Last Updated on March 12, 2023 by Dave Farquhar
Want to repair a Marx 1209 transformer? There are two schools of thought. One is that small, sub-75 watt transformers from starter sets aren’t worth fixing because they are so cheap. The other is that since they are so cheap, you have nothing to lose by trying.
Marx didn’t design its transformers to be fixed, but the design is extremely simple. The hardest part really is getting the case apart and then getting it back together. If Marx had designed them to be serviced, like its competitors did, they would have cost more, so we wouldn’t have as many Marx trains to enjoy today. So it’s easy enough to forgive Marx for this.
Let’s dive in.
Safety first
The first first step is to unplug the transformer. Never work on a transformer while it’s plugged in. The next step, before you take a transformer apart, is a safety check. That way you know what you’re getting into.
Disassembling the Marx 1209 transformer
If you’re missing nuts on the binding posts, the top nut is a simple knurled 8-32 nut. The nuts that go up against the case are simple common 8-32 hex nuts.
The first step is to straighten the four tabs on the underside, shown in the photo to the right. Larger Marx transformers go together the same way, but may have six tabs instead of four.
To open the case, slowly pry up with a screwdriver. You’ll have to work your way around the case.
If the power cord is in bad shape and you need to replace it, snip off the power cord now. Not sure what bad looks like? I provided a photo. If the cord has crumbling insulation and/or exposed wire like the photo above, replace it. Sometimes the cord is fine outside the transformer but not inside, or vice-versa. So examine the cord thoroughly, inside and out. A short inside the transformer is still dangerous. You could install a 3-prong cord for extra protection if you want.
Completing the disassembly
That’s enough safety preaching for now. Let’s get back to the repair.
The handle engages the wiper inside the transformer, so don’t try to take the handle off before opening. You can see how it works in the photo to the right. The design is a bit clumsy. But since every transformer design had elements like its transformer wiper assembly patented, every company had to find its own way to do some of these things.
Flip the lower portion of the case over and straighten the four brass tabs. These tabs hold the core in place, though there’s a bit more work required, that we’ll get to in a minute, that is necessary to free the core so it will lift out.
Fixing two common problems
But before removing the core, let’s get two common, simple to fix problems out of the way now. First, make sure the fiber insulator is still present and in good shape. If it’s torn up or has fallen out, replace it or shore it up with some electrical tape. Lack of an insulator will cause the transformer to never fully power down when you slide the handle into the “off” position.
Next, make sure the wiper is still soldered to the wire. This is a very common failure. If the transformer hums but doesn’t output any power when you move the handle, it’s one of two things. Either this wire is broken or the handle isn’t properly engaged with the wiper. Re-solder this connection if the wire is broken, and if the wire is damaged badly, replace the insulation or replace the entire wire. Insulate the connection with some heat shrink tubing if you re-solder it. This gives a bit more safety, and it reinforces the solder joint.
Give all of the wiring one final inspection. If you don’t need to replace the power cord or paint the case, you’re done at this point. You can skip the next section and proceed to reassembly.
Replacing the cord and/or repainting
If you need a new cord, a cheap way to get a cord is to cut the end opposite the one that plugs into the wall off an inexpensive extension cord.
Replacing the cord or painting the case requires you to remove the transformer core. To free the core, remove the nuts holding the posts to the side, then pop the post assembly out. Now you can simply lift out the core.
If you’re going to paint the case, paint it now for best results and let it dry for at least an hour before continuing. I like Rustoleum Painter’s Touch paint.
Replacing the power cord
Replacing the power cord requires some soldering once you remove the core.
You have some work to get at the power cord’s solder joints. Untie what’s left of the cord, then unwrap the white tape and slide down the rubber band away from the solder joints. Now what you’re looking at should resemble the photo to the right.
Set the rubber band aside, then desolder the old cord and pull it out. Clean up the solder points as necessary, as some of the old solder may have dripped down onto the frame. If it looks like the photo above, it’s clean enough.
Thread the new cord through the case. Slide the strain relief onto the new cord.
Slide the rubber band back onto the cord behind the strain relief, or, better yet, use about an inch of 3/4″ heat shrink tube. Push everything back far enough to get it out of the way. Then thread the wires into the existing holes and solder them into place.
Once the solder joints cool, slide the rubber band or tubing into place. Then wrap a couple of turns of electrical tape around the rubber band, or shrink the heat-shrink tube. Fit the core back into the case. Adjust the position of the knot if necessary to make everything fit. Then remove the core, and put the posts back into place. Attach the wire from the core to the post furthest from the wiper. Put the wiper back into its holes. Double check your alignment. We’re almost there.
Reassembling the Marx 1209 transformer
If everything fits, flip the case over and twist the tabs to hold the core tightly in place. This reduces vibration, making the transformer quieter.
Line the top up. There are four tabs to line up, plus the sides, plus the notch that attaches the wiper to the handle. Work slowly and carefully and double-check everything. Test the transformer before you bend the tabs back using a light bulb or a motor. You don’t have to put the case back together very tightly at this stage.
Once you’re sure everything is working, close the case up firmly. If you can’t get the tabs into place by hand, a tap on each corner with a hammer can force them into place
Twist the tabs instead of trying to bend them back into place. Put a small piece of 1/4″ heat shrink tube on the tab and shrink it if you want to secure the case better.
These transformers are fine to use with accessories as-is, but if you’re going to use them for trains, put a circuit breaker between the transformer and the track. They are UL approved so they are safe but the circuit breaker protects the transformer from overheating in the event of a derailment. And of course, they’re fine for completing a boxed set, if you like collecting Marx sets. The 1209 came in a lot of early/mid-50s electric sets, such as set# 4222.
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.