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DCC install help? MRC 1907 in Atlas S4 Locos and rolling stock
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Active Member 86 posts kengoudsward Active Member 86 posts
kengoudsward

I'm having a wee problem with this install and hope somebody can give me some advice.

This decoder is designed as a drop in replacement specifically for this loco - however, it seems that my particular model might be an older version or something as there appears to be one less screw hole than expected by the decoder.

the red arrow shows the screw which is installed, but there's no hole in the chassis to accept a screw at the purple arrow.

main.JPG 

here's a view of the bottom of the board - as you can see, it makes contact to the brushes via a tab that gets pushed down onto a brass section mounted in a purple block.  The screw mount on the left in the picture (toward the front of the loco) attaches to the chassis. You can clearly see that there is no hole on the chassis for the screw on the right

 

clip.JPG 

it's a bit hard to see in these pictures but the "empty hole" on the board has a ring painted around it that is the EXACT same colour of purple as the motor mount block - which leads me to think that perhaps those two pieces should be connected electrically. Can anyone confirm this? I could easily add a wire connecting the hole contact and the tab that hits the brass bit.

by the way - I'm a little bit confused by this whole concept of "drop in" boards because I'm used to thinking in terms of direct wiring using Digitrax DH123 decoders (see below) so if anybody could explain/translate the connections using Digitrax colour coding that would be a huge help to me - in particular, it's the motor connections (grey and orange wires) vs chassis connections, and the concept of "ground". I tend to think of the chassis as ground, but the DH123s don't actually have a ground connection, per se. I suppose in that case it makes more sense to think of one of the track leads as ground.

20wiring.jpg 

thanks in advance,

Ken,

GWT&P
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DKRickman Prolific poster 3,069 posts
DKRickman

Hope this helps

If you can find the 1907 instruction manual, it might help.  On mine, I had to modify the frame a little to make some room, possibly for the speaker?  As far as the screw holes, the way the motor is made, the top contact is insulated, while the bottom brush is connected to the frame.  Therefore, when you screw the decoder to the frame, that completes one side of the circuit.  The spring on the bottom of the decoder touches the top connection for the other side of the motor circuit.  DO NOT add a wire from the "spare" hole to the top brush, unless you can clearly see that the two are already electrically connected.  You could inadvertently cause a dead short across the motor.

In DCC terms, "ground" is the blue common wire, NOT the chassis or either rail!  The truck leads are the red & black wires (though on this loco they're both black).  The motor contact and screw holes are the orange & black leads.

Ken Rickman

Danville & Western HO modeler and web historian

http://southern-railway.railfan.net/dw/
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kengoudsward Active Member 86 posts
kengoudsward

Thanks Ken! Yeah i was

Thanks Ken! Yeah i was thinking the chassis was probably connected to the other end of the motor. It's really weird though why that one hole definitely seems to have a purple colour code. Do you recall if your loc chassis had two screw holes? and if so did they appear to both be just into the chassis or was the purple one electrically isolated?

I will try to find the manufacturers documentation for the 1907 online
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bapguy54 Regular poster 171 posts
bapguy54

I believe the bottom motor

I believe the bottom motor brush is connected to the frame. If so, you need to solder a wire to the nut and put a piece of tape between the motor and frame. This wire then goes to the decoder.If the motor is not isolated from the frame, you'll fry the decoder. Joe
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DKRickman Prolific poster 3,069 posts
DKRickman

Not quite

Quote:

If the motor is not isolated from the frame, you'll fry the decoder. Joe

Actually, no.  Since the frame is not connected to anything else*, it's fine to leave the bottom motor brush connected to it.  The decoder is designed so that all the connection points are at the frame/motor polarity.  There is one catch - you MUST use plastic coupler boxes.  Metal boxes and metal couplers would result in a short and a fried decoder.  But given that the coupler boxes are plastic, it's no problem.

* I'm not just guessing here.  I completely disassembled the trucks on my S-1 and carefully tested them for any possibility of a short circuit with the frame.  I'll stake my S-1 on the statement that it's okay to leave the motor connected to the frame in this case.

Ken Rickman

Danville & Western HO modeler and web historian

http://southern-railway.railfan.net/dw/
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kengoudsward Active Member 86 posts
kengoudsward

well I'm having a heck of a

well I'm having a heck of a time trying to locate a manual or schematic for the MRC 1907. If anybody happens to have any paperwork for this part it would be very appreciated if you could upload a photo or send me a PDF.
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