DCC Loco Conversions - 7 - Worth the Effort? – Part 5 of 5

Discussion in 'DCC Control' started by Jim Freight, Jun 29, 2021.

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  1. Jim Freight

    Jim Freight Full Member

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    14.2) Magnet

    Another point to consider especially for older locos whether Hornby Dublo or Triang is the state of the magnet, early magnets were notoriously unstable and could fade away and cause the motor armature to take increasing current until it burned out. Also dismantling these motors without using a keeper (a steel plate across the magnet's poles) could result in a serious loss of its field strength. So if a weak magnet is suspected due to high current it needs re-magnetising.

    Alternatively replace with a NEO equivalent, but be advised they are often stronger than the original motor design catered for and can cause cogging at low speeds, i.e. the armature jumps rotationally in steps rather than evenly at low speed. This is more an issue with smaller shunting locos than heavy express engines run at speed.

    Sometimes piggy backing small NEO magnets to the original magnet or it's frame can aid strength if you have the space.

    14.3) Armature

    Another aspect is the type of armature the motor has, the historical standard for DC motors has been the 3 pole armature, that is the rotating part has 3 arms with windings that are energised in sequence as it revolves by the brushes passing current to them via the commutator (slotted copper ring).

    Later developments in pre-DCC motor design used 5 pole motors, the main difference for us is how smooth the loco runs at low track speed. Manufacturers generally get over this issue by having a high gear ratio between the motor and the driving wheels. The higher the gear ratio the more revolutions of the motor is required for each revolution of the loco wheels. So motor jerkiness or cogging is much reduced through the gears by the time it reaches the driving wheels.

    There are exceptions, recent conversion of two old British Trix A2 Class locos. It was difficult to adjust CVs to run them slowly. The motors are very large and rotate relatively slowly with a low gear ratio between motor and driving wheels, but then again they are not going to be used as shunters. The windings are large with fine wire and despite their size draw only a modest current.

    Contemporary wound armature motors are very much more efficient especially those with skew wound windings which cleverly improve torque and smoothness at all speeds.

    Coreless motors do not suffer from cogging, but as these are typically in DCC Ready locos I will not consider them further here.

    14.4) Alternative Motors

    Much is said about swapping out the large pancake 3 pole motors in old Lima locos with ex CD drive motors as the Lima motors tend to have poor low speed characteristics even if you can get them to run slowly. I prefer to discard the original loco wheels, especially if they are one or more of “pizza cutters”, unplated or have rubber tyres and replace them with Ultrascale wheels. The downside is that Lima locos have very little weight as they relied on rubber tyres so I increase the ballast weight within them significantly. The Lima motors are strong, the extra weight tends to tame their boisterous nature at low drive voltages as well as improving traction.

    I would also look to swapping out tender drives where possible, e.g. the Hornby Railroad 9F mechanisms combined with the detailed models with tender drive, (oh why did they do that?) make a much superior model. There are many detailed wrecks around due to Mazak rot to make this a cost effective solution.

    15) Lights

    Some pre-DCC locos have running lights or head code lamps.

    Filament lamps in older DC locos may be controlled by diodes with lamps fitted at both ends to be directional e.g. Hornby diesel head code boxes, or continuously lit as head code lamps on a British Trix A3 or Jouef Class 40.

    In both cases they are just connected to the pickups in parallel to the motor so brightness is linked to loco speed. These cannot be left in place as the raw DCC track voltage is likely to be too high and they will be on all of the time which can cause body damage through overheating.

    Given the choice of warm white and cool white LEDs, warm white should be used for oil lamps and tungsten filament lamps and cool white for modern image white running lights.

    It is a lesser known fact that the current rating limit for driving a filament lamp can be somewhat less than for an LED so check the decoder data sheet before trying to connect the filament lamp to a decoder function output.

    A resistor is required between the LED and the decoder, I generally use one 0.125W 680 ohm resistor per LED. However if only one LED is illuminated at a time, e.g. head code lamps on a diesel then I just fit one in the common connection to the decoder.

    When using LEDs in place of filament lamps you may have to orientate them differently because although a standard 0.2inch LED fits a holder for the LES (Liliput Edison Screw) or plain capped lamp the direction of peak light from an LED differs from a filament bulb. An LED emits it's peak illumination from the domed end, a standard filament lamp emits peak light from the side of the glass bulb. Sometimes this matters, best to confirm before finishing the wiring with heat shrink wrapping of joints.

    16) Summing Up

    The issues above I will address in detail as appropriate when I start the presentation of a wide variety of conversions I have undertaken, usually with good results. Occasionally some are just beyond hope, either by design or old age, I will cover successes and failures.

    Upgrading pre-DCC locos can be chosen for many reasons, the main ones can be economy, lack of a DCC Ready version or just sentimental nostalgia. I take a pinch of economy as some cost far more to convert than they cost to buy, but it is mainly the nostalgia of being able to run them freely with my new RTR locos.

    One should bear in mind that achieving a performance match with contemporary designs is unlikely, they often have motors well visible over steam loco foot plates, no daylight through bogie frames, and extra thick glazing just to name a few of their charms!

    Just take them as they are, veterans of days gone by which can be handled easily without fear of leaving a confetti of details on the track after a running session.

    Next, my views on decoders, "Decoders & Me"

    Jim

    Image: Bachmann Wickham Trolley fitted with a Digitrax DZ126T decoder which is to be disguised as a folded tarpaulin sheet.

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