How to DCC Chip a SPUD


31 August 2021
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This popular ready-to-run motor bogie of 1980s design has driven many 'home-brew' models. It can be upgraded to digital control standards, too, as Phil Parker explains.

Is it easy to add DCC?" asked an RMweb member after reading about my Planet diesel build in the May issue of BRM.

The locomotive runs on a Tenshodo SPUD, so conversion is possible, but a little tricky. You need to put breaks in the connections from the pick-ups to the motor. It's not impossible, but hardly a plug-in solution, either. Tempted by the challenge, I starting investigating how I'd go about installing a DCC decoder.

Tenshodo's Self-Propelled Underfloor Device arrived in the late 1980s when digital control (we didn't call it DCC back then) meant using Hornby Zero One and installing a chip bigger than the motorised unit you'd be wiring it to. While the SPUDs have proved popular, Command Control was less so.

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As the years have passed, many thousands of models owe their movement to a SPUD power unit and in that time, the design hasn't changed. While digital control has evolved and taken off, Tenshodo hasn't re-designed its unit to include a socket for direct DCC fitment.
This means digital control of a SPUD will involve hard-wiring. The job is made harder than a traditional locomotive because you can't cut into the wires between the pick-ups and motor – there aren't any. A single metal piece on each side provides pick-ups and feeds the power to the motor.

Once I looked into the job, I'm pleased to say it's not difficult if you are happy with using a soldering iron. Half an hour should see the job done and the model reassembled.

Work in a clean area because the SPUD must be taken apart and you don't want any muck getting into the mechanism. In doing so, you can wave goodbye to the warranty.  Also, keep an eye on the tiny screw that holds the unit together – it's very easy to lose, although not essential as I have found in the past.

1

My test subject is the SPUD I fitted under the Planet Industrials locomotive in the May 2020 issue of BRM. The plastic infills are because I accidentally ordered spoked wheels when I should have bought plain ones.

2

Undo the small cross-head screw, then carefully unclip the baseplate. Both axles will fall out and the motor will be exposed. Keep the wheels, baseplate and screw somewhere safe for the moment.

3

The motor is held in place by friction. Gently easing it out of the plastic housing by levering under the brass worm gears should remove it.

4

Wheel pick-ups and contacts to the motor are single pieces of metal. Looking inside the housing, you can see the flaps that bear on the motor contacts.

5

Bend the flaps up and then cut them short with some sharp snips.

6

I have a Gaugemaster eight-pin decoder. Although it's designed to plug into a locomotive DCC socket, the plug can be removed. A bonus is that the wiring harness can be soldered in place before plugging the more fragile chip in.

7

Only four wires are needed. Red and black are soldered to the pick-up strip. Orange and grey go to the motor contacts sticking through the plastic housing. Polish all the metal with a fibreglass pen to make sure it's clean before soldering the wires into place.

8

Plug the chip into the harness and test that the wiring works. Then the trickiest part of the job is finding space inside such a tiny model. The chip just squeezes into the bonnet and with care, all the wires can be tucked out of the way. Installation complete!

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