Chasing a short

It wouldn’t be one of my builds without some sort of disaster at the end. On #10, I dropped the boiler on the way to the paint booth, breaking the smokebox off completely. For 622 the disaster is a short that has developed in the frame since the last time I tested the locomotive.

At first, I thought it was just a missed cross-wire, and indeed the cylinder cocks presented one. However, even breaking that in half and completely removing one side from the model left me scratching my head.

Maybe it was the yoke? I unbolted the cylinders and half-removed the whole assembly; it doesn’t come off entirely anymore because of the pipes to the air cylinder. Nope, the meter still showed a short.

Perhaps I had trapped some debris against the firebox with the epoxy for the valve gear. I filed away most of the vestigial casting, and ran a knife through the gap.

Maybe the paint is failing on the sides of the firebox weight, and it is shorting through there. I scraped a little paint from the bottom of the weight, and found it was still isolated from each of the two frame halves.

That left the frame itself. The last time I chased a short around the frame, it was due to some debris in the hole for the tender coupling. I cleaned that out, but the meter steadfastly refused to yield a clean result.

Thinking it could be the gaps at the tail of the frame, which I never liked, I went back to making those wider, ultimately filing through enough that I could break the epoxy and lift a little sliver of each layer away.

Still 4 Ohms.

The only thing left was major butchery. I filed the rest of the way through the rear cross-member, and tested it. Still a short, even with slips of plastic ensuring things that I thought were isolated indeed were.

With the rear cross-member broken, I tested the top and bottom of the remaining layers, and found that they are shorting too. They’re not as bad as the whole frame – 8 Ohms, but still shorting.

So here is what I think is going on. Part of the isolation between the frames is provided by a layer of tissue paper, as despite the design allowing for .005″ of plastic, I had much less space than that. Perhaps the tissue paper has broken down as I have washed the frames over the summer.

Like Robert Frost, I see two roads diverging. On the one hand, I could redesign the frame – literally back to the drawing board. On the other, I can saw through the remaining cross-member, which provides a lot of the structural integrity of the model. Unlike the poet, I am not sorry I can not travel both, but I fear that is what will happen in the end.

14 thoughts on “Chasing a short

      1. If it’s a problem now wouldn’t the tissue paper be a problem later as well?

        Maybe something else as an insulator? I’ve heard good thing about nail polish. That might be able to get a thin coat applied?


      1. Rene,
        I was just thinking about using the etched pieces as a mold and simply pour the resin in-between the two etched pieces. I don’t even know if you can get resin that thin and controlled.

      2. That was more or less the thought behind the epoxy I used. The tissue paper was there to assure isolation while the epoxy set. Interesting idea, but difficult to control!

  1. Hi Craig, Thanks for the suggestion. The actual insulation is meant to come from a liberal layer of epoxy, and the tissue paper is supposed to be just belt and braces. All future insulation will come from a layer of plastic that I can see!

    1. Did you test the epoxy by itself to see if it is conductive? I wonder if epoxy changes conductivity over time?


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