Sunday, November 14, 2010

First Post - That one in a thousand

A friend in Chicago who does his own installations called to tell me about a problem he was having with an engine that he had installed a decoder in several years ago and now was not working.  He had tried everything he knew how to do but was stuck.

So I had him send me his Kato U30C with a Digitrax DZ123 decoder.  When I got it I took the shell off and did a quick inspection of the installation.
  • He had used an Aztec Trackmaster milled frame - OK
  • The all of the wires appeared to have their insulation intact and not pinched anywhere - OK
  • The motor turned freely when spun by the flywheels - OK
  • The decoder was attached to the frame with a piece of double stick tape - OK
I then put it on the test track and sure enough, it would not respond to either DCC or DC.  I reset this decoder by programming 08 into CV 08. Then I got it to move on the default address of 03 but the LEDs were acting strange.  The rear LED was acting like a strobe when the engine was in reverse and the front LED was on all of the time.  I also noticed that this engine seemed to need a higher throttle setting than normal and I knew that the momentum settings were zeroed because I had reset the decoder.

Then I saw the smoke !

I immediately removed the loco mechanism from the track and removed the decoder from the double stick tape that secured it to the frame.  There was a burned spot on the bottom of the decoder.

As I had seen decoders burn out before, I thought that what it was a went about replacing the decoder.

In the course of doing this I had the mechanism completely apart and then back together again so I felt sure that everything was right.  This time before I put the wheels back on, I held the mechanism in my hand and placed each side of the frame on a rail.  I turned the throttle up and everything seemed OK, the motor ran and the LEDs responded as they should.  It was then that I noticed that the decoder was warmer than it should be.
I then decided to take a current reading on this motor.   I did this by placing my amp meter in series between the motor and a DC power pack.  I was surprised to see that this engine was drawing at least 400ma and up to over 1 amp when the throttle was advanced to near full.  Normal for an N scale Kato engine is between 200 and 300 milliamps. 

This ended up being a defective motor which over time, ruined the decoder.  As it turns freely, there must have been something wrong with the rotor windings.  This is not something that can be repaired and the motor must be replaced.  It must be really rare, that this happens as I have worked on many engines over the years and this is the first time I have seen this.  Replacement motors for Kato engines are generally available if you look around.  The motors are the same for almost all of the Kato engines but you may have to change the flywheels.

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