DCC Loco Conversions - 10 - General Techniques - 1 of 2

To avoid excessive repetition and keep each conversion to specifics for that loco I now outline my general techniques for converting locos to DCC whether pre-DCC or not that 'ready'.

Will it Fit?

Firstly can you fit a particular decoder in the space you have available, assuming it can drive a typical load current indefinitely and handle infrequent peaks due to potential loco mechanical problems, e.g. jammed valve gear, or being stalled by an obstacle like tools or elbows!

It is amazing how tricky this can be even in DCC ready locos, extra space can be gained by shortening wires or in the case of those with a harness which plugs into the decoder surplus can be cut back to the connector. But ensure the cuts are clean, no stray strands that may short cut ends to each other, the loco chassis, or a metal body, remember that on many locos old and new at least one rail with have an electrical conductive path into the chassis.

Irritatingly a least one DCC ready loco I have bought provides space by instructing you to remove a ballast weight from inside of one side tank, DCC requires good power pickup and DCC ready locos are generally made as heavy as possible to achieve this. Especially when bodies are lightweight plastic and traction tyres are not an option for useful tractive effort because they reduce the number of wheels available for power pickup.

The best option in that case is to select a decoder that is physically smaller and remove the socket if need be to maximise the available space.

Even a big steam loco such as the Hornby Railroad 9F has so little space for a decent specification decoder removal of the socket provides the space. (Removed sockets are worth retaining for converting large internal volume available pre-DCC locos with decoders, especially DMUs and diesel locos).

Most decoders I have used are sleeved, but not all so be wary of what they may touch especially with older locos such as those made by e.g. Hornby Dublo and British Trix which often have metal bodies, do not rely on the paint to act as an insulator.

When insulating with tape avoid using ordinary PVC insulating tape as in my experience has two problems. First it is rather thick, it was never meant for use on small diameter wires and second the adhesive often becomes gooey with time (a very short time if its already been sitting in your toolbox a few years) which not only means it detaches from the wire but it also leaves a sticky mess.

For protecting decoders I recommend using Kapton tape (Ref article 9) on areas it may come in contact with. Be aware of any manufacturers warnings about ventilation before wrapping a naked decoder with tape or especially with shrunk on heat shrink sleeving.

I only use a PVC tape to insulate the insides of metal bodies where space is tight, and there is a sufficiently large area for it to stick on. I also occasionally use black tape to disguise the magnet end of a motor sticking onto a footplate, e.g. Tri-ang 3F tank loco.

Interference Suppression Devices

This is a tricky one as determining whether they should be left in place or not requires specialised radio frequency (RF) test equipment to check for emitted electrical noise.

Interference suppression devices usually comprise of one or more components, from one capacitor across the brushes to include an inductor in each feed to the motor. These are to kill short sharp voltage spikes that can radiate using the rails as antenna to interfere with TVs and radios. These were essential in DC days due to crude motors and receivers with poor built in protection from interference. These days there are strict limits on emissions allowed from devices, plus receivers are required to filter out a certain level of RF noise.

Typical 'vintage' examples :-

Hornby Dublo Ringfield, brown disc a capacitor, yellow cylinder an inductor.



Triang, brown square item on the left, and grey cylinder on the right capacitors, yellow cylinders, inductors.



Hornby Dublo, inductors, one for each brush, probably from a 0.5 inch motor.



Lima power bogie. square white box is a capacitor.



Triang-Hornby HST Ringfield power bogie capacitors, that were only hooked in place, not soldered!



EU directives stated that suppression devices were compulsory and the decoders must be capable of running the motor with them in place, this may still apply to the UK now that we have left the EU. If concerned you should check on the requirements of your country.

Please note that I do not accept any liability for any issues arising from you removing the interference suppression devices, that is your responsibility. It is also likely that any warranty on the loco will be voided.

With DCC the decoder acts as a barrier between the motor (a noise source) and the track (acting as an aerial), however data on this is scant to the layman. My view looking at many sources is that if the loco runs on DCC with the interference suppression devices fitted, leave them in place. If erratic running occurs usually when BEMF is enabled remove them. DCC ready locos should have decoder compatible suppression already fitted, (probably to suit their own specification decoders though), pre-DCC will not.

Wiring Techniques.

Most people have their own preferences on how they perform wire stripping and soldering, these are mine and give me the results I require, I won't be there to sort out any issues on your railway, so do as you please, I really do not mind!

Tools and materials were detailed in the previous article of this series to keep this article more tightly focused on actually doing the job.

Basic Soldering

I use a modest temperature controlled iron for most jobs and a basic uncontrolled 25W iron for the odd heavy connections.

It is preferable to use wire that has tinned copper stands as these tin very easily, however old stock can be tarnished in which case I apply liquid flux prior to tinning with typical 60/40 rosin cored solder.

Also I have found that some decoders have un-tinned copper wire and this tarnishes very easily and certainly needs addition of liquid flux, if badly tarnished to black then it is better to replace the wire if possible.

Good quality PTFE insulated wire has silver plated copper strands, top quality gold but that's rather over the top for locos.

Prepared wires should be lightly tinned and then held together and heated together with a lightly tinned soldering iron tip. When shiny additional solder is applied to the joint direct from the reel, not carried there on the soldering iron tip. If all is well it flows freely and merges the two wires together. Ideally the outlines of the two wires should be visible after soldering and the whole joint bright and shiny with a small fillet between joint and the wire.

A finish that looks like a sphere of mercury on a smooth surface will be what is termed a dry joint and there will be very little contact and grip between the joined items, with intermittent electrical contact at best. When the joint dries shiny with a fine fillet at the edges the connection will be sound as both surfaces were sufficiently wetted by the solder to create a good bond.

A closer look at the Lima power bogie illustrates a spherical potentially dry joint, typically touching these causes them to detach from the metal they are soldered to. This can also be source of problems when joining a wire to a chassis pickup plate, especially if it is concealed, remember DCC decoders react adversely and quickly to poor connections.



And just for good measure from a recent DCC conversion of a British Trix Warship, probably the worst wiring I have ever come across, very likely original wiring. I do not know why else the whole loco was wired like this, another from a different source almost as bad, most other Trix have been okay. At the very least rewire a mess like this, clean all terminals with fine abrasive and use liquid flux for good wetting of fresh tinning. Do not attempt to just reattach wires to such contaminated terminals.



Ensuring that your tinned soldering iron bit is kept clean helps enormously, I prefer to use the coiled metal type of mesh to clean the bit on, a damp sponge is not so good as it cools the iron and worse still if you forgot to wet the pad molten or burnt plastic from the pad adheres to it.

The next tip is not just my opinion but my avionics training, never, ever, blow on a molten soldered joint, it causes embrittlement of the outer surface of the joint and so weakens it.

Rugged wiring is essential for long term operational reliability of your DCC system. I build my railway to outlive me, I do not want to save up issues for when my faculties become poor, I want to still enjoy my railway in 20 years time.

Okay, repeating myself, do as you wish, I know plenty of folks who will pooh pooh what I say on the basis that they have never had an issue by doing it their way, I'm just giving you food for thought. It will not be me wishing I had done it better.

Next, I discuss making connections between decoders and motors in :-

DCC Loco Conversions - 10 - General Techniques - 2 of 2

Discussion always

Jim

Return to Index