Dead-Rail Conversion of an MTH 2-Rail O Scale 4-6-2 K-4S

Presents the dead-rail (battery power, radio control) of an MTH O scale, 2-Rail K-4s steam locomotive with PS-3.0.

Introduction

I obtained this 2-rail O scale MTH 4-6-2 K-4S (MTH 20-3473-2) on eBay circa November 2020. This model is unusual because it’s a 2-rail version with “scale wheels” and is equipped with a PS-3.0 control board that can operate in either DCS or DCC mode.

Box information on this locomotive

The Good News: No extensive 3-rail to 2-rail conversion was necessary, and no new DCC decoder was required.

The Bad News: This locomotive contained a PS-3.0 board in the tender I had not seen before. Also, this was my first conversion of a 2-rail MTH locomotive. I had a few issues to learn!

Close-up of MTH PS-3.0 board for the MTH K-4s

Well, let’s seen how to proceed to convert this loco to DCC dead-rail operation.

Analysis of the Electrical Connections

I’ve done several MTH 3-rail conversions with PS-3.0 boards. Still, this locomotive was designed quite differently: it has a switch to select 2-rail or 3-rail operation (a potential problem) and another switch for DCS/DCC operation (easy to take care of).

Bottom view of tender showing switches and electrical pick-ups. Before dead-rail modification the grey wire connected the rear left wheels’ voltage to the tender frame, and the brass spring connected the front right wheels’ voltage to the black wires inside the tender.

This is a bit complicated. With this original design, the tender frame assumes the voltage from the tender’s uninsulated rear left wheels (whose right wheels are insulated) via a grey wire connected to the tender frame, which under 2-rail operation is “DCC track left” inputs on the PS-3.0 board. The right track’s voltage is picked up through the copper pickups connected to the tender’s uninsulated front right wheels (whose left side is insulated) and is connected by black wires inside the tender. In 2Rail operation, the 2Rail/3Rail switch will connect these black wires’ voltage to “DCC track right” on the PS-3.0 board.

Under 3Rail operation, the left/right rails are connected electrically as ground or “DCC track left” (and the frame is now either ground or DCC left rail voltage). A grey wire from the locomotive (which is electrically connected to the locomotive’s center roller pick-ups) is connected as “hot” or “DCC track right.”

In the original design, several grey wires are connected to the tender frame to pick up the “DCC track left.” Our job is to completely isolate both DCC track right and track left so that the DCC amplifier that we add will be the only source of DCC, completely isolated from the tender frame, which will become our battery ground.

We always want to operate in DCC mode, so we need to disable DCS operation permanently.

The following images demonstrate the modifications I made to isolate all DCC from the frame and permanently enable DCC operation.

These images show several important conversion steps:

  1. Cut and seal off the two wires connected to the DCS/DCC switch. This will permanently enable DCC operation.
  2. Cut and seal off the grey and red wires to the center posts on the 2Rail/3Rail switch to ensure the DCC track’s total isolation right and left from any other electrical connections. This step will ensure no unexpected connections because of this switch’s setting.
  3. Disconnect the two grey wires mounted to the tender frame by one of the mounting screws holding the PS-3.0 in place. One of the grey wires goes to the underside connector on the PS-3.0 board, and it needs to be connected to the DCC Amplifier’s “DCC Track Left” output. The other grey wire that is electrically connected to the tender’s left wheels should be sealed off. This step electrically-isolates the tender frame from any other electrical connections, allowing it to act safely as a ground.
  4. Provide DCC “Track Right/Left” connections from the DCC amplifier (which we will add) to the two grey (DCC Track Left) and two red (DCC Track Right) inputs on the PS-3.0 board. We mentioned one of these connections in step 3, and the other pair of DCC “Track Right/Left” inputs go to the “Track” connector on the side of the PS-3.0 board.
Wiring modifications to isolate DCC from the tender frame and permanently enable DCC operation
Isolation of DCC track left from the tender frame

We need to move the PS-3.0 board forward slightly to make room for the battery, antenna mounting, battery switch, and charging plug. Also, we need to bend down the right side of the PS-3.0 board to provide sufficient clearance for the RF receiver/DCC amplifier that will be mounted on the inside top of the tender shell above the PS-3.0 board.

Also, I removed the two super-capacitors on the PS-3.0 board. The locomotive will then immediately turn off when battery power is turned off: we have no worries that power will be temporarily interrupted as with track power. I like the locomotive to turn off when I disconnect power. This is not a required modification!

Moving the PS-3.0 board forward to accommodate the battery
Cuts of 2Rail/3Rail switch wires and charging plug mount
The 2Rail/3Rail switch center posts are disconnected
New DCC connections from DCC amplifier to two plugs on the PS-3.0 board
DCC connection to the underside of the PS-3.0 board. This image’s purpose is only to show one of the two plugs where DCC inputs to the PS-3.0 board.

Dead-Rail Additions

The tender modifications to add a 14.8V LiPo battery, antenna mount, battery switch, and charging plug can be seen in several images above. There is nothing unusual about these additions.

I used a Tam Valley Depot DRS1, Mk IV receiver with a U.FL external antenna plug rather than my ProMini Air receiver and separate DCC amplifier because of space considerations. The Tam Valley’s DCC “Track Right/Left” outputs are connected directly to the two “track right/left” inputs on the PS-3.0 board (on the side and bottom connectors of the PS-3.0 board), as shown in the images above.

Conclusions and Warnings

I cannot emphasize enough the need for complete isolation of the tender frame ground from the DCC voltages output by the DCC amplifier that provides inputs to the PS-3.0 board. If you inadvertently leave a connection of tender frame ground to DCC left (from various grey wires), you may cause a severe short circuit, or the PS-3.0 board will not operate properly. Trust me, I know from a couple of bitter experiences…

Still, this was a fun and reasonably-easy dead-rail conversion, especially so since I didn’t need to modify the locomotive at all.

