Kato SD90

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Kato SD90 with Soundtraxx DSD-AT100LC Decoder
One of my train buddies had bought four identical engines (different road numbers), Kato’s Union Pacific, SD90’s.  He wanted to run them on our layout at the show and possibly at home.  He also wanted to have sound in at least one.  The cost of several would just be too expensive.  This engine then, was to be the one engine that would be run most of the time.  I had done several others for this train buddy and had put things in them like ditch lights and red running lights for some NS SD70’s.  He asked to put ditch lights in this, the lead engine, but no cab lights or other running lights.  And interestingly, we have avoided cab lights because they are more of a novelty and a use for one of the six functions that Digitrax has in their new decoders.  They are important to me to show the capabilities of DCC so I put them in mine.  Running lights, especially truck lights, are also something that has not caught on; however again, I do it to mine to show off.
Decoder Selection
Since we are limited by space, I wanted to put in just one decoder and picked the Soundtraxx DSD-KT100LC.  A quick call to Soundtrax found that this is no longer in production.  The substitute was the DSD-AT100LC.  The particular model (820041) was a 2nd Generation EMD sound with selectable horns.  It had amps matched to the engine and was a drop-in replacement for the factory installed light board.  On the other units I will use a Digitrax DH123-PS (short plug) and leave the lights alone.

Critical in this unit to some degree, there was a lot of room, for whatever reason, under the Q-fans at the rear of the unit.  It looks like they originally designed the undercarriage and weights to fit an SD70 and didn’t “up-size” it to match the larger SD90 cowl.  In any event, I was looking for some air holes for the speaker(s).  The fans are always a good choice as they point up and under normal circumstances usually have open grills.  Not so lucky in this case.  Even though the fan detail is marvelous with 10 blades showing through the 52-inch grillwork and open, at least at the top, these were closed units on the bottom.  Options included drilling holes or otherwise opening the bottom of the fans without disrupting the top grills and molding or, as I chose, cutting off the fans and replacing them with a detail set from a detail manufacturer.  This was actually simpler than I had expected.  The fan set is four pieces with a separate three-hole piece just right for mounting details and hiding the ugly saw marks from the now detached fans.

The drop in was longer than the original board.  I was going to use LED’s, as I usually do, and Kato had already provided them.  I just had to unsolder them and then solder them onto the board WITH a proper sized resistor since the Soundtraxx board is built to accommodate electrically 12-16volt incandescents.  They do not recommend LED’s because there is still a 1.5v AC ripple even when the light is supposed to be off.  Some LED’s will have a faint glow with this residual voltage.  There are several solutions; the best is to not do anything about it.  Most prototypes will use “Rule 17 dimming” anyway.

The next space consideration was the size of the speaker(s).  I was going to use 2 one-half inch speakers but read the “NOTE” on the instruction sheet for the DSD-AT100LC which stated that the decoder was designed for a single speaker.  So, I elected to use an elongated speaker which was easier to mount, more power output, more surface area than the two that I originally planned on as well as easier to enclose.  The speaker would not fit in the space under the fans without a little shaving.  I didn’t want to shave the speaker so I shaved the enclosure.  This is a separate removable cowl from the main, outer hull.  This made it easier to file and shape, kind of…

I spent an awful lot of time shaving and even thought that I should have just cut off the area and filled it with the proper width of styrene.  The issue would have been the retaining pins for the inner fan cowl to the outer hull.  These were in a spot that I would have to cut off.  I thought it best to continue with the filing.  I also felt that it would be a tighter fit for the speaker enclosure…maybe not.
Now that the choices had been made it was time to make these choices work.  After carving out the spot for the speaker, sever the rear light pipe about ¼ inch from the edge of the third fan hole.  This will allow the addition of an LED, shrink-tubed (a new verb!) onto the end of the light pipe.  I polished the end of the light pipe before attaching the LED.  Cut the leads back to a length that provides comfortable solder length without interfering with the speaker enclosure.
Now, directing our attention to the speaker.  This one has four mounting holes.  While good for mounting, here we must fill them to remove unwanted air leakage.  I used plastic 2-56 screws.  You could use any left over sprue plastic and just glue them in.  I couldn’t find any plastic that size (.080).

