Union Mills DCC fitting

[Richard E]

Fitting a decoder to a Union Mills loco turned out to be a lot easier than I expected. I had seen a guide somewhere but that involved milling out the inside of the tender under the coal load. This was, apart from plugging a decoder into a DCC ready loco, my first conversion.

 

So turning to my model:

 

The first thing to do is separate the loco and the tender. Note that there is a connecting wire between the two, it is trapped by the drawbar screw on the loco. Pick up is simple, the loco is live to the right hand rail and the tender to the left hand rail. This actually makes the DCC conversion very easy in my view. Simply undo the tender drawbar from the loco and the wire will pull out:

 

 

The next step is to remove the drive unit from the tender. Again just undo the drawbar screw but note the thin washer between the chassis and the body, don't lose it! The wiring is simple, a resistor from the tender pick ups (the motor body is used as a proxy for these as it is live to the track) to one motor brush and a wire direct to the loco from the other motor brush:

 

 

Because of the way this is wired all I did was remove the resistor completely and unsolder the wire from the other motor brush. For those who want to retain the resistor in circuit just unsolder it from the motor body, use some small heatshrink to insulate it when reassembling.

 

Now this is where it can all go wrong. I was lucky as I bought, a while ago, a pair of CT Elektronic DCX77zD decoders specially for my Dean Goods. Unfortunately these don't seem to be available at the moment but the DCX77L is; this is the same decoder but with the connections on the short side of the board instead of the long side, it is a couple of mm longer and a mm narrower as a result. I've measured the available space under the coal load and the DCX77L should fit without a problem. For comparison I've shown below (from left to right) a DCX77zD, a DCX74zD and Gaugemaster DCC22.

 

 

I've then fitted the decoder soldering the red wire to the motor body and the decoder motor output orange and grey wires to the motor. The black decoder wire is left at full length. If anything I've left the wires to the motor a bit long on this one. you can also see that I have left the yellow and white auxilliary outputs in place but cut back and then covered by a short length of heatshrink to insulate them. The tape on top of the motor is original, it is actually put there by Union Mills.

 

 

At this point strip and tin about 8mm of the black decoder wire. Refit the drawbar to the loco fitting the decoder black wire into the small hole above the drawbar from which you removed the original Union Mills wiring. Place on your programme track and set up and test. If all is OK refit the tender body and drawbar making sure the wires are clear of the tender wheels and, most importantly, the tender drive gears. That is why shorter decoder leads are more desirable for this conversion.

 

The only external difference is a slightly finer wire from loco to tender:

 

 

Overall I found this a very easy conversion, the only downside is that it requires, ideally, a very slim chip and they are a bit dearer than most and difficult to find. I've found the DCX77L in stock in the UK today at £32.00. I've recently dealt, in person, with the shop selling these and they are really helpful.

 

EDIT - I've edited to re-post the photos. Note that you need to alter the CV's to limit the maximum voltage to the motor if you remove the resistor.

 

Edited August 3, 2022 by Richard E
re post pictures

[Antonov124]

I know this is rather late, but is this description of DCC fitting to Union Mills locomotives the same for all UM locomotives. I ask because I have a Union Mills D16 and I’ve just purchased a Zimo MX616 to, hopefully fit to it. Any information would be helpful. 

[doug22150]

I've fitted Zimo MX 616s into all of my Union Mills locos and only one of them required work to increase spece on top of the motor.  I'm now adding Stay-Alive but this requires removal of the "coal" from the top of the tender.

 

Running with Zimos is superb, with no tinkering with CVs.  I like to achieve very slow running and because some of my track is slightly uneven have opted to add stay alive. Locos. thus fitted with crawl almost imperceptibly without faltering.