DCC Loco Conversions - 11 - General Techniques - 2 of 2

Decoder Connections

I will solder wherever possible, fitting connectors into a 4mm scale chassis requires a lot of volume, so given a loco without a socket I will in general hard wire, that is solder the decoder wires directly to the locos motor brush arms and pickups.

Typically decoders with wires terminated with an 8 pin plug are cheaper than those without, probably because most are sold with a plug, just carefully cut off the unnecessary leads at the decoder so they cannot short together. At the plug cut off or de-soldered the wires you actually need, the fewer the number of wires the easier it is to route them. (I often remove surplus wires even if a socket is fitted).

Where adjacent wires are cut at the decoder cut them to different lengths, that will minimise inadvertent shorts between the cut ends, alternatively cut each at angle of 45 degrees so that adjacent wire ends take on a saw tooth pattern.



Often I will take a socket out of a DCC ready loco to install a better 3rd party decoder which is often larger, this is particularly the case with Hornby and some Bachmann locos.

Joining Wires

Soldered joints between two wires for decoder installation can take two forms, the slim and simple lap or the stronger but bulkier hook type.

I have tried to photograph the steps but it proved rather more difficult than the actual soldering, I have used larger gauge wire than I would use for decoders as it is easier to illustrate, note that the wire strands should still be partly visible on the outer surface of the final joint.

Hook Joint

For the hook joint the tinned ends (1) of two wires are formed into hooks (2), then hooked together, and lightly squeezed to grip each other (3). Whilst held under light tension the joint is heated by the iron and solder is flowed into them from the reel, resulting in (4). This is easier and stronger than a lap but will generally need to be cut out for any re-work.



Lap Joint

The lap is where the tinned ends (1) of the two wires to be joined are laid against each other, non-corrosive flux (ref article 9 - Soldering Materials) is applied and then touched with a lightly tinned soldering iron bit to allow the solder between them to fuse together (5).

The joint fuses in a sizzle of flux almost instantly when the solder on the bit touches it, note that the solder on the iron is only to allow rapid heat transfer to the joint and not to add solder.

This does take some practice but makes for a slim joint for tight spaces and is easy to de-solder joint if re-work is required. Taping one wire to a scrap of wood with masking tape provides a useful third hand.

Joints should be sleeved with heat-shrink sleeving, but test the installation before shrinking the sleeving into place.

Stripping PTFE

Generally this is virtually impossible to perform with typical wire strippers as the nature of the material is very different to general purpose PVC covered wire, PVC insulation is very easily cut through, and melts very easily.

PTFE on the other hand has a structure more akin to a fabric, it is also very tough so the layer of insulation is much thinner than PVC which for a given outside diameter you can have a much larger cross-section of copper wire compared to PVC coated wire.

It's tough nature and very high melting point, a typical soldering iron will not melt it , makes it very useful in tight spaces especially within a loco chassis and linking to a tender.

Whilst preparing this article I tried stripping some PTFE with the budget and RS strippers I showed in (ref article 9 - General Tools), the budget stripper was just not sharp enough to penetrate the PTFE. The RS stripper actually made a very clean strip, ah, perhaps things have improved over the last 40 years. Tried again it would not penetrate the PTFE, the reason was that strand like slivers of the PTFE totally jammed up the toothed stripping blades and need extraction, so be warned.

The way I was taught to strip PTFE comes from my early avionics training, it takes a little practice but can yield high quality results, do not be tempted to use a scalpel directly around the wire as it will nick the soft copper wire.

Doing so makes it liable to failure where there is vibration and reduces the available wire cross-section for passing current. Although the current taken by a loco, even an old 4mm scale one will be 1 amp or less remember that DCC short circuit current may be much higher and the wire you may be using very thin.

The way, shown in steps in the image below :-

1) the wire as cut

2) crush the last 2-3mm of wire flat with a pair of small pliers, you will notice that the crushed insulation will look like multiple strands of fabric and tease the crushed insulation into two tails 180 degrees apart with a scalpel taking care to avoid damaging the copper wire.

