F3 DCC install

I have a significant number of USAT A and B units, and I am using QSI Titan decoders, so I considered two different paths:

  • One way would be to use a QSI Titan in the A unit, and power the B unit from it also. Since the Titan has dual amplifiers and high current capability, I'll be able to have a speaker in each.
  • Another way is a Titan in each unit, and 2 speakers in each unit.

I tried two speakers in a single unit, and that sounded great, so I decided on a decoder per loco. The QSI has a "balance" control for every individual sound, so I biased the horn and bell to the speaker in the top and the prime mover sounds to the speaker in the tank.

There's a number of advantages to this:

  • First, you don't want to send high frequencies into your ballast, they just get absorbed, and keeping a direct route from the speaker to your ear preserves the fidelity and higher frequencies of the horn and bell.
  • Likewise sending the prime mover down into the ballast is ok since it's mostly bass, and the speaker sort of "couples" to the track by it's proximity, and this enhances bass.
  • Another plus that aids realism is that different sounds come from different places on the loco, and you can tell this from 10 to 15 feet away.
  • The last one is hard to describe until you hear it, the prime mover coming from 2 speakers makes it sound more real and bigger.

By the way the Visaton speakers are much cheaper if you buy them at Parts-Express.com. Here is a link to the Visiton speakers (remember to order 8 ohm speakers)

Visaton at Parts-Express


I show 2 methods of adding speakers, my first attempt just fitting speakers into the locations provided, no enclosures, no baffling.

The second method show 3D printed enclosures for the speakers, what an improvement! Thanks to Dave Stubbs for the excellent design.

I'll address lighting first:


OK, the lights are:

    • Rear headlight
    • Cab light
    • Number boards
    • Front headlight (except B unit)
    • classification lights

As with many USAT Diesels, there are early and late versions of the electronics. The early versions have the white plastic "pancake" smoke units, that are square, thin, and are a heating element only.It's the square white thing between the 2 fans.

IMG 1870


The lighting on earlier models tend to use the larger 2 pin connectors shown in the picture below, at the upper right edge of the "T" shaped board. Also, the smoke unit power board is different.



In the picture below, you see a more modern board, notice the 3 pin connector at the lower right.

usat board

The later versions tend to have more LEDs and fan driven smoke units. The connectors for lighting tend to be finer pitch.

The earlier version is documented below, and my guess for the change, besides the obviously nicer fan driven smoke unit, is that the regulators for lights cost money and the 3 volt lamps were getting scarcer.

In the picture below is an example of the 3 terminal series regulators on the weights. These are producing the regulated voltage for the 3 volt bulbs.  Usually there are two, one on each weight. There is an advantage to this system, in that at about 5 volts DC, all these lights are at full brightness.

weight cleaned

Because of the specialized bulbs used, and difficulty in adapting LEDs, I mention the voltage and current drawn, so you can provide this with a DCC decoder.

Rear Headlight

The rear headlight is a GOW bulb, there is a piece of silver metal tape over it inside, presumably to keep any light coming back into the body. There is a nice clear lens on the outside. A nice warm white 3mm led fits in here fine. I strongly suggest replacing the incandescent bulb with a LED. The stock incandescent lamp is supplied with a regulated 3 volts and it drew 39 ma.

Cab light

There is a 3 pin connector on the main board, it powers the cab and number boards. The number board "board" has a connector that goes to the cab light.

The cab light is in a plastic housing that screws to the roof. A 5mm led will fit fine here, but need to determine if I want this to come on with the number boards, as the cab light plugs into the circuit board in the nose, and then a 3 pin connector comes from there. If you replace with an LED, be sure to get a wide angle LED, like the square surface mount ones.

Number Boards

The number boards are GOW (grain of wheat) incandescent bulbs, mounted on the same circuit board. They are located close to the number boards, so if to be re-done with LEDs, getting high dispersion leds, or 2 LEDs per board, or some kind of dispersing filter is needed.

