As some of you might remember (see this earlier post) I had the intention to fit a Digitrax SDN144PS sound decoder in an Atlas GP-40 engine. At that time I had no idea how I should manage to fit all the stuff (decoder, speaker and capacitor) into the engine. And I am still not sure if I will manage. Anyway, I thought I should update you regarding the status so far.

I soon figured out that the only place for the speaker would be in the fuel tank, facing downwards. A bit of the frame would have to go, to make room for it. I also decided to skip any ideas about a speaker casing. No space for that and me, with no previous experience of sound decoders, not knowing what would be required to make a casing worth while.

So I disassembled the locomotive and started to grind away at the frame, using my Dremel motor tool. The speed (RPM) regulator of the tool broke when I was halfway done so I did the last part using a Dremel bit in my drilling machine. Not very effective, but eventually I was done (I really need to get my self a new Dremel). Here is a shot showing the “hole” where the speaker will sit.

Not the neatest of milling jobs, but hopefully it will serve its purpose. I also made a hole in the fuel tank. I did not only drill a set of small holes for the sound to get out, but a large hole, the diameter of the speaker, so that the speaker fits inside the hole rather than behind it. In this way I gained the thickness of the fuel tank material and did not need to remove as much of the frame (hole not as deep as otherwise required). Here is the hole in the fuel tank.

Below is a picture showing how the speaker will sit in the fuel tank, flush with the bottom of the tank.

Last I made a groove in one of the frame halves, for the speaker wires. I also removed part of the frame at the very top. This to make room for the decoder.

The wire groove is the diagonal cut above the place where the rear truck goes. You might also notice that the top of the facing frame half is a little shorter than the other one (to the right of the text “Atlas”). I still need to do the same cut on the other frame half.