Project
This is a complete battery power, radio control, sound installation in a Bachmann On30 Forney. The entire system fits under the coal load with the battery barely visible where it protrudes on the floor of the driver's cab.
Preparation
Since this model already has a factory-installed non-sound decoder, the hope is that installation can be simplified by use of existing wiring to motor, headlight and driver wheel rail pick-ups. It's also convenient that the fuel bunker is a separate molding attached to the frame by 4 screws.
 | Lifting the lid, we find a relatively large circuit board with a standard 8-pin decoder socket, decoder and speaker. None of this is reusable, but the expected 6 wires are nicely accessible, despite non-standard color coding, which fortunately is labeled on the circuit board.  |
Begin by clearing the clutter and planning efficient use of available space. Laying the battery flat on the loco frame and using space in the cab accommodates a 500 mAh battery. Obviously, the speaker must be mounted elsewhere. Internal space within the fuel bunker is 40 mm long, 40 mm wide and 22 mm high. Since the battery is approx. 6 mm thick, the challenge is to fit a sound decoder, S-CAB radio receiver, BPS battery power supply and speaker in the remaining 16 mm above the battery. Several arrangements are possible, but I eventually decided on a major rework of the BPS circuit board to make it a "middle layer", with radio receiver and speaker mounted at the top of the assembly. This gives good speaker acoustics and keeps the radio antenna clear of metal surfaces and wire tangles.
Modified BPS board
BPS has 2 standard arrangements. The long configuration can be cut in half and the 2 halves mounted back to back to form a short configuration.
BPS-L
This is the standard, one-sided BPS circuit board
This is the standard, one-sided BPS circuit board
| Because the Forney space is square, 2 halves of the BPS board are arranged side-by-side, wired with a few extra connections, and glued with epoxy, which was never considered in the original design. I remember something about "necessity being the mother of invention". It's not something I expect a user or even an experienced installer to attempt, but having done it for this project, it could be a special-order item. |  |
Installation
At this point we have an installation plan and a modified BPS circuit board. Let's determine if everything fits.
| First, add (using super glue) a styrene plastic frame to hold battery (the slot at bottom) and modified BPS board above between the side tabs. The trail fit looks good, but there's still a third layer of components and some awkward wiring to fit into the remaining space. |  |  |
| Here's a closeup photo of the completed installation. And yes, . . . . . . the lid fits. The battery is visible though the open cab door, but there is no externally visible change to the model. |  |
Wiring
What to do with a tangle of wires is often considered too late in an installation. For this model, the entire installation must fit a space measuring 1.575" square (40 mm) and 0.79" high (20 mm). The planning process included component placement as well as arranging wires to minimize both quantity and length while still being able to make the required connections. The final arrangement is illustrated below. There are 3 unused decoder function outputs (green, yellow and brown wires) where leads have been shortened, but are still accessible for (unlikely) future use.
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