Here’s the final video of the fully assembled dead-rail locomotive. The PS-3.0 provides a number of DCC functions including:

  1. Directional lighting on/off (F0)
  2. Bell (F1)
  3. Horn (F2)
  4. Start-up/Shutdown (F3)
  5. Passenger/Freight Announcements (F4)
  6. Marker/cabin/firebox lights on/off (F5)
  7. Speaker volume (F6)
  8. Smoke unit on/off (F12)
  9. Smoke unit volume control (F13)
  10. Numerous other features (F0 through F28 are all active). See the Users Manual for extensive details.
Locomotive with final dead-rail installation

Thanks for dropping by!

General Battery Issues

Introduction

One of the most vexing challenges for me when doing dead-rail conversions on O scale steam locomotives is what battery to select. As nearly as I can determine, lithium polymer (LiPo, see LiPo Wikipedia) batteries are the almost-universal choice among dead-railers. So, the choice of battery technology was not much in question for me, but the battery voltage and size is still a choice to be made. I’ll deal with each of these in turn.

Battery Configurations

I’m not sure how I fell into the 14.8V “camp” for LiPo batteries on O scale dead-rail conversions. Maybe it’s because CVP pushes them for their Airwire products (CVP Airwire). I think CVP has valid admonitions about using higher voltages in regards to radio control range performance and cooler operation (see for instance CVP G3 Decoder User Guide, page 12). And, and almost all of the O scale operating modes (2 rail DC, three rail AC, TMCC, DCS) seem to have 24V or so maximum operating voltage, which is getting close to damage threshold for some radio receivers such as the CVP Airwire receivers. So, would 11.1V (3 LiPi cells in series, “3S” as discussed later) work? Probably just fine in most cases. The plus for 14.8V LiPo is that vendors offer a variety of battery physical and power configurations at 14.8V, which I’ll address next.

There is probably a good deal of lore and religion surrounding the specifics of which brand and configuration of LiPo battery to choose for dead-rail. I’m going to try to stick with what I have tried, not what might theoretically be “better” or “worse.”

Let me start with the less controversial part of my decision process: configuration. LiPo batteries come in a large variety of sizes and configurations (see for instance this site), but in my personal experience with dead-rail, the LiPo’s seem to have a single cell size roughly that of an AA cell with 3.7V output with a charge capacity of around 2000 to 2600 mAh. The individual cells are connected to achieve a total output voltage of about 14.8V (four connected in series, thus the term “4S”), but what varies is the total charge capacity and its ugly handmaiden – physical size.

This is the bear: We want large charge capacity for longer running times, but we in O scale must fit the batteries in often-tight spaces (at least compared to G scale) such as tenders where we also put radio control receiver boards, sound cards, speakers, etc. Of course, HO scale has even more severe volume constraints, but with approximately one-sixth the mass the locomotives must pull.

Personal experience here: I tried to fit an eight-cell (two rows of four vertically-stacked cells, 2.6″ x 2.8″ x 1.4″, 6000mAh, CVP BATT2) configuration into several O scale tenders, and that battery pack just flat-out would not fit. Even though this configuration was only one cell high, O scale tenders are just not tall enough to fit even one cell oriented vertically – the cells must lie sideways to fit. This cell-length limitation is important to remember for O scale. You might find space in diesel locomotives, but not steam locomotive tenders. These limitations were a big disappointment for me since I wanted to cram a large storage capacity battery pack in the tender and run “forever” (forever being at least four hours).

Sigh… Backing off in size, I have found that 2x2x1 LiPo battery packs will fit in O scale tenders with the individual battery cells running along the length of the tender. See the Figure below (pardon the body parts). You can see that this configuration also comfortably fits the width limitations of O scale tenders.

two_by_two_by_one_battery
Figure 1: 2x2x1, 14.8V battery configuration that fits. For reference, the CVP G3X receiver board between the battery and speaker is approximately 4″ long.

A note of caution: you cannot stack too much on top of this configuration before you lose vertical clearance inside the tender. For instance, I thought this was going to work:

two_by_two_by_one_battery_stack
Figure 2: This didn’t fit!

Stacking the receiver board on top of the battery would be a useful space-saving strategy, but this configuration would not clear vertically in all tenders I tried (Big Boys, Cab Forwards, Challengers, and Alleghenies). The receiver board manufacturers would probably dislike my mounting electronics on top of batteries, even with sufficient clearance.

Battery suppliers

My experience is confined to three battery suppliers of 2x2x1 LiPo battery packs: CVP, Tenergy, and “HJE.” All come with a Protection Circuit Module (PCM) that provides:

  • Overcharge, over-discharge protection
  • Overcurrent protection
  • Short-circuit protection
  • Voltage- and current-balance
  • Temperature protection

Suppliers:

Recent experience

Example of a 1x2x2, 4S1P (4 Series, 1 Parallel) battery configuration (from Tenergy.com) on a dead-rail install of a PS3.0-equipped (w/ DCC operation) MTH Virginian Triplex. This battery configuration reduced the thickness sufficiently to fit alongside the PS3.0 board where the original PS2.0 battery pack was mounted. The PS3.0 replacement board was obtained from Ray’s Electric Trainworks.

More creative 14.8V battery configurations are possible that include one battery-diameter thickness solutions such 1x2x2 (Tenergy.com) with dimensions of 131 x 36 x 23mm (LxWxT), so it’s approximately one battery-diameter thickness, two battery-diameters wide, and two battery lengths long (~5.2″). See the picture above for an example of using this thin configuration in a very tight mounting configuration on an MTH Virginian Triplex with a PS-3.0 board operating in DCC mode. See this blog for more details.