Tuesday, January 6, 2015

Kato SD80Mac slow running

At a recent train show one of my buddies from the Ntrak club I belong to asked me to take a look one of his Kato SD80Mac locomotives.  His complaint was that while it did run, it was slower than other identical locomotives.  I verified this on my test layout and it seemed to get worse the more I ran the locomotive. I then took the shell off to test the motor for current flow and this is what I found.

Apologies for this blurry photo.

The kapton tape was coming into contact with a rotating part of the motor as indicated by the arrow on both sides preventing the motor from turning freely.  This put an overload on the decoders motor output.

This photo shows how the installation should look. The kapton tape comes over the side of the frame on each side and stops before reaching the open part of the motor.

This type of installation is common to several Kato models.  They are generally an easy DCC installation but it is a good practice to always check for free rotation of the motor when doing an install.

When I put the decoder on my test fixture, the voltage on the motor output of the decoder was zero when connected to the test motor but was normal with no load. There was no sign of anything burned on the decoder.  A reset was programmed into the decoder but this did not clear the problem.  In this case the decoder needed to be replaced.

Before installing the replacement decoder the current draw of the motor was measured by connecting the motor directly to my bench DC power supply as shown in this photo.  It is rare but I did once see a defective motor cause a decoder to burn out and that became the first post on this blog.