Lionel KW diode installation

Lionel transformers use a selenium rectifier disc to produce a jolt of DC voltage to activate their train whistle. These discs degrade over time, so a decades-old transformer often produces a pretty anemic whistle–even one of the bigger transformers like the 190-watt Lionel KW. Replace the disc with a diode for a cost effective and reliable fix for that wimpy whistle. Here’s a step by step guide to a Lionel KW diode upgrade.

The diode controversy

A lot of people put 40 amp diodes in these transformers, which is serious overkill. The KW doesn’t even produce 40 amps. A 3 amp diode would theoretically be more than sufficient, as the original copper or selenium disc has about a 2-amp rating.

I use a 6A10 diode (examples include the NTE 5815, NTE GI756, or ECG 5815) which shouldn’t set you back much more than $1.50. Any 6A10 diode will do. You can also step up to a 10A10, which is 10 amps. Sometimes a 10A10 is cheaper than a 6A10. You probably could even use a 1n5400, but I like having some extra headroom. I don’t like the idea of paying $15 for a diode but I don’t mind paying a dollar.

To those who argue the 40-amp diodes look like factory replacements, they aren’t what Lionel would have used. Lionel would have used a 1n5400. With some care and a steady hand, a diode modification looks like a professional did it. Mine doesn’t, but my shaky hands are legendary.

The procedure

There are four screws on the top of the KW, but once you remove those, it’s not exactly clear how to open the case. The trick is to pry off the handles.

Examining the handles, you will notice there is a metal ring between the two of them. Carefully slide the tip of a large slotted screwdriver between the metal ring and the bottom handle. Repeat on the other side of the handle with a second large slotted screwdriver. Slowly twist one of the screwdrivers and work it under the ring, then repeat with the other side. Then you can gently pry upward with both screwdrivers and the top handle will pop off. Remove the top handle, the ring, and the bottom handle and set them aside. The smaller orange handle lifts straight up without tools. With all of the handles removed, the top Bakelite portion of the transformer case will now easily lift off.

Once you’re inside the KW, it’s easy to access the old-school disc rectifier. Pry off the speed nut with a standard screwdriver.

Remove the two whistle buttons and the springs and set them aside. You’ll see the top panel is held in place by four tabs, and likely only two are bent. Carefully straighten the bent tabs with a pair of locking pliers and lift up. Don’t take the transformer completely apart, just lift it up enough to give you some room to work with.

Solder the striped end to the steel plate and the other end to the brass arm. The steel plate is hard to solder to, so it may take some patience.

Pry off the speed nut holding the disc rectifier, then remove the rectifier, the insulating washers, and the stud.

Solder the striped side of the diode to the mounting plate, and the other side to the copper or brass arm that the stud went through.


To reassemble, just reverse the disassembly steps. If you don’t want to risk breaking the tabs you bent, you can use heat shrink tubing to hold the brackets together. Cut a small piece of tubing large enough to cover the tab, then slide it over the tab, and shrink it with a heat gun. Repeat for the other four tabs. Replace the springs and buttons, then slide the top cover back on, replace the four screws, slide the bottom throttle on, replace the ring, then press the top throttle on. Press the whistle handle on. Then test your whistle. It will start with much less hesitation and may even run at a higher volume than with the old disc rectifier.

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