3) grip the two tails, each with fine pliers or broad bladed tweezers and pull apart, it's just like pealing a banana from two faces at once until there is sufficient bare copper wire for the intended connection.



4) trim off the two tails of PTFE insulation.

5) lightly tin the exposed copper wire.

6) cut off the crushed tip off the copper wire and the prepared wire is now ready to be used.

Heat Shrink Sleeving

This is well worth using and can be bought in modest reels (ref article 9 - Sleeving) or in short cut lengths from eBay and other general electronic component suppliers. I generally use black to help hide the wiring from view. It is also available in resistor colour code colours which for our use can be used to mark e.g. fine black wire with red, orange or grey rings.

Useful too, as strain relief of wires attached to moving connections whether solder tags or motor terminals on diesel bogies.

Heat shrink sleeving un-shrunk can be used to bundle wires going past e.g. a motor to feed lights, as a strain relief when decoder wires are soldered directly to motor brushes on XO4 Tri-ang motors and similar fitted to some Dublo and Trix locos.

This leads me to connecting decoders to motors.

Continued Below

 

Continued (Article split 25/10/24 due to expansion of "Lap Joint" above)

Connections to Motors

Generally these older motors have replaceable brushes and have the pickup feed connected to the motor by a formed metal tag which is sandwiched between the brush spring wire and the brush.

This connection is another potential reliability source of trouble to DCC and so should be discarded where fitted and the decoder orange and grey wires soldered directly to the top of the brush arms.

It must also be remembered that the other brush has electrical contact via the brush spring to the chassis and one rail so it must be insulated from both brushes otherwise decoder destruction will be fast. I prefer to use PTFE sleeving as it resists soldering iron temperatures. Alternatively when using PVC, solder to the brushes before sleeving the spring wire. Another option would be to use heat shrink sleeving.

The first image shows an X04 motored Triang conversion, and the second an early power bogie conversion.

In both cases both of the spring wires are insulated with PTFE sleeving, the wires from the decoder are soldered directly to the brushes (in the case of the power bogie the copper connection from the right hand wheel set has to be cut away too).

To avoid the wires pulling on the brush arms and affecting the spring tension they are restrained to the spring wires by un-shrunk heat shrink sleeving, shown black on the power bogie and white on the X04.






The metal arms of the the brushes are likely to be tarnished with age so I clean the top faces that I am to solder to with some 400 grit abrasive paper, remove the resulting dust and apply some non-corrosive flux (ref article 9 - Soldering Materials) and then tin them ready for attaching wires.

The soldering of the wire to the X04 motor brush shown above is how I want all the joints to be, note the outline of the wire is clearly visible, a fine fillet of solder joining it to the brush arm and all is shiny.

This is more complicated for Hornby tender drive locos and Mainline/Bachmann split chassis designs as the way the chassis connection is made is varied and often well hidden, including extra long frame screws or carefully crafted metal brush springs. I will deal with those in more detail with specific conversions.

Split chassis designs may require much more work, detailed in actual conversions.

Further complications arise with Hornby Dublo integrated and Ringfield motor assemblies.

If in any doubt always check for electrical isolation between motor brushes and both rails with a resistance meter (or ohms range on a multi-meter) before going any further.

Heat Shrink Sleeving - Application

This can be shrunk using a small hairdryer, I use a 1200W dryer with a rectangular nozzle that was going spare, and use it on the high heat setting. To avoid damage to the loco by excessive heat and to reflect heat onto the opposite side of the heat shrink tube, thus speeding the shrinking I use some kitchen foil folded so it is 4 layers thick. It can be formed easily to protect and focus the heat readily.

