I just installed a DH126 decoder into an older blue box Athearn locomotive. I followed the instructions as noted in a video from the Digitrax website. The locomotive ran well. Then I installed an LED light with the resister to the blue and white wires and everything was working perfect, including the lights and the actual locomotive. However a few hours later, I could not turn off the light, I would press function zero on my Zephyr Xtra and all that would happen is that the light would get brighter and dimmer, it would not turn off. So I reset the decoder and my Zephyr and it made no difference. So by chance I had a second DH126 decoder and I plugged it into the harness and the light was working perfect and also the locomotive would run perfect. However about 10 minutes later, smoke started coming out of the decoder and started to melt the blue plastic wrap and now the light does not work on the second decoder, but the actual locomotive still runs.
To summarize the first decoder will not turn off the light, function zero will make it brighter or dimmer but the the loco runs well and the second decoder started smoking and the light won't work at all, but the loco runs well. I'm completely confused.
please help?thank you,Pablo
Was the motor current draw checked before installing a decoder?It seems to me that you have a decoder toaster engine on hand.
It's not the motor drive that's fried, it's the function output, otherwise the loco would not run. No amp issues here, it would have to be a defective motor to draw more than the 1.5 amp rating of the decoder.
There must be a short somewhere between the functioon wire and the frame of the locoo. Neither side can touch the post in the front where the headlight is typically mounted on a Blue Box loco. One piece of that is riveted tot he frame and connects to the rails via the truck bolster, the other springy tab if left in place connects from the top motor clip to the other terminal of the old light bulb, via the truck tabs. I assume this has been removed to hook up the decoder. The LED and resistor must be connected to the white and blue wires from the decoder and nothing else, not touching the frame or the truck clip or anything else.
What LED and what value resistor did you use? Too low a resistor will blow the LED and that usually take the function output with it. The functions are usually goof for 30ma each, but if you used a small enough resistor to exceed that, the LED wouldn't last long as most can't take much more than 20-25ma.
Don't toss the decoders, you can wire the LED to one of the other function wires and program it to work on F0. But check the wiring first, don't just swap in another decoder, the same thing will happen unless you fix the wiring fault.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Thank you for the responses so far, Randy I do agree with you in theory.
some additional info...I never did put the shell on the locomotive as I wanted to test the engine before doing so. I soldered the resistor on the positive LED anode and then I connected the resistor to the blue wire common from the decoder, and the LED cathode negative from the LED connects to the white wire from the decoder.
I have a couple questions , Firstly when I connected the LED to the blue and white wires I just twisted them for now, if one of those exposed wires accidentally touched each other or accidentally touched the frame, would that cause the problem?
Secondly, if the gray wire at the bottom of the motor tab accidentally poked through the electrical tape when I isolated the frame, would that cause this problem?
The LED are 3MM golden whites, with 1K OKM resistor
What baffles me, is that at first everything was working perfect, then all of a sudden these bizarre symptoms started happening on both the decoders
First case, yes, if either of the wires touch the frame, that would blow the function.
Second case, if that happened, it would blow the motor drive and the loco would no longer move. Or else take off at full speed as soon as the power was turned on.
For original style BB locos, swap the clips top to bottom, the top clip doesn't have the extra part that is bent out to make contact with the frame. ANd get some Kapton tape if you don't have any, and put a layer under the motor, covering the part of the fram that used to be contacted by the motor clip. Kapton tape is much stronger and a lot less messy than electrical tape. And a lot thinner.
I believe you're right, because I was just testing the locomotive,I temporarily twisted the blue and white wires to the LED and there was bare wire exposed and I believe these wires accidentally came into contact with each other and caused a short, blowing the light function on the decoder. I spoke to a guy from Tony's Train Exchange and he has the same theory. Expensive lesson to learn...taking short cuts can result in short circuits