meaningful stay-alive.

If you want to push the boat out with stay-alive, then we can substitute the Tantalums for a SuperCap (17000uF in this case), which fits in fairly easily too - just have to be careful placing the LifeLink - it won't fit directly on top of the decoder, as that makes everything too high when fitting the body back on, but you can offset it to the side a little and it will go on OK.

Ok, more effort than it should have been , but overall it isn't too hard to convert like this, and the results are very good.

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