The common post on a Marx transformer

Last Updated on February 10, 2019 by Dave Farquhar

When you want to phase transformers, it’s good to know the common (in Lionel terms) or base (in American Flyer terms) post. It’s a shame that Marx didn’t label which of its posts was common. So here’s how to find the common post on a Marx transformer.

It’s a good thing this is fairly easy to figure out, because Marx transformers are dirt cheap. I bought one for exactly one dollar at the last train show I attended, and the vendor wanted to sell me a box full of them for $5.

Two-terminal transformers

On smaller Marx transformers with only two terminals, the hot post is almost always the one closest to the handle, which will usually be the one on the right. That means the common/base post is the one on the left. The reason is very simple; this arrangement used the least amount of wire inside the transformer, which kept costs lower.

I have disassembled 50-watt Marx 1209 and 709 transformers, as well as a 25-watt 329, and can verify all of those transformers use this arrangement.

Four-terminal transformers

For transformers that have separate sets of posts for accessories, the arrangement is a little bit different. These transformers have a pair of posts on the left for the trains and a pair on the right for accessories. I checked both a 50-watt 1249 and a 75-watt 1559, and the common or base posts on those are the top posts. In each pair, the lower post is the hot.

Three-terminal transformers

Marx also issued a few transformers that only had three posts. In this case, the common is the center post. I’m surprised Marx didn’t use this arrangement more frequently. Perhaps some people found it confusing.

Other transformers

If you have a different Marx transformer from the ones I wrote about and want to verify, turn the throttle up about halfway, then connect a light bulb (a streetlight accessory is handy for this) to the accessory posts to verify both the bulb and transformer work. Then proceed to use every possible post combination until you find the combination that doesn’t light the light. Once you find that combination, you have your common posts.

Best practices

Label the common post on all of your transformers using a piece of masking tape. Then when you need to use more than one transformer to power a larger layout, it will be easy to wire them.

I also phase my transformers and then label the top of the plug with a piece of tape so that when I need to add a transformer to my layout, or build a temporary multiple-transformer layout, my transformers are already in phase.

Once you identify the common post or base post, you can follow American Flyer’s method of transformer wiring. And here are some tips on wiring Marx accessories, to save a bunch of wire and effort.

Parting words

If you’re wondering about other transformers, including those made by that other company that had a factory in New Jersey, here’s a collection of transformer pinouts. If you’re wondering if a particular transformer you have is adequate for accessory use, here’s some advice on accessory wattage.