Digitise Bachmann Branchline 31-225 - Rebuilt Royal Scot 4-6-0 'Black Watch'

[MrWagner]

Hello,

I would like to digitise my favourite locomotive. But I don't know if it will work as expected and if there are any problems. It is an older model from Bachmann. As usual, there are no sliding contacts made of thin metal. The whole body of the locomotive consists of two metal parts that are isolated by a small gap.

I have a Lenz Standard + V2 decoder.

I put the locomotive in almost all its individual parts, cleaned it and now I want to install the decoder. The decoder has already been tested and attached the naive plan of how I would do it:

1.1.) I would solder 0.8mm diameter cables to both metal parts on top. There should be enough space between the metal parts and the locomotive housing.

1.2.) I would also solder cables to the motor and insulate them with shrink tubing so that they do not touch the metal housing. Hopefully there should be enough space between the two metal parts so that I can lead the cables out

1.3) Solder the decoder and first test it without the housing

If that works, I would then put the case on.

2.1) There is very little space in the locomotive housing. There is just enough space between the metal housing and the rear wall of the driver's cab that hopefully the decoder will fit in. I would place the decoder here. I would put shrink tape over the entire decoder to isolate it

2.2) Testing. I don't know if I need a capacitor, it could be installed later. There should still be space in the fire box.

I: Wiring Plan

II: 2x Overview pictures

Regards

[Jupiter 1707822591]

Looks good to me. You will not need the suppression capacitor, the decoder has it built in

Just protect the decoder from the chassis. Leave the decoder facing out in free air if the bulkhead is plastic, helps with decoder cooling.

[ColinB]

I have done a number of these on locos that I bought years ago when they first came out. You need to machine away the metal around the motor contacts as normally the two halves of the chassis would normally push up against these with spring contacts. Just remove enough metal to allow you to solder a wire on each motor terminal and then insulate it. What I also did was use a six pin socket for the DCC decoder, there is enough space at the back of the loco for it to fit. Somehow you need to attach wires to the two halves of the chassis, I used solder tags that fitted onto the existing screws that connect the two halves of the chassis together. By using a six pin socket it means you can fit either a DCC decoder or just a DC header. The advantage of this is when you complete putting it back together you can test it on DC to check you got the conversion right and check you have the necessary isolation between chassis halves and motor. Once happy fit the decoder, I used a £20 6 pin Zimo one, most of the other normal ones don't seem to handle the current, these do. Hope this helps, I have converted these and a number of A4s, the setup is basically the same.

[MrWagner]

Thanks for the feedback and suggestions. I have now soldered cables. The cables could be a "bit" thinner and I could have used a bit shorter and thinner heat shrink tubing. In principle everything fits, I don't necessarily have to file down the metal block at any point. I would not like to change the original locomotive in any way if it can be avoided.

Only now I have noticed that there is really a bit of space missing in the case for the decoder and cable. The rear wheels are also a bit in the way. I cut out a piece of decoder template out of cardboard, there is only a few millimeters of space (especially what appears to be on top of the metal / engine block). But my decoder is not small either.

There still seems to be enough space between the metal block and the driver's cab. At least I can clamp two cardboard templates in between here. As mentioned, the Lenz decoder seems to be a bit too big with 25x15x3.8mm. I think a smaller decoder would help me a bit here. As mentioned by Colin, I'll take a look at a Zima decoder. The "MX615" with 8.2 x 5.7 x 2 mm looks very good. Otherwise it would also make sense to use a thinner cable and thinner heat shrink tubing, so that the cables are not so stiff. If I have to remove something, then the back of the metal block would probably be the best place.

A six pin socket would be very useful. Unfortunately, I haven't found such tiny plugs, I have to get myself a smaller decoder first. I cannot currently imagine that there is still space for some kind of plug.

[RB51]

@ MrWagner, images are held back for Moderation before being approved or otherwise. If approved they are usually released on to the forum pretty swiftly. Your duplicate posts have been removed. R-

[Fishmanoz]

What you have described, and what Colin has given you a conversion for, is known as a split chassis design (not a split body, the body being what you lift off to get to the chassis).

Split chassis designs were used some time ago to avoid any wiring. Power is picked by the wheels on each side then goes from axle to chassis and on to the motor terminals which contact the chassis.

The problem for DCC conversion is that it is absolutely essential that the connection from wheels to motor terminals must be broken on each side, no exceptions. Unless of course you like to see a one-off puff of smoke from your decoders. Any connection between track input and motor terminals will blow your decoder.

Now you can understand the reason for Colin machining away part of the chassis, which is connected to track power, to break that connection to the motor terminals. If you can break the connection from chassis to motor without machining, then fine, but you must break it.

Once that connection is broken, conversion is simple. Whether you use a plug and socket or solder your decoder directly, decoder red and black wires connect one to each side of the chassis and decoder orange and grey to the motor terminals. If the loco runs in the reverse direction to what you were expecting, swap the orange and grey wires (swapping red and black makes no difference). Remove the capacitor across the motor terminals and don’t replace it.

That’s it, all done.