After slightly more than a dozen of these conversions of the older Atlas/Kato and Kato models, I have devised a relatively fast and simple method.....compared to my first few.
So if anyone else has been collecting these little slightly used beauties from places like Dan's Trains (I once got 4 on close-timed auctions for an average of $28 each), this is how I now do them:
At first I was replacing the single incandescent lamp in the middle with a single LED. But after some experimentation, I found I liked:
1. melting* back the headlight bars to the mounting brackets in the shell. A hot iron does this best (although some forum members argued unsucessfully with me about this) as it is very quick, leaves a mirrored "cut" and does not create plastic dust everywhere. This gives space for the decoder and two LEDS on the original plastic platform atop the motor. I have to use nippers to trim off the ridge of melted plastic created on the top remaining end of the bar.
2. I like NCE's D13SRs best as they have the best performance for the price. I have also used tiny Digitrax 123s and 125s
3. Pull out the incandescent bulb and I bend the posts of the LEDs so they will sit on top of the black tabs of the grey plastic board and then with a chisel point soldering tip, just heat them until they "set" down into plastic. Then I trim the LED posts and put on the resistors etc.
4. I use the same chisel point to smear over enough meted plastic of the grey board to lock in the flat copper motor contacts where they rise up alongside the board. This is handy because they are now held in place when I cut the copper tabs where they contacts the track power strips with a dremel.
5. Finally, trim the decoder leads to length solder them up and tape down the decoder with kapton tape.
* I do all melting in the outdoors and with a respirator because the grey plastic on these models is especially vile when melting and it's probably full of nasty toxins. The headlight bars are not nearly so toxic smelling.
Now, the headlight bars come down just before and after the ends of the D13sr decoder and pick up the light perfectly....and I have switchable forward and after beams unlike the original single incandescent bulb.
This particular model is a GP7 and has the nice solid catwalk for a weight. The older models have that fussy detachable catwalk and free-mounted weights that sit on the light bars. A tip with those is to start the day ahead by gluing in the weights with some latex caulk so that they don't keep falling out during manipulation of the shell. If necessary, they can be pulled free in the future.
Now, I don't think I know all the answers, but it is my 2 cents after a dozen installs in these particular models, and I have it down to a pretty smooth operaton now.
Nice job. I'm surprised you have enough headroom to set that decoder on top of the board in there.
On these locos I have sometimes epoxied the LEDs onto the ends of the light bars. One good thing this way is that the LEDs are so far into the middle of the shell there is no escaping light through cab windows or grills on these models.
Otis enough headroom
enough headroom
Thanks.
There is juuuuuust enough. The space from the catwalk to a reversed dimple/bump inside the top of the GP7 (and others') shell is 2.6 cm. The D23sr on top takes up 2.4 cm. If I think there will be a problem on some of these conversions, I will melt down the black tabs holding the grey board in place and set the LEDs down another couple of mm that way.