Tomix DE10 DCC Pt 3: Wiring

Read Part 1 of this article.

Read Part 2 of this article.

As a preface let me say this: The results of this install are mixed. On one hand, I got everything to fit, with room to spare! Which is excellent news. I also learned a lot, which is also excellent news. However, several things went wrong with this install. I broke a handrail (minor—it’s an easy fix, but annoying). The headlights don’t work right (bad decoder, will have to send the decoder back for repair—again). And the motor makes a funny clicking sounds when the loco travels in reverse (might be easy to fix, and may simply be a lack of lubricant—but it might also be worse). But it works! And it fits! And that’s something.

Last time, recall that I burned out one of the bulbs. This turned out to be a valuable lesson. Although the bulbs that Tomix used for these lightboards are rated at 60ma, and although the function outputs of the DZ125 decoder are rated to 500ma, this turns out to be a bad combination. Incandescent bulbs have a much lower resistance when cold than when warm, meaning that when first switched on, a bulb draws as much as 10 times as much current as it does when its warm (ref here; see also inrush current). That means that 60ma bulb can pull over 600ma when first turned on, far in excess of the 500ma rating of the decoder function output!

So the stock bulbs needed to be replaced with lower-current bulbs. Finding bulbs with the same physical characteristics as the Tomix bulbs but DCC compatible electrical characteristics (namely: 12–14V, ≤50ma, 3mmØ) turns out to be really hard. Those cheap grain of wheat bulbs on eBay won’t do (70ma!), and the easy-to-get bulbs from Radio Shack are too big. These bulbs from MCM are the only ones I could find. I bought a bunch more than I need, and so if you’re following along and want a couple without having to pay through the nose for shipping, I’d be happy to sell you a couple from my stock at cost (which turns out to be 65¢/bulb + first class postage).

DCC lesson over, on with the rest of the install!

First, remove the old bulbs from the lightboards, and the diodes, (on the one board) the capacitor, and all that excess solder. Tomix used an ever-loving ton of solder, and I ended up overheating the circuit board, lifting one of the traces slightly. Do be careful, and perhaps remove that solder in several passes. You can keep or discard the bits as you see fit: We won’t reuse anything for this install.

Here the new bulbs are installed. I threaded the leads through the two holes closest to where the board mounts in the frame, bent them back towards the leading edge of the board to hold the bulb in place, and used a moderate amount of solder to hold it down. I haven’t clipped the leads in this photo, not yet.

After removing the old components, but before soldering in the new bulbs, in order to prevent a nasty short, we have to cut the traces on the boards. I simply used a needle file to remove enough of the material that the traces the bulbs are soldered to won’t touch the frame.

Just to make sure everything is still good, here I’ve test fit the decoder in the frame. The decoder sits just flush with the top of the frame, although I’ve had to trim a bit of the shrink-wrap from both ends of the decoder. Notice I’ve used a small pair of finger-nail scissors to trim that shrink-wrap on the wired end of the decoder to give the wires a little more flex.

This shot illustrates how the pickup and motor wires will be routed. I cut a notch in the side of the frame for the motor wires to slide down. I used shrink-tubing to isolate the motor pickups from the frame. The pickup wires fit in a neat little existing gap in the frame beside the plastic spacer to the right. The pickup itself is a little brass clip that just sits in the frame on either side of that spacer, and I simply soldered the pickup wires directly to the brass pickups. No soldering to the frame! This is the most hair-raising part of the install, as you have to trim the decoder wires pretty short, and it’s hard to measure exactly…and indeed, at least one of them got cut just a tad too short, necessitating a little creative re-juggling of the wire routing under the decoder.

Test, test, test! This first test reveals that the install is going well: Nothing to go back and repair. It also serves as a reminder that a decoder cannot magically transform a badly running loco into anything but a still-badly running loco. A little CV tweaking can’t hurt though! I’ll offer some sample CV values in a future post here.

This is it! You can see now how the channels we cut before guide the wires. I had to widen the gap between the frame halves just to the right of the decoder a bit to fit four wires through there, but that was no big deal. I soldered the lighting leads in ‘backwards’ to alleviate stresses on the solder joint, and left enough slack that the lightboards can be pulled straight out without having to pull the frame apart. I don’t know that there’s really any advantage to that, but I had the room to do it so why not? A little scotch tape (don’t tape the motor!) help hold the wires down.

I’d post a pic of the final product, but it doesn’t look any different than the stock loco from the outside!