Digitrax and double power truck problem?


VA3TOM

VA3TOM

Member
I am not sure if anyone has done this or why this is a problem.

I have a single unit with both trucks powered (Q Car can motor on each truck). I am trying to put one decoder in, but I have smoked two of them. I have double checked the wiring, even tried another harness in case there was a problem with the first one. As soon as power is applied, the decoder buzzes and starts to let the smoke out. The trucks will power and move without the decoder (00 setting on the digitrax controller), so I know they work.

I have another unit that has two trucks powered with one decoder, but this one is giving me a problem. Maybe after a nights sleep!

Thanks!
 
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Hmmm...how much current draw do both motors produce? Have you actually measured the current draw at full stall? It sounds like the decode is probably a 1 amp or maybe 1.5 amps and the two motors are drawing more current than that on startup. I would start with this issue first and make sure that's not what's causing the smoke.
 
Yes, it is a 1.5 amp decoder. I tried to measure the draw in DC mode, the loco only draws .5 amp (not stalled). What puzzles me is the decoder will buzz and smoke as soon as the power is applied (as soon as contact with track power is made). The engine is not moving, no forward signal yet, no load, just sitting idle on the track the decoder will smoke. I have another loco with the same setup and it is fine. I am sure the can motors are isolated from the frame, and I have checked with an ohm meter to verify this.

This is a batch of 4 new dh123d decoders I got on ebay, I don't know if there is a problem with them or not. I may pull a good one from a loco I know is working and try that (at the risk of smoking another one).
 
I've had problems with a similar unit... turned out the dang thing is not electrically isolated internaly! Of course I let the smoke out of a couple decoders first before I looked into it... sheesh!
 
Tom, if you're getting decoder failure as soon as power is applied, it does sound more like a short than an overload problem. I'd check into Rico's idea that motors may not be really electrically isolated. It's always possible to get one or two bad decoders but I'd bet against it being a decoder problem. Also, any chance you have a short anywhere from the motor back to the decoder? I smoked three decoders on a P2K E-7 until I finally traced all the wiring and found one of the track pickup wires had a small bit of insulation rubbed off in just the right place. Well, I guess it was the wrong place in terms of $45 going up in smoke. :)
 
Rico said:
I've had problems with a similar unit... turned out the dang thing is not electrically isolated internaly! Of course I let the smoke out of a couple decoders first before I looked into it... sheesh!
Click to expand...

Yeah I second this one, especially if one of the MOSFETs is burnt worse than the other on the chip. I had a Rivarossi E6 chassis that wasn't isolated that basically did the same thing.

With the decoder disconnected, grab a multimeter and check continuity between all four motor and power wires. Other than across the motor wires, everything should be completely isolated. Also move the trucks around while doing it to check for intermittent things like metal truck sides touching the frame.

Mark
 
I will double check the motor isolation tonight. These a new Q car power trucks, and the motor has a wire to each brush, so I assume the armature is isolated. I tried to check for continuity while assembled, and the motor seemed isolated, but I will disassemble the unit and check each truck individually. I may send a note to Quentin to see what he thinks about these trucks.

I thought about a short as well, so I redid the wiring with a new harness in case the original one had a problem. That's when I lost the second decoder.

Anyone know how to get the smoke back in to make these things work again?
:rolleyes:
 
I am thinking I was taken.

I got 4 decoders in an auction on ebay. One decoder was chipped at the plug end, so the seller sent an extra one. I thought that was more than fair and after testing them with the LT1, I posted positive feedback.

After wiring and burning 3 up, I thought it was something I was doing wrong, but the the fourth decoder worked OK. On the second engine the decoder smoked right away again, so without changing any wiring I replaced the decoder with a new one from the local HS and it worked fine.

I don't know if there was a bad batch, but this auction was no deal. Cost me over $60 for one working decoder.

Support your local Hobby Shop.
 
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