The 3 pin connector on the main board uses the black wire for common, the red wire powers the cab and number boards, these are GOW, and I reversed the polarity to them, they still lit. (means no regulators or diodes on the circuit board in the nose to these). These were supplied a regulated 3 volts from the original circuit board. They drew 135 ma at 3 vdc. This would require a 126 ohm resistor at 2.3 watts.

Front Headlights

The front headlights are GOW also, and are supplied a regulated 3.57 volts (in the forward direction only) from the original circuit board. At this voltage, the 2 bulbs drew exactly 100 ma. That would make a 164 ohm resistor at about 1.6 watts.The front headlight will be replaced with an LED for sure, simple.

Classification Lights

There is a 2 pin connector on the main board that powers the classification lights, which are a bipolar red/green led. (bipolar is 2 leads, color changes with polarity changes) That is a bit tricky on DCC, it can be done with a interesting circuit with 2 outputs and 2 resistors.

Please note, I have never seen classification lights that are red on a loco, they are either off, white or green. When running light, many diesels could display a red light on the rear loco, but it was NOT a classification light, and often another light in the headlight enclosure.

The 2 pin connector gets track voltage, and is connected to the 2 bipolar bicolor leds just above the number boards. The wires are black and white. If the white lead is positive, then the leds are green, and if the black lead is positive, the leds are red. There must be a current limiting resistor on the small board in the nose of the F unit. At 20v DC, these drew 57 ma. OK, so fine, my AC is 20v rms. Hooked it to the decoder directly, 57 millamps, great.

I've bought some 3mm common anode red/white LEDs, so may just hook these up with white on with the front headlight, and the red tied to a function button for manual control.

Speaker install, using provided mounts and no enclosures


The idea here is to do the simplest install, using the provided factory mounts.

Speaker in the fuel tank:

First, what the heck is this removable piece in the tank?

Apparently it can be inserted into the tank 2 different ways, one way makes the outside wall flush,and the other way:

Anyone who figures this out, please let me know. The best theory I have heard is that this piece simulates an optional slide switch to turn a sound unit on or off. Makes sense since the speaker would live there and there is also a mounting boss, possibly to bolt a sound unit to.

Speaker selection for the fuel tank.

The fuel tank is 20mm deep, and the speaker "cutout" is 66.34mm (2-1/2"), and you could go to about 75mm (3") if you cut the lip off, but I'm doing 10 of these locos, so was looking for a solution that took the least modifications.

The sound from the fuel tank will be the prime mover rumble, and it has room for the largest speaker, and also, you get more bass when you are close to the rails.

I picked a Visaton FRS7-8, cutout 61mm, depth 30mm.8 watts. All the other speakers with any bass were deeper, and cutting a large hole in the chassis to clear the magnet would be dicey, the "chassis" is not real strong to begin with.

Other people have used a Visaton K64WPT, cutout 60mm, depth 19mm. Clearly this would fit also, but the bad news is that it is only 2 watts and they recommend a 300 hz high pass filter.. This clearly tells me no bass or even mid bass. I want the maximum bass, even though this speaker fits more easily. 

So I bought the FRS7. Note that some suppliers may or may not append the impedance (the -8). Be sure to get the right impedance for your sound unit.

In the picture below, I had already tested using some strong medical scissors to cut away some of the flange. (bottom left corner), pretty hard to do with tin snips/scissors, and you risk distorting the speaker frame.

You need to trim all the flange away, until it is 66 mm in diameter. I did some initial cutting with heavy scissors on the 4 corners, and then finished up with a belt sander.

The way I sand down the edge is a belt sander with a shelf to hold it perpendicular:

Notice that the iron filings get everywhere, I normally wrap the backside of the speaker with blue painter's tape. A blast with compressed air removes these, or press some tape on it and lift the filings off.

Be careful you don't push the speaker into the belt too aggressively though! You may catch the edge of the speaker frame in the belt, you can see how I know this!

It can happen easily! Take light "cuts".

Now you need to prepare the tank a bit more, see the 3 square "nubs" inside the speaker "circle"? Trim those down flush, I used a cutoff wheel running slowly. The foam surround of the speaker will keep the proper clearance between the speaker and the "grill".

