Tomix EF81 “Hokutosei”
Here’s today’s subject: A Tomix EF81. This one, no. 98, is a special edition, not easily replaceable—and not DCC-ready, either. In this article, I will cover how to disassemble the locomotive—which is very easy—, and mill the frame to fit a TCS CN-GP decoder—which isn’t terribly hard.
The shell removed
The shell comes off in the usual way: Gently pry the sides of the shell away from the frame near each of the trucks, and the shell will just pop right off. Set it aside.
The lightboards in situ
The grey plastic bits at each end clip onto the frame on the sides. Removing them is a matter of prying each of the clips away from the frame. These clips hold the frame together, and the lightboards against the frame. Removing them reveals the lightboards, which come out next.
The trucks removed
After the lightboards have come out, remove the two powered trucks by squeezing them gently to one side, and rocking them off the frame. The center, non-powered truck is held in place by a plastic clip, which releases the truck when squeezed. Be careful not to lose any of the springs: two in each powered truck and one large one for the center truck.
The locomotive in parts
What remains are some small plastic details pieces. The couplers and coupler mounts slide off the ends of the frame. There are two small black plastic clips that fit between the trucks that come off with some small gentle prying. At this point, split the frame, and remove the motor and worm gears, paying attention not to break the drive shafts that connect these three parts. The shafts may well fall out on their own, which is fine. Except for the frame-halves, set all of the parts aside where you won’t lose them. (Everything is clearly marked, so you don’t have to worry about what goes where and in what direction when reassembling. Thank you Tomix!)
Inspecting the frame-halves reveals that there is very little space to install a decoder—indeed, less space than the much smaller DE10 affords! For this reason, the decoder will certainly have to be located in one of the lightboard pockets. However, the available space is quite small. Lightpipes attached to the inside of the shell fit into the small holes in the gray plastic clips, and so sit just in front of the LED on the lightboards; whatever modifications we make, we must be sure not to interfere with those lightpipes, else we will have to modify more than the frame.
The CN-GP decoder test fitted
To ease this process, I have selected the TCS CN-GP decoder, as it is designed as a lightboard replacement decoder—you (typically) install it in place of the existing lightboards. It comes in two parts, one for each lightboard. Here, I have test-fit it into the EF81 frame—This conversion shouldn’t be too hard!
A comparison of the CN-GP and the existing lightboard
But where the smaller of the two part is a perfect fit (some Kapton tape will be needed to avoid shorts), the larger is a few millimeters too long, as this photograph makes clear. The frame will have to be milled to lengthen the lightboard pocket.
Drilling a hole for motor access
I like to do the easier things first. The CN-GP is not a drop-in decoder: Naturally, the motor must be isolated from the frame. The motor has two metal tabs which press against protrusions in the frame. I will use some tape to isolate the motor tabs from the frame. To get the motor leads from the decoder to the tabs, I’ve drilled a small hole from the top of the frame to the location of the protrusions. I’ll run the lead through this hole, and solder a small piece of brass as the contact point, so that I don’t have to solder anything directly to the motor.
The channel for wires
The shell is a tight fit to the frame, so channels must be cut for the all the wires that must be run between the two decoder parts, or from the decoder to the motor. Be careful, as the frame is rather thin in places.
The unmilled frame, showing the position of the stock lightboard (in purple)
Here is the frame, unmodified. The purple line indicates where the stock lightboard sits; there is a rest on the right (front end of the frame), with a nub to hold the board in place. On the left (middle of the frame), there is another rest, with a nub to make contact with the contact plates on the lightboard.
The unmilled frame, showing the eventual position of the CN-GP (in purple)
This is the same shot, but now the purple line represents, roughly, the shape of the CN-GP. Notice that there are bit protruding down from the board, and that the board must sit quite a bit further back in the frame.
Material to be removed to accomodate the CN-GP (in red)
And this figure is the same as the previous, but with the addition of red marks showing what must be milled out of the frame for the decoder to fit. Notice that the CN-GP is designed presuming that the frame will make contact with it on the top and the bottom, like a pair of pliers, but that the Tomix model uses a plastic clip to press down on the lightboard. When milling, be sure to leave a small bump or nub underneath the CN-GP’s contacts, and that you don’t mill out so much that the plastic clip won’t be sufficient to hold the decoder down securely on that bump.
The milling complete
In the final photo, I’ve completed the planned milling. Crucially, the milled-out area must be square in all three dimensions—the decoder must be held in place by a point on the frame that touches the contacts. If the cavity isn’t square, then the decoder will be held by its edges, there will be no electrical continuity between the frame and the decoder.
That’s it for now. I need to test that my milling is sufficiently square for the decoder to make proper contact with the frame, but what remains is to finish cutting channels for the wires, and then installing markerlights, both of which I will cover in the next part.