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Installation of an MRC 0001636 decoder board & speaker assembly in an Atlas GP40

by Jim Reising


Some time ago I was asked to participate in an experiment. Aztec’s John Claudino needed someone with DCC experience to assist getting his new MRC PNP Sound Decoder frames ready.

We took an Atlas GP40 (the board fits several other locos, I am told) and the MRC decoder/speaker and, after some trial and error, ended up with onboard N scale sound and an easy conversion.

The MRC decoder replaces the Atlas light board, and the speaker goes into frame’s milled out fuel tank.

The procedure is like other Aztec DCC conversions; you send your loco’s bare frame to John (Along with some money!) and about a week later you get back a milled frame and clear instructions on how to proceed.

The GP40 disassembly is straightforward. If you’re nervous about it, take some pictures to aid in reassembly, but it shouldn’t be a problem.

The very first thing to do is to put some tape on the frame in the area where the contacts hit the board. Although on close inspection it looks like there is no chance of contact, failure to do this will cause a major problem (DON’T ask how I know!).

The rest of the reassembly is as easy. Take your time and route the wires as shown in the accompanying pictures carefully.

The clear insulation is said to be fragile but I had no problems with it. I’d be particularly careful to insure the wire stays in the milled channel near the spring power pickup – repeated abrasion here would not be a good thing. Once the shell is back on this should no longer be a problem.

MRC suggests drilling some holes in the plastic fuel tank molding to better allow the sound to escape the speaker but I noticed no difference with or without the tank molding. The speaker actually fires into the bottom of the motor anyway, it has to because of the way the speaker connections are set up.

MRC includes comprehensive instructions [see link at right] with the decoder and when you have followed their instructions you will end up with a new dimension for your layout.

To those of you who think they might want to rout out their own fuel tank, good luck. This is not something I would tackle willingly when I can get Aztec to do it for a couple bucks. Getting the depth correct could be REALLY tricky.

I have a few caveats:

I run a Digitrax Super Chief with DCS 100 command station and DT 100 radio throttles.

Because the decoder was built for MRC’s own Prodigy DCC system there are some incompatibilities. My DT 100 has functions 0 through 7, and they worked nicely, but MRC’s system has 19 functions. I WAS able to access them through CVs, but this is pretty awkward.

I couldn’t use the radio throttle to control the loco. It worked correctly while plugged in, but once untethered there was no response from the loco.

My GP40 has the original high speed motor, and MRC has set up their decoder to apparently apply a lot of voltage as the lowest start voltage because the loco acts like a rocketship – and apparently in this iteration of the decoder, can’t be modified. The loco takes off at 2 (of 100) on the DT 100. Programming worked but actually made it worse so I returned the CV to the original value - the lowest possible setting.

It’s kind of cool the way they have the sound set up; you advance the throttle and hear the diesel sound rev up and then the loco starts to move. Unfortunately I THINK that’s as far as it goes, the sound emulates the Diesel’s transition but then does not rev up again, and at that point because of the noise of the mechanism you likely couldn’t hear it if it did. And as you ease to a stop you hear the squeal of brakes applying.

Before this, if you’d have asked me if I wanted sound I’d have looked at you as if you were nuts. Now I admit it’s pretty neat to hear the engine idling (with occasional air compressor and other sounds), switch the bell on, give a couple blasts of the horn, and hear the prime mover rev as the loco begins to move. It IS a new dimension, and one in which I will participate.

To sum up, I think the problems I encountered are minor and will likely be dealt with as the sound decoder matures.


MRC Decoder Installation Instructions

Click here for MRC 1636 decoder installation insturctions.


Installation instructions are in PDF format. To view and/or print them you must have Adobe Acrobat Reader installed on your computer. This is a free program available through Adobe’s Web site. Click on the Adobe logo below to download.


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To those of you who think they might want to rout out their own fuel tank, good luck. This is not something I would tackle willingly when I can get Aztec to do it for a couple bucks.