Dead-Rail Conversion of an MTH UP 4-12-2 2-Rail locomotive with the New, Smaller ProMini Air Receiver

I have posted several dead-rail conversions of O scale 2-Rail MTH steam locomotives equipped with a PS-3.0 controller capable of operating in DCC mode. These locomotives are convenient for dead-rail conversion because they come fully equipped with good sound, lighting, and smoke effects – all controllable with DCC. However, I have received numerous questions asking for clarification.

So, what’s new in this post?

The goals of this post are to show off a dead-rail conversion with my new, much smaller ProMini Air receiver (1.1″ x 0.8″) coupled to a small DCC amplifier, the DRV8871 (1.0″ x 0.8″), and, to explain the conversion strategy for O scale, PS-3.0-equipped MTH locomotives. I have chosen the PS-3.0-equipped MTH UP 4-12-2 2-Rail locomotive (MTH 22-3641-2) because it has a small, crowded tender, making for a challenging installation of the required dead-rail components: battery, ProMini Air receiver/DCC amplifier, antenna, switches, and charging plug.

Some conversion details, such as power connections, are left out to reduce cluttering the critical points.

Introduction

The photo below shows what we are up against: a very crowded tender!

The original, very crowded tender electronics

The challenge is how/where to locate the dead-rail components.

Dead-Rail Conversion

Since this locomotive is fully configured for lighting, sound, and smoke effects, and all of the control electronics are in the tender, I did not modify the locomotive!

We’ll turn our attention to the tender.

The most challenging aspect of this conversion is battery location. After some fiddling and considering other battery configurations, I decided on a flat 14.8V Tenergy battery, mounted in the tender as in the photo below.

Battery location using a 14.8V Tenergy battery

This location required slightly bending the PS-3.0’s heat sink to provide battery clearance.

I also moved the speaker platform forward and removed the plastic speaker enclosure to make room for the battery.

I moved the speaker platform forward to provide room for the battery

The wiring of the 2Rail/3Rail switch is at the heart of our conversion. Since we will not operate on 3-rail track, we will repurpose the 2Rail/3Rail switch to retain the original 2-rail track-powered operation or use the new battery-powered amplifier output connected to the ProMini Air receiver. See the diagrams below for the original and final wiring for repurposing the 2Rail/3Rail switch

The original switch wiring for 2-rail operation. The right 2RAIL post is not connected!
The original switch wiring for 3-rail operation. All wheels become “Track Left” and the center-rail pick-up rollers become “Track Right.”
The final switch wiring for 2-rail operation. Track-based “Track Left” and “Track Right” are fully retained.
The final switch wiring for radio-control operation. Now the outputs from the ProMini Air receiver’s DCC amplifier supply “Track Right” and “Track Left” to the PS-3.0.

I modified the wiring to the 2Rail/3Rail switch to accommodate DCC inputs from the ProMini Air receiver’s amplifier. The photo below shows the first step: moving the gray wire soldered to the right center post of the 2Rail/3Rail switch to the front right post.

The next step is the hard part: figuring out the re-wiring required. To aid in the discussion, let’s talk about the capabilities of the MTH PS-3.0 controller. This board is designed to pick up signals through the locomotive and tender’s wheels, and, if operating on three-rail track, the center-rail pick-up rollers. To accommodate either 2-rail or 3-rail operation, MTH provides a 2Rail/3Rail switch on the underside of the tender chassis.

Consequently, when you set the switch to “2Rail”, the gray wires, which are electrically connected to the left track, provide input to the “Track Left” of the PS-3.0.

Next, the gray wire directly connecting the “Track Left” input to the PS-3.0 board is separated from the other gray wires and soldered to the right-center post of the 2Rail/3Rail switch. Now, the center-right post provides the “Track Left” input to the PS-3.0 from rail “Track Left” when you set the switch to “2Rail.”

Moving the gray wires and creating a single Track Left input to the PS-3.0
The Track Right (red)/Left (gray) connections to the 2Rail/3Rail switch to the PS-3.0 board

Since we will NOT be operating in 3Rail mode, we can repurpose the 2Rail/3Rail switch’s 3-Rail connections to provide the DCC inputs from the ProMini Air receiver’s DCC amplifier.

