Kato Mikado Decoder Install -- Weird Intermittent Short

[RBrodzinsky]

This weekend I decided to take the time to install an ESU LokSound Select Micro into my last DC-only Mikado. I have successfully installed a handful of Lenz decoders into others, so I wasn't really worried about the wiring and process, really just making sure everything fit neatly. The other activity was to change all 4 drivers (they were early style). This was my first time changing the drivers that the drive-rods connect to, and that was slow and tedious, but got them all working well and tested while everything was still DC.

So, I got the loco-side all wired up, following the same instructions I've always used, and none of the wires leading to the tender were showing any shorts (in particular either motor lead to either side of the frame). Got the tender all wired up, decoder connected; and checked for shorts. All ok.

With the tender body off, go to test track to see how it behaves. Turn on track power, hit F8, and the sound starts just fine. Light -- check. Notch the throttle up - starts to move, then jerks. I hear the "shorting" sound, and pull the plug. Check all the frame to motor wire for a short -- nothing. Put back on track, runs as if everything is fine (but, a little jerky at low speed). When I bring it to a stop, the short sound occurs! Pull the plug (no dummy am I -- kill power fast -- save expensive decoder)

Now I have a hard short on the Fireman's side motor lead to frame! And this is where it starts to get interesting. I take the cab and boiler off, and can confirm the short. But, if I rotate the fly wheel about a quarter turn, the short flips sides! And, the short is only in about a 5-10% arc on either side. The rest of the travel, the motor to frame are completely isolated.

SOOOO -- I am guessing the motor is bad, maybe a section of coil wire is making contact with the frame? The one thing I have never done is disassemble the whole loco frame on any of these. Before I do, though, does anyone have any other ideas or suggestions I can try? And, of course, if I do need a new motor, Kato doesn't have any in stock (I do see that they have traction-tire drivers in stock, for $12 each -- a 50% increase from a few years ago)

[peteski]

Wow!  I have never ran into this situation, but your description does seem to indicate that one of the armature windings is shorting to the armature and to the metal body of the motor.  It is a 5-pole motor so so the small angle of the shorting is probably related to the commutator segment which is attached to the shorted winding being in contact with the brush.

I haven't taken a Mikado apart for quite a while so I can't offer any directions, but as I recall it was fairly easy task.

[RBrodzinsky]

Yep - it is the motor.  I carefully "spread" the frame (without full disassembly) so that I could see into the motor area.  There is one wiring area where there are two broken wires, and they are probably making contact in their rotation with the frame.  When running on DC, the short is so fast, doesn't cause an issue. Similarly, on DCC, when it got moving, the short is momentary, but when I slowed the motor down to a crawl, the short was enough to stop it right there.

Using very fine tweezers, I pushed the wire back into the winding it came from, and the short occurs less often (checked with multi-meter). But still reproducible at the same points in the rotation, just not each rotation, now. 

Now, do I want to try the full disassembly and replace the motor....

[mmagliaro]

PM me if you need an original  Mikado motor.

[RBrodzinsky]

PM me if you need an original  Mikado motor.

Thanks. PM being sent