Next create a box.  Here we just need three pieces of plastic, all 0.630 inches wide.  Cut them to length to form the bottom and two sidepieces.  We will have to seal as we make; so use silicon adhesive that will stay soft.  Recommended “gasket” cement (RTV for room temperature vulcanizing, Permatex or generic brand) from a local auto parts store was used.  The trick here is mounting the capacitor.  I usually mount it very close to the speaker and in the enclosure.  On this enclosure the plastic touches the speaker magnet.  Not much room here.  I had to mount it externally.  Just keep it close.  Also, remember to drill the holes for the wires and pull them through BEFORE you glue on the final cover.  You may want to mark them for plus and minus.  A lot of posts on this subject suggest that you should try it both ways and then mark the better sounding polarity.

At this point you should be ready for the electrical.


This decoder is considered a “drop in”.  While it fits in the space of the other board, it is far from the intended NMRA “plug in”.  First, lets consider the motor connections.  To take out the light board you must squeeze the connections to the motor to release them from the holes.  You would then drop in the new board.  I left the mounting tape on the board for insulation and and extra, but unneeded, adhesion.  Next, cut back the left and right, front and back, wires from the track pick ups off of the trucks.  Solder them into the holes provided.  I pushed them up from the bottom and soldered on top.

Carefully straighten out and flatten the connections to the motor.  Then bend them over with needle-nose pliers just above the “tabs” towards the board.  When you do this they should both just fit over the board and in between two through holes (called through-vias in the printed circuit board business) and solder to one or both.  You should pre-solder or tin the motor connections with a dab of solder to make it so that you are not putting a lot of heat on the board.  Note that these vias are connected to the motor “tabs” on the side that would ordinarily be used for Atlas engines.  A stroke of genius, I might add, by Soundtraxx engineering to allow this board to be used for both manufacturers’ engines.


Add a 510-ohm, ¼ watt resistor, in series with both LED’s.  I usually connect the resistors to the board if there is enough room.  The rear LED will have to be connected by wires to the board because the speaker and enclosure is now in the way.  You might consider putting a connector between the board and the LED.  I did not as these usually fail over time with corrosion.

Separate one-wire sockets are on the board to connect the speaker.  The jury is out over whether this is a good idea considering my experience with long-term corrosion.  You really need to do a lot of repair work to get an appreciation for what I’m talking about here.  Brass, copper, even tinned copper oxidizes constantly.  They are more susceptible when there is a current flowing.  If you want to avoid the problem you could solder the connections but I would wait until I tested both polarities before soldering.  I soldered.  The connections had popped out every time I opened her up and I was afraid that over time this might happen and cause a short.

Connections done, lights in, I test along the way.  I had already tested the motor and pick-ups before adding the lights and speaker.  I had already programmed in the address and ran it around the loop a couple of times also.  Once with the 03 address and again with its new 8284 address.  It ran rather rough considering the quality of Kato engines.

Button it up

Considering that my normal path is to almost overhaul an engine, even if brand new, I continued with my now famous Train Buddy™ regimen of engine service.  This consists of basic lubrication with three kinds of lubricant at appropriate places and gauging and repairing any deficiencies in gauge with the wheels.  It may also require truck interference testing, especially on large, 6 wheel truck diesels or articulated steam.  Removing any interference (even Kato is not infallible), I am ready to test again with the shell off before I commit.  This is a valuable test, since; many times a unit will fail when the cowl is pressed into place because of wire or board pinch and compression.  Sometimes wires even get pressed into the flywheels, drive shafts or the motor itself.  While we all try to be very careful here, it happens all too often.