Now, the speaker should sit down inside and not project above the fuel tank housing:

I dribbled hot glue around it to hold it in place:

Not pretty, but it works!

One last modification: you will need to cut a slot in the chassis to allow the wire to clear the magnet, which blocks the original hole.


Cut the slot away from the front of the loco:

Speaker in the body shell:

In the shell, coming out of the 2 grills just behind the cab, there is a 50mm mounting spot, with a circular ridge:

The Visaton VI-FRS5-8 - cutout dimension 46mm, 84db sensitivity seemed to be exactly what I needed. There is plenty of room for a substantial magnet, so speaker depth was not an issue: (apparently a K50 will fit, but it needs an enclosure)

Placing it in the mounting spot shows that the "ears" kind of get in the way of one of the 3 mounting screw locations:

Since I had to trim one "ear", I just trimmed them both, using heavy scissors. Then I sanded it down until the metal edge was gone and all you see is the black mounting "pad".

It fit perfectly, be VERY careful to pick your screws so they do not penetrate the shell !!


Below is the "assembly line" for a Santa Fe freight ABBA consist:

 assembly line


Speaker Installation with 3d printed enclosures


Luckily I have a creative friend, Dave Stubbs, who comes up with elegant solutions, and here are his speaker enclosures.

Fuel tank speaker box and QSI mounts


Open loco and remove main board and smoke board. I remove any regulators on the weights as if I do run smoke, it will be directly from the decoder. I also remove the smoke units in the roof and the regulator board. I also wind up removing the little 2 connector board on each side.

Note all screws are 3mm in diameter, so only the length will be mentioned.

Speaker Boxes:

Mount the speakers before you solder the wires. You can do it after but it's a bit tight.


The speaker mounts on the outside of the plate, you see the recess for the back of the basket, and you see the countersunk holes on the outside, i.e. when you are looking at the front of the speaker, the countersunk holes in the corners are facing you. Use four 6mm screws.




Now mount the speaker plate in the spacer wall with four 6mm screws. Note the speaker plate goes into the recess, which is only on one side. Set this assembly aside.










Prepare chassis with base:

Mount the QSI standoffs with four 8mm screws on the base plate. The screw heads fit in recesses in bottom of the base plate.



Take the inside base plate and fit it over the 2 bosses in the center closest to the 1/2" hole for the speaker wires. On some locos, the base plate won't go in until you clip off the a reinforcing rib on one of the bosses. It will only go in one way

Now drill 5 #33 holes, There should only be 5 available.

(if you are not using the QSI mounts, be careful the ones you drill. 2 are at the corners of the plate, 2 more are NOT at the corners but the next set in. Double check this by aligning the speaker walls, only one way will line up the holes.)

The 5th hole you see that does not line up with anything, that is for your speaker wires, enlarge it now to fit your wires.

Now deburr all holes, the enlarged one on the base plate, and both sides of the 5 you drilled in the chassis, I use a single edge razor blade.

Mount speaker box:

Now you mount the speaker box from underneath by running four 12mm screws from inside, through the base plate, through the chassis and into the 4 holes in the speaker box.

be sure to route the speaker wires through the hole you drilled in the chassis, and the matching hole in the base plate.




You need to remove the post and the raised ring in the fuel tank to clear.


Mount QSI (or other decoder):

use four 8mm screws to mount the decoder.

Roof mount speaker box:

Mount the adapter plate to the mount in the roof. There are two 6mm screws and one 5mm screw... the 5mm screw goes in the center hole, the one closest to the center-line of the roof.




Mount the speaker to the speaker plate, you will see the shallow cutout for the mounting "ears", use two 6mm screws


Now solder wires to the speaker, I use about 12" long.

Mount the speaker plate to the wall section, with four 10mm screws, be sure the end of the wall section with the groove for the speaker wires faces away, i.e. the open end



now mount the speaker assembly to the body mount/adapter. In the first run, there was one hole offset, so if they don't line up, rotate 180 degrees, use four10mm screws


finally mount the cover on the bottom with four 6mm screws


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