F3 DCC install

I have a significant number of A and B units, and IO 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 a true "dual motor" 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 it's going to be a decoder per loco... they do sound great.


The first 2 sections show speaker installation for the fuel tank and then the roof of the unit.

Next I address lighting

Speaker in the fuel tank:

First, what the heck is this removeable 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 clealy 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 agressively 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 slow away from the front of the loco:

Speaker in the body:

In the shell, coming out of the 2 grills just behind the cab, there is a 50mm mounting spot, with a 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 VERYcareful to pick your screws so they do not penetrate the shell !!


OK, the lights are:

  • Rear headlight
  • Cab light
  • Number boards
  • Front headlight
  • 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. The lighting on these tend to use the larger 2 pin connectors shown in the picture above.

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. Notice the 3 terminal series regulators on the weights in the pictures above, these are producing the regulated voltage for the 3 volt bulbs. There is an advantage to this system, in that at about 5 volts DC, all these lights are at full brightness. Unfortunately, the PWM power output of many power supplies (like the Crest Trackside units) makes the regulators crazy.


The rear headlight is a GOW bulb, there is a piece of silver metal tape over it inside, presumeably 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.


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.


The number boards are gow 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.


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.


There is a 2 pin connector on the main board that powers the classification lights, which are a bipolar red/green led. That is a bit tricky on DCC, it can be done with a interesting circuit with 2 outputs and 2 resistors, but on my road, these were either off, or white. They MIGHT be red if the loco was running backwards with no train attached.

The 2 pin connector gets track voltage, and is connected to the 2 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.


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