Test again with the cowl or shell in place.  Test the lights (you should already know from previous testing whether they will stay lit as suggested in the Soundtraxx documentation or go out).  If they don’t go out and you want them to, then a simple diode, capacitor arrangement will insure that you get full DC with no AC component.  This takes up more room in the shell and must be carefully insulated.  A short or incorrect resistor value will destroy the light output functions on the decoder.

Ready to go.  You might want to add other light functions but this decoder doesn’t have any.  Its major feature is sound, so enjoy.  If you want ditch lights, beacons and the sort then you could use a TF2 or TF4 from Digitrax that is very space conscious to add light functionality.  Just make sure to program them with the same address.  In order to do this you must separate the decoders electrically from each other while programming.  The programming track for Digitrax is not designed to handle programming more than one decoder at a time.  You could also lash this up with a front engine that has all those features and get a great combo.  My train buddy John will be running four of these beauties together.  Can’t wait for the next show to wow the youngsters and oldsters with this sound combo.


One last note.  The standard program for this would need to be modified for the prototype.  The three horn sounds are somewhat industry standards.  Look at the horn that came with the unit.  A single bell (horn bell here) means probably a Wabco.  Three bells of varying sizes, sometime in two different directions, is a K3 (Leslie usually) and Five bells, ether in one or both directions is a K5 or five-chime (horn-speak for five separate musical notes or tones) horn.  While there are some exceptions to this, usually related to era or road, the manufacturer of the engine has done their homework and has picked the right horn for that engine.  Again, many have gone through a progression of horns from Wabco, single note in the early 50’s to late 60’s to K3 and/or K5’s up until recent times.  Only good prototype history digging, along with a picture of the unit that you are modeling, will give you the correct prototypical answer (if you even care).  In this case it was a K3 so Soundtraxx CV115 was set to 1 which is a “medium” horn.

These are the other CV changes that I made from "factory default".

  • CV2 (start) = 14  this makes the engine just start to move when the speed is "01".
  • CV3 (acceleration) = 10  this makes the engine hesitate a little when starting out as though it was dragging a long line of cars.  Allows the sound throttle to match the locos speed better.
  • CV4 (brake) = 10  again used to match sound and speed.  This means that even when you turn the throttle completely down it will drift for three or more feet before it comes to a complete stop.  The faster it is going the longer this distance is.  (real world you know.)
  • CV115 (Horn) = 1 This is the setting for medium horn.  According to Soundtraxx this is a 3 chime (bell or note) horn.  According to the model, that is what is physically modeled on this engine.  My research verifies that this is correct.  Don't change this setting (unless you just want to hear the other options.)
  • CV116 (notch) = 11  Assuming that the top speed of the engine is when the throttle is at 99 (I didn't verify this) and there are 8 notches and they are linear then splitting the 99 % range of the throttle into 8 pieces gives us a notch setting of 11.  This shouldn't change.
  • CV120, 121 and 122 (volumes) = default  (for the bell and exhaust it is set to 128 and the horn is 192. 

These can be changed while operating.  Look in the book for "operations mode programming"

Test and Refine
While ordinarily I would have caught this sooner, it was only after I ran it one last time that I noticed it.  The rear light would go on faintly every time you blew the horn!  I called Soundtraxx again, this time to get a sanity check.  Was this normal, did I do something wrong or was the board defective?  As it turns out, for some odd reason, on this board, and only under the circumstances mentioned with an LED, this happens.  Well, I set out to “fix” it.  My solution was crude but worked.  I put a 240-ohm ¼ watt resistor across the leads of the rear LED.  This creates a resistive load for the driver transistor and also a voltage divider circuit.  Too small of a value (220) and it will keep the light from coming on at all.  Too large of a value (310) will not have any affect.  You can do this test for yourself carefully put a 500 ohm potentiometer across the LED with the lid off and vary it until you get the best results.  Measure the potentiometer value and replace it with the next size commercial fixed resistor.  Now for the (ugh) 73 part detail set.

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This page was last updated: 2015-06-21