Last Updated on January 18, 2018 by Dave Farquhar
Sometimes you want to lock a Marx motor in one direction. Some of them have a switch for doing so but many do not. Here’s how to find the switch, and what to do if yours doesn’t have one and you don’t want to modify it.
Drop out the motor
To do any of this, first you’ll have to drop out the motor. Usually it’s two screws, one on either side of the locomotive. Remove those screws, then slide the motor forward toward the front of the train, then down.
Finding the switch
If your Marx reverse unit has a switch, it’s a brass pivot at the top of the reverse unit. It has two positions. Turn it from one position to the other to lock or unlock the reverse unit.
Locking the reverse unit without a switch
If you don’t have a switch, you can still lock the motor in one direction without one. Marx motors without a switch have a small hole in front of the driver wheels. Inside, you’ll see a small piece of brass bar. If you reach in with a toothpick, a small screwdriver or a similar tool, you’ll find you can move that brass bar toward the front of the train or the back of the train.
To lock it in one position, stick a toothpick or wire tie or something else small, thin, and non-conductive down into the hole to hold the brass bar in one position or the other. Holding it toward the back of the train locks it into reverse. Holding it toward the front locks it into forward. You can cut it flush with the hole and wedge it into the hole to make it stay in place, but I just cut it a bit longer than that and secure it with a piece of tape. That holds it while I put the motor back into the locomotive. Once everything it back together, gravity takes care of the rest. And with this method, when you want to restore normal operation, it’s easy to pull off the tape and pull the piece back out.
Why you can lock the reverse unit with a toothpick
This works because when you block the brass rod’s movement, when the reverse unit goes to cycle, it drops down momentarily and then pops back up into the same position it had been.
When you look at the reverse unit outside of the motor, you can see how it works. The brass rod moves a rocker switch in the lower part of the mechanism. The concave path the rod takes causes it to alternate between sides when the solenoid activates. But when you restrict its movement, it can’t travel the full path in its opening, so it pops back up where it was.
The Marx reverse unit is a simple but clever design. Its simplicity makes it rather reliable. It also makes it easy to defeat when you want it to not change the train’s direction.
Adding a switch
You can add a switch to lock out the reverse unit if you’d like. It’s not a particularly hard operation, but some people prefer to leave their trains original. My cutoff is $50. If the locomotive is worth less than $50, I don’t feel bad about modifying it. To the right person, a $10 locomotive with a lockout switch might even gain a little value.
If it’s worth more than $50, I leave it original, since most collectors want their trains original. I’d rather not turn a $50 locomotive into a $25 one.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.