Repair a Marx 1209 transformer

Want to repair a Marx 1209 transformer? There are two schools of thought. One is that small, sub-75 watt transformers 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.

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.

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.

Marx 1209 disassembly
There are four tabs on the underside that need to be bent straight down to open the case. Larger transformers are similar but may have six tabs.

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.

Bad cord
Examine all wires. If wire has crumbling insulation and exposed bare metal like this one, it’s time to replace it. This cord is no longer safe.

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 to the right. If the cord has crumbling insulation and/or exposed wire like the photo to the right, you’re going to need to 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.

That’s enough safety preaching for now. Let’s get back to the repair.

Marx 1209 opened
Here the case is off. You can see how the tab in the handle engages a slot in the wiper.

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.

Marx 1209 core tabs
Straighten these four tabs to allow the core to lift out

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.

Marx 1209 insulator
Make sure this fiber insulator is in place and in good condition.

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.

Marx 1209 wiper
Pay attention to the wire connector on the wiper. This is the most common cause of a dead transformer. I already repaired this one. It’s best to insulate this connection if you resolder it. If you see any breaks in the insulation, replace it or slip some heat shrink tubing over it to re-insulate it.

Next, make sure the wiper is still soldered to the wire. If the transformer hums but doesn’t output any power when you move the handle, it’s either because 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, you’re done at this point and can proceed to reassembly.

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.

Marx 1209 core removed
Here is the core after I removed it. The wiper simply lifts out. Ensure the connection to the base post is also secure and the wire is in good condition.

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.

Marx 1209 cord connection
The power cord attaches here via two solder joints. De-solder these joints to replace the cord. Or, if the wires inside are in OK shape, you can splice onto the existing wires.

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.

Marx 1209 cord connection holes
Here I removed the old cord and cleaned up the holes. Thread the new wires into the holes, tin the wires, add a bit of fresh solder, re-insulate, and this transformer should be good for a few more decades.

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 to the right, it’s clean enough.

Marx 1209 new cord installation
When inserting the new cord, be sure to put the grommet in, then tie a knot in the cord. The knot is important to protect the solder joint from strain.

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 that it’s 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.

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.

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