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VR DERM by Trainbuilder

Updated: Dec 11, 2020

2nd production run (2017)

The second run of Trainbuilder DERMs came from the factory with a new spud drive located under the chassis. This liberates room in the guard's compartment for some decent sized speakers. In this example, a Reverberator speaker from DCCSound was fitted, along with an ESU 58820 LokSound V5 8 pin Micro decoder. Each lighting function output (except for the interior lighting) was protected by a 470 ohm resistor.

F0f = white wire (engine end headlight)

F0r = yellow wire (non-engine end headlight)

Aux 1 = green wire (non-engine end red markers)

Aux 2 = magenta wire (engine end red markers)

Aux 3 - not available from the LS5 Micro decoder

Aux 4 - ditto

Aux 5 - Interior lighting

Motor calibration - you get out what you put in:

The "Auto-calibrate" feature present in the LokSound 5 decoders made the motor control worse than the "default" settings available in the LokProgrammer software 5.0.16, ever since 5.0.0! The "default settings" available in the LSP software made the motor sound "grindy", delivered little drive torque and the motor shuddered badly when it came to a rest. The BEMF was such the motor also over-ran when the speed was set to zero. Strange stuff.

In comparison, the auto-calibrate feature worked very effectively in the older V4 decoder environment, so perhaps something was lost in translation with the LokSound 5 decoder development.

After much experimentation the following CV settings resulted in very good motor control under DCC control:

CV51 1 Regulation parameter K slow, defines slow speed influence of BEMF. This partilcular motor is an outlier, and so this parameter needed to be set as low as possible to avoid jerky performance.

CV52 1 A related parameter to CV51, set very low for the Trainbuilder spud motor.

CV53 130 Regulation reference, defines the ceiling for motor speed.

CV54 30 Regulation parameter K, has to be quite low for this spud motor type. The auto-calibrate feature reduced this value to zero, but in practice it did not improve the control at all. It was CV51 & CV52 that needed adjustment, not CV 54 and this was not managed by the auto-calibrate feature.

CV55 100 Regulation parameter I.

CV56 150 Regulation reference slow speed, basically defines the "kick" to get the motor turning at speed step 1. 150 was a minimum, but any higher resulted in too much BEMF regulation and poor control.

Basically the pulse width of the controller waveform feeding the motor from the decoder is minimal near zero motor speed, compared with a wider pulse (more power) at higher motor speed. Motor control parameters need to account for different motor designs, which may not be optimal, due to properties of inductance and current ripple response, which start to interfere with the signal near zero motor speed. So it's good that ESU offers a large number of CVs for the end user to have access to, in order to optimise their model's motor performance.


The model is disassembled by removing four screws under the chassis that holds the body in place. The non-drive end coupler is also removed.

There is a large weight over the drive bogie, which in this case has been removed to make way for the speaker, decoder and stay alive circuit. In the image below, the 4 remaining wires are for powering the headlight, markers and interior lights.

In the image below, the above lighting wires have been connected to 470 ohm resistors, except for the LED light strip which comes with inbuilt resistors to protect the LEDs from drawing excess current.

The model is then reassembled with the body reconnected to the chassis and coupler refitted.

1st production run (2011)

The Trainbuilder DERM (1st production run) was specifically designed to accommodate an ESU 16x25mm oval speaker in the #2 end. The speaker should be sealed to its custom enclosure with MicroKrystal Klear. The motor and pickups are soldered in position.

The headlights and markers are driven by 4 LEDs, a white and a red LED respectively at each end. The group of transistors and voltage regulator is incompatible with DCC, so require removal from the LEDs. At each end of the DERM, one white and one red LED should be wired in series with one 470 ohm resistor onto one output function from the decoder (yellow -ve for #1 end, white -ve for #2 end). Then the +ve common from the two groups of LEDs is connected to the blue wire of the decoder.

A 3 pin micro plug & socket between the decoder and lighting systems enable separation of the body from chassis. The interior lighting comes as a separate system which runs off DCC track power. It can be left alone from the decoder, as there is an on/off switch located under the floor. However it can be wired in as a separate auxillary function to the decoder via the purple or green wire (-ve) and the +ve will be again the blue common return wire.


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