Posted by: bgtwindad | June 1, 2011

Bachmann F7A/B Decoder Install

Untitled by BGTwinDad
Untitled, a photo by BGTwinDad on Flickr.

I just completed this install of a pair of TCS M1 DCC decoders into a Bachmann Spectrum F7A/B set. This was a challenge because it required some milling of the frame to make room for the decoder and to isolate the motor. Still, it wasn’t all that hard, so if you’re looking to expand your skills into installs that require frame mods, this might be a good place to start.

I was so excited to get this one back to its owner that I forgot to take some “pretty shots” of the pair, so you’ll have to make do with the test-track photo above.  This pair was in Southern Pacific livery, and was a fairly nicely detailed, smooth running pair.  The Spectrum line from Bachmann is a big step up from their train set models, and is generally a good value buy for the cost conscious.  While the design is not “DCC Ready” in the sense that a fair amount of work is required to hook it up, at least it has a split frame and the mods required are minor and straightforward.

The A and B units share the same chassis, which makes the B unit a bit easier since the decoder will easily fit into the chassis cut-out for the A-unit cab.  Plus, there’s no headlight.  Tim “diezmon” Diez provided an excellent tutorial on the TCS website for the B unit, and for the most part I simply followed his instructions.  I highly recommend you review that page, as I will mostly be detailing the variations I made from that plan.

My B-unit install...

The B unit install went exactly as Tim outlined it, and aside from the juggling act required to reassemble the frame went very easily.  To attach the red and black wires, I drilled a small hole in the top of each frame half and miraculously managed to solder the wires into the holes.  I don’t recommend this method.  Much better to drill, tap and screw the wires to the frame.  Fortunately, the MicroTrains #1059 coupler drill and tap set – and the screws included with the 1015 couplers – work very well for this application.  You do have to cut the screws short, though.  I think the screws are available separately if you want them.  Another method that a local modeler tipped me off to is to wrap the wires around the frame mount screw bosses.  I haven’t tried this but maybe it would work.

The frame I had was slightly different than Tim’s in that the bottom motor brush contact was a spring compressed between the motor brush and the side of the frame.  I’m not sure that milling out the bottom of the conductor-side frame is necessary on those models, but it is easy and probably good insurance.  You will definitely have to mill the tab Tim marks off the top of the Engineer’s side frame.

The A unit requires a little bit more work.  First, there really isn’t room beneath the cab for the decoder, so I milled out the opening above the rear truck gears to slip the decoder in.  This also allowed me to replace the headlamp with a 3mm LED.  I’ll contradict myself here by saying that if you retain the incandescent lamp, or if you use an SMD LED, you *might* be able  to squeeze a small decoder into the cab area.

The milled out space for the decoder...

Aside from milling space for the decoder and installing the LED, the install goes exactly like the B unit.

Installing the LED is simple.  You’ll need a warm white LED, available from a hobby supplier like Richmond Controls (the one I used), or from an electronics supplier like DigiKey or Mouser Electronics.  You’re looking for something in the 3000K color temperature range.  Higher temperatures will have a more bluish tint that will not replicate the old headlamps very well.  You’ll also need a resistor.  The value is not terribly critical, as long as it is above about 600Ω. I used a 1KΩ resistor and found it plenty bright.  You simply connect the blue wire to the resistor, the resistor to the anode (positive terminal) of the LED, and the cathode (negative terminal) of the LED to the white wire.  You can also put the resistor in the white wire instead, which is what I did.

Wiring the LED and resistor...

It is critically important that you get the polarity of the LED correct.  The cathode is the longer pin on a radial-lead LED, and will also be nearest the “flat spot” in the LED case.  On a surface mount LED the cathode will be marked with a dot or other marker.  The cathode must be connected to the white wire, not the blue.

You will also have to clip the leads short on a radial-lead LED to make them fit.  This is where knowing about the flat spot is useful.  The LED does not go in the hole where the original lamp went, but simply lays horizontally over the front truck in the cab area.  You may want to use some opaque material to block the LED light from entering the cab.  My friend liked the glow of the lit cab, so we omitted this step.

Once you have everything wired up, all you have to do is tape it down and slip the shell back over the frame.  Both units have strips of black tape along the sides that provide a black background to the side windows.  I forgot to remove these before milling, so I got the sticky side all covered with metal fragments.  Instead of replacing the tape with more black, I used Kapton tape, which I think gives a better “see-through” appearance to the windows.  Beauty is in the eye of the beholder, though, so feel free to use black, or to carefully remove and re-use the factory tape.

As should be expected, this procedure will work not just with the TCS M1 decoder, but with any similarly sized NMRA-compliant wired decoder.  The Digitrax DZ125 would be a good alternate if you prefer that brand, as would the NCE N12-SR.

Additional photos and a few more details are available on my Flicker Set.

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