I first removed the wiring on both of the 3Rail posts on the switch.

Removal of the 3-Rail wiring connections to the 2Rail/3Rail switch. After removal from the switch post, the two black wires MUST be connected together to ensure that rail-based “Track Right” is supplied.

I sealed off this wiring, preserving the connection of the two black wires since they both contribute to “Track Right” from the locomotive or tender wheels.

Sealing off the 3-Rail wiring

Then, I soldered two wires with a plug to these “3Rail” switch posts that will connect to the DCC Track Right/Left outputs of the ProMini Air receiver’s DCC amplifier. With this modification, when the switch is set to this position, it connects the PMA amplifier’s DCC output to the PS-3.0. This now completes the conversion of the 2Rail/3Rail switch to a 2Rail/RA (for radio-generated signal) switch. That was the hard part.

Wiring for DCC inputs from the ProMini Air receiver’s DCC amplifier so that the “3Rail” switch setting now becomes the selection for “Radio Control DCC.”

The signals originally picked up from the rails come in two “languages” that the PS-3.0 controller understands: DCS and DCC. To accommodate this capability, MTH provides a DCS/DCC switch on the underside of the tender chassis. The DCS commands are a proprietary MTH invention, and for our purposes, do not interest us. DCC is important to us since the ProMini Air receiver is designed to receive wireless DCC commands, which are an NMRA standard.

We can set up the wiring for permanent DCC operation and repurpose the DCS/DCC switch for Battery ON or Battery Charging. When you set the unmodified DCS/DCC switch to “DCS,” the two black wires activate DCS mode, which we no longer need. When you set the DCS/DCC switch to “DCC,” these two wires are not electrically connected, which is what we want permanently.

The first step is to remove these two black wires and close them off to prevent them from shorting together.

Removal and insulation of the DCS wires for repurposing the DCS/DCC switch as a Battery ON/Charging switch.

Then, three wires are soldered to this switch:

  1. Center posts: Battery +. This post provides battery power that will either supply power to the PMA Rx and DCC amplifier or receive charging power from the charging plug, depending on the switch position.
  2. Back posts: PMA Rx/DCC amp power +.
  3. Front posts: Charging plug +
Final connections for the switches

The right and left posts are soldered to each wire to ensure a low-resistance, high amperage connection. The rest of the power connections are standard and not discussed here.

OK, we’re finished with all wiring modifications; now, let’s turn to adding the antenna and charging plug by first drilling holes on the bottom of the tender’s chassis and mounting the antenna and charging plug (see photo below).

Antenna and charging plug mounts, and repurposed switches

The antenna mount has a wire connection carrying RF output from the antenna to a U.FL connector plugged into the ProMini Air receiver.

The charging plug has a “+” power connection wired to the battery ON/Charging switch. All power “-” connections are on the “-” posts of the charging plug.

Finally, I mounted the ProMini Air receiver and its DCC amplifier over the speaker after removing the plastic speaker cover to provide sufficient battery clearance.

Mounting of the small ProMini Air receiver and DCC amplifier

The small size of the ProMini Air receiver and its DCC amplifier make this mounting strategy possible.

Demonstration

The video below shows the “proof in the pudding,” where the locomotive is controlled by the new stand-alone ProMini Air transmitter integrated with a WiFI-equipped EX-CommandStation that receives throttle commands from a smartphone app.

Demonstration video using WiThrottle app connected to PMA transmitter integrated with a WiFi-equipped EX-CommandStation that transmits to the onboard ProMini Air receiver.

Author: Darrell Lamm

I earned my Doctorate in Physics from the Georgia Institute of Technology in 1982, and before retiring in 2019 I worked for 37 years at the Georgia Tech Research Institute. My last position was Chief Scientist of the Electro-Optical Systems Laboratory. Like many people, my love for model railroading began at an early age, and I rekindled that interest starting in 2017.

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