I've got a BNSF Kato SD70MAC with a Digitrax decoder. I don't run trains that often, and occasionally, when I turned on track power after it had been off for a while(like a week or more), about half the time, this engine would short out the whole layout. Also, occasionally, it would be running fine, and it would hestiate, and then keep going just fine. It wouldn't always hesitate in the same place, but it most of the time it would do it in relatively the same place. So last night I get my layout running for the first time in a month or so, and first thing, this engine shorts out the whole layout, and then keeps dying in the exact same place at low speeds on the layout. It was the only engine doing this, and I could see the headlight flickering, so I was pretty sure it was in the decoder. I leave the engine on the track, and pull the shell off. I'm moving the decoder around a little bit, and I see the board short out. On the back half of the decoder, right before the frame drops down for the rear headlight, the decoder was rubbing the frame, but only occasionally, causing the short which would short out the layout, and also causing the bad decoder contacts. I could move the board and actually see the sparks from it. I didn't have any Kapton tape, so I slid a very thin piece of styrene between the decoder and the frame. Problem solved. So my question is, on future installations of decoders into Kato engines, should I place Kapton tape between all spots on the frame where there is less than .010 clearance between the decoder and the frame, and has anyone else had this problem?
My first question is the spot that is shorting on the pc board where a trace has somehow been exposed or is it a lead coming off a component? Either way I would say the Kapton tape is a better way to go rather then shimming and putting pressure on the board and if you feel safer adding Kapton tape in those extra areas then better safe then sorry. I have 35 of the K1B's installed in SD70MAC's, AC4400's, and C44-9W's(all have the same mechanism), not to mention all the 40's and 40-2's that have the same decoder, and I have not had one issue like you have come across. Now with Atlas engines I have due to sharp edges on the frame that cut into the coating on the PC board and then to a trace causing a short.
I didn't take the decoder out, but it must have either cut or rubbed into a trace or two. You can actually see a small spark when it shorts. The styrene puts very little pressure on the board. It's .008 styrene, which probably isn't much thicker than the Kapton tape. From now on, I'm just going to tape these areas prior to install. I was just wondering if anyone else had these problems, and if they were having the problem where to look to fix it.
the instructions for all of my digitrax decoders for the SD70mac show kapton tape on the rear of the frame by the light. I have about 18 of them now no shorts. SD
That's funny because any K1B manuals I've have they only show Kapton tape on the frame where the motor clips are. I know the K1C shows to use the tape in 3 places but this is because of the design of the mid production SD40-2's and how the board pretty much sits on the frame.
you don't need kapton tape. just take any tape thin enough. i usually use tamiya masking tape since this is what i usually find first.