I let the airflow heat up whilst away from the loco and then hold it about 20mm away from the tubing to be shrunk, only heat long enough for the tube to shrink to the contours of the joint. PVC wire insulation is likely to soften a little, very cheap and thin PVC insulated wires can start to melt.

If in doubt try out on an off cut away from the loco to see what works for you. Do not use a hot air gun designed for paint stripping unless it has a specific low temperature setting, as it will be capable of burning your loco as well as the wire insulation.

Time to Convert Locos

These conversions will range from simple to fiddly and to needing small machine tools, so please be aware of my five "Worth the Effort" articles before taking a hacksaw or worse to your favourite old timer.


Discussion always

Jim


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Connections to a Split Chassis

A common issue with converting locos with split chassis to DCC is where to make sound and therefore reliable connections for track power to a decoder, this particularly difficult at times with Mainline split chassis and I suspect early ones that migrated to Bachmann too.

This is compounded by almost all the volume inside the loco being filled with ballast which unfortunately being Mazak is not as dense as lead or the denser alloys used in current models so often the loco cannot afford to lose a single gram, once again Mainline chassis in particular.

It is difficult, I consider impossible, to solder to the Mazak due to the composition of the alloy and the large mass of metal requiring a high wattage soldering iron with a high rate of energy delivery.

A simpler way is to make a loop of stranded wire and anchor it to the chassis by one of the screws holding the chassis halves together, although simple it is a potential source of intermittant failure later on for locos which are by design, difficult to get running satisfactorily with DC, let alone DCC.

Potential issues which can cause power pick up issues include

• The strands of wire being broken or cut through by the screw threads.
• The screws not being tight enough for their primary chassis joining role.
• The screws can come loose as the plastic parts they screw into disintegrate or crack.

The alternative is to use solder tags but they typically need to be tiny, less than 10BA or M2, which I could not find, so I now use this approach to make simple ones of my own.

The Tags

I simply take a short length of 32 SWG tinned copper wire bend it loosely around the shank of a 1mm drill bit, (or tightly around a 1.5mm) twist the loose end with the other end still attached to the reel, flow some solder into the twist, add the screw and then cut the tag from the reel, done.



Fitting the screw (M1.4 x 3mm) before cutting from the reel makes it easier to position the tiny screw over the pre-tapped hole, as I found out after taking this image, the wire acts as a handle.

I added loops to the free end of those for the 38xx conversion but there is not room in a Collett so a lap joint sufficed.

It is easy to make a quick soldered joint to these tags when mounted using liquid flux as I always do, the copper wire restricts the heat transfer rate to the chassis metal which ensures a good solder flow without the heat of the iron being dissipated into the mass of the chassis part.

The copper wire is more resistant to damage than fine wire strands.

Recommended Screw Set

The most useful metric self tapping screw set for these Mainline loco conversions is this one from Amazon, which currently even includes very useful tweezers, quite well made too:-

MEIYYJ Small Pan Head Self-Tapping Screws PA Phillips Head Micro Screws Tiny Self-Tapping Screws Assortment Kit Pack of 1000pcs (Black)

https://www.amazon.co.uk/gp/product/B07S4FNB5N/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Many other of these sets are worth having to hand to replace missing screws on locos (especially countersink head type), or less exciting domestic things like spectacles.

Attaching the tags to the chassis

The pilot drill size is critical, in this case 1.2 mm, the mixed boxes of small screws from Amazon are intended for use in plastic, one false move and they shear, yep got that tee-shirt now!

It was also better to use one screw for the cutting the thread in the metal, usable for at least two holes and another for the job as the thread can easily get clogged and the soft head damaged, tricky unclogging a 1.4 mm diameter screw thread. A damaged cross head is a potential problem, if not when fitting but may worsen when releasing the screw at a later date.

Example of Use

Front of a Collett chassis.




Summary

These have worked well and have the bonus of being customisable to suit the conversion in hand, long with a loop at the free end for a Mainline 38xx, short for thier Collett.

Discussion always

Jim


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