Kit: MS500N; SugarCube8; 2x 470uF Flat Tantalums for stay-alive
It is time to revisit the Farish Class 25 now that the MS sound decoder range is available. The hope is to keep the factory PCB
and simply plug in an MS500N 6pin sound decoder.
Certainly no problem with the decoder - so simple, and so tiny, we can barely believe that this is actually a sound decoder!
As always though, we have to consider the speaker, and we don't like going for anything too tiny. In this case we measure up,
and it looks like a SugarCube6 will fit in the fuel tank, with some milling.
With the milling done, we reassemble the chassis and begin work cutting the plastic frame so that the speaker can sit neatly
The fit is pretty neat, so we work on getting the speaker wires up to the top of the chassis - careful routing and insulation
must be considered here to avoid any short-circuits.
As it turns out, we've milled enough away that we can go with a deeper speaker, so switch first to a SugarCube7, then right up
to a SugarCube8 instead - the bottom of this sits nice and flush with the bottom of the fuel tank.
A little fettling of the frame is a good idea to avoid the speaker wires getting pinched.
With the speaker firmly in place, wires routed up, we can connect these up to the decoder.
You may have spotted the blue and grey wires from the decoder, which are for a direct connection of stay-alive capacitors. Up
to 1000uF (16V) can be connected directly to the MS500 without any extra components, so we opt for a pair of flat 470uF
Joined in parallel, the Tantalums sit neatly on top of the circuit board. Blue wire for positive side, and grey for
The body goes on comfortably, and we can disguise the speaker with a little black acrylic paint.
In conclusion, yes, the speaker installation still requires some effort, but the MS500N certainly makes the the electrical side
so much simpler!