I recently successfully installed a sound only decoder and a speaker to Atlas FM H-15-44 road switcher. I would like to share my experience with other model railroaders who are interested in giving mute locos onboard sound.
The inside of the loco hood is filled up with a motor and diecast weight and is left a very little room. So you have to do some modification work to the chassis or other things to accommodate a decoder and a speaker.
This Atlas loco has a factory installed motion decoder #345. I could have taken it out and put a sound / motion decoder like Soundtraxx LC PNP decoder for Atlas road switchers. But the original decoder has a nice feature: LED front and rear maker lights. I did not want to give up this beautiful green and red light glowing on the sides of the loco and decided to add a sound only decoder
I studied sound only decoder products available in the market. Actually you do not have much choice for sound only decoders while there are more motion & sound decoders coming out lately. I picked up a Soundtraxx DSX with FM exhaust sound and single chime horn (Tony’s part #825244).
The good thing about FM H-15-44 is that the hood is tall. This give me room over a gearbox and allows more speaker enclosure volume for better sound. It also leaves a just enough clearance for DSX sound decoder over the factory-installed motion decoder.
The following are the steps that I took.
I first programmed the Atlas motion decoder to modify the function key mapping. The default setting for turning on and off the marker lights is function key #2. This conflicts with the DSX’s default function of horn. My other loco sound decoders also use this #2 for horn or whistle function so I changed the Atlas decoder’s CV (54) to remap this to function #5 (value =16).
Take out the front (in case of my loco UP #1325) weight. Trim away the top half as shown in the picture. This reduces the loco weight approximately 70g, 15% of the total loco weight and may well influence the traction.
Make a speaker enclosure on top of a 20mm x 35mm oval speaker. Because it is necessary to leave room for headlight and marker lights, I shaped the enclosure like the picture. The radiator bulge on the front sides gives you enough width for the speaker (shell internal width of this part is 22mm), but it is not long enough. So I cut out the corner of the speaker and make the enclosure shaped to fit the cut out.
I replaced original headlight lamps with LEDs. I added a 1000-ohm 1/4 watt register in the positive line to each LED. There is no need to replace a LED in the future because LED lasts very long so I glued it to the top of the speaker enclosure.
The lead wires from rail pick up could be directly connected to the DSX. But I use the same loco ID for both motion and sound decoder, so I placed a miniature connector in the middle of the wire and put it under the loco floor. This allows access only to the sound decoder without taking out the body shell. The H15-44 has a skirt on each side and hides the connector.
I put back the milled front weight. I then place the speaker on top of the front weight. I taped the DSX decoder down on top of the Atlas decoder and connected all necessary wires. I shortened some original wires and made new wires for sound decoders as short as possible. I added a connector in each speaker lead wire for easy maintenance.
Placed the body shelf back over the chassis. Now I can fiddle with the DSX for better sound effects. Some CVs of Atlas decoder and DSX are overlapped but fortunately those function like exhaust and horn volumes are mapped on separate CVs. So I modified CVs just as if a single decoder in the loco. I may unplug the lead wire connector to the DSX and separately program it in the future.
Now the loco looks as if nothing had happened, but it sounds like a real thing with beautiful marker lights!!