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The great thing about On30 is the ability to add realistic sound. The equipment is large enough to put speakers large enough to utilize today's DCC sound decoders. In the smaller scales sound becomes sophisticated white noise because of the small speakers. On30 allows large enough speakers to produce authentic sounds. How to wire a Soundtraxx Tsunami follows the speakers.
The tender space allows two 1-1/2 speakers. Two dissimilar speakers are being utilized. I have tested two of the same and the two different speakers produce a better sound.
These and similar speakers are available from electrical houses on the net.
Mouser electronics has a large complement of speakers in their catalog.

The left speaker is one sold by Tony's Train exchange and the right is a Philmore TS19 from an old AHM steam sound card.
Drills:
1/16
1/8
5/32
7/32
1/4

1/4 inch paper punch Center punch
Wire stripper
Phillips head
screwdrivers
NEW razor blade
Compass

Drill Press
Tools Needed
Rubber cement
GE Silicone II caulking cement
I use a 35 watt soldering iron. Use a rosin flux only for electrical connection. I use Radioshack rosin paste. You will need flush cutters to clip small wires after soldering to neaten up.
Strip the tender chassis down. Mark which side is the red side. Put the marking closer to the outside. I drilled holes through my mark. Take off the tender electrical boards also.
I drew up a speaker hole template and rubber cemented it to the frame. I put a piece of 1/4 thick balsa under the frame to protect the frame projections while drilling.
Click here to download the template
Adobe Acrobat required

Mark the holes with a center punch. Drill 1/16 diameter pilot holes. Drill the holes out with the corresponding drills.
Mount the speakers on a 1/16 thick piece of balsa. This isolates the speakers on an inert material to help prevent the speakers from  vibrating,
The balsa breaks easily so be careful.
Use the compass to mark the hole points on the balsa. Run the compass around to make a groove and cut through the remaining balsa with a NEW razor blade. The holes should be the open diameter of the speaker face. Punch out the 1/4" dia holes with a paper punch. The two small holes indicated by the yellow are drilled with a 1/16" dia. drill.
Glue the speakers to the balsa with the Silicone II caulk. Put a bead all around the speakers. Glue the balsa to the frame with clear glue. I use Bond 527.
I use Dallee connnectors. Connect the speakers with wire in series with a jumper from the minus terminal on one speaker to the other speaker's plus terminal.  Solder the wire connector across the remaining terminals on the speakers.
Also need are 1/16x3 balsa sheet and small diameter wire in red, black and purple.
Mount the tender bolster pickups with the soldering lugs facing the front. Run a red and black wire from the back lug to the front lug. Weave them through the 1/16 dia holes. Clip off one set of the red and black pickup wires on the locomotive main board. Solder the other set to the front lug.
Put the tender back together. The tender trucks have an indexing lug. Solder the opposite speaker wire connector to the decoder purple wires. Check the decoder wires on the DCC Bachmann plug and make sure they are the same as the decoder you are using.
Readjust if they aren't.

Put electrical tape on the bottom of the control board to prevent electrical shorting. Just set it on top of the speaker.
Installing Speakers
I am installing dual speakers. The more area the better the low end. Using the tender body as a sound box gives more sound box volume and improves the sound. One 1-1/2 dia speaker could be installed on the balsa pad.
Installing a Soundtraxx Tsunami
This is the Bachmann decoder. We will steal it's NMRA plug. This will effectively neuter it. It isn't a very good decoder. In this day of great low cost decoders Bachmann could have done better. You could also buy NMRA plug blanks.
Press the plug into a sheet of 1/4" balsa while soldering. This prevents the pins from moving while they are being soldered. Believe me they move and cause a real mess. Unsolder the original leads.
Cut the decoder leads 1-1/2 inches long and tape the unused wires to the decoder with transparent tape including the uncut tan wire on the other end.
Strip the wire ends 1/8 inch. Tin them and cut them back to 1/16 inch
While pressed into the balsa solder the first row of wires. Check the connections making sure they are not touching each other. Next do the second row.
The Dallee connectors are pushed onto the speaker wires. Then a bead of clear hobby glue is placed around the wires and onto the connector. I use Bond 527 cement. Allow that to dry thoroughly.
Press the decoder plug into the locomotive circuit board the red wire into the female 8 on the board. Stuff the capacitor into the tender. I am using the oil bunker so it covers the decoder. A coal or wood load will have to be built up to cover the decoder. Reconnect the tender to the loco.

The perfomance of the Tsunami is outstanding. The chuff sync and start are about right. Low speed is great. Make sure if you install sound that you do it on a day that is free. Sound is great.