Repair a Marx 709 transformer

Last Updated on October 24, 2022 by Dave Farquhar

Internally, a Marx 709 transformer differs little from a Marx 1209, so those instructions work for actual repair. But disassembly is a bit different, so I’ll walk through the disassembly process now.

Safety first! Before you take a transformer apart, do a safety check. That way you know what you’re getting into.

Initial disassembly of the Marx 709 transformer

There are no screws, as Marx didn’t intend for anyone to be taking these units apart and servicing them. But you can do it. It’s just a series of tabs and slots that hold them together.

There are two tabs on the top of the transformer. Use the thinnest flat-bladed screwdriver you can find to carefully bend these tabs up, as shown to the right. They’ll need to be straight up for the case to lift off. It’s easier to finish the job with pliers once you get the tabs bent up to the point that pliers can fit underneath. Bending these tabs is by far the hardest part of the disassembly process. Once the tabs are straight, the cover lifts straight up.

Gaining access to the core

With the cover removed, you can see the core and the wiper. Unlike later Marx transformers, the handle and wiper are an integrated piece. Check the connection between the wiper and the hot post on the side of the transformer closest to the handle. Also check the insulator to the left side of the transformer. Replace or resolder the wire if necessary, and repair or replace the insulator as necessary.

Common failures include a damaged insulator and loose or broken wires. If your Marx 709 never shuts down and always outputs voltage, it’s the insulator. If it never gives any voltage but it hums and shows other signs of life, it’s a wire.

Replacing a Marx 709 transformer power cord

Flip the transformer around to check the power cord. If the cord is frayed, you will need to replace it. Also check the wire connection to the base post, clearly visible on the far left side of the picture to the right. Reinsulate or repair the wire as needed. You can use shrink tubing or liquid electrical tape. Shrink tubing makes for a neater repair but requires more disassembly.

To remove the core, there are two tabs on the underside of the transformer (inside the two holes in the center) you will have to straighten. After straightening those tabs and removing the posts from the side, the transformer core will lift out so you can replace the power cord. Replacement is exactly like the Marx 1209.

Reassembling the Marx 709 transformer

To reassemble, replace the posts, replace the transformer core, twist the tabs on the underside, replace the wiper/handle assembly, then slide the case straight down.

The metal is pretty heavy gauge, so the tabs don’t bend back down very easily. To re-bend the tabs, I find the easiest way is to gently tap them down with a small hammer.

And that’s it. Enjoy your newly refurbished transformer.