Testing prewar and post war transformers.

[c50truck]

I apologize if this topic has been covered before, but I did a search and could not find a topic previously discussing my question. I have several transformers from the late 30’s and late 40’s. They are Marx and A.C. Gilbert brands. How can I test these units BEFORE I plug them in to be sure they are OK? A Marx’s transformer has a brittle cord, should or can this be replaced? Do these units either work or not? What about three prong plugs? I do have a volt/ohm meter and understand basic electric currents. Any Help would be appreciated. Thanks

[mackb4]

Be sure you update the bad cords with UL approved plugs.Someone else may know how to test them before you plug them in for continuity,but I don't. I think most transformers test at 14-18 amps with juice going thru it.

[willpick]

Most of the transformers from the prewar era were built to last. I would replace every cord for safety's sake, unless you want to preserve the original look. To check the windings, use your ohmmeter on the X1(lowest) scale. The primary(120V) should read a few ohms(more than 3 or 4). the secondary(track voltage) Is a little harder to check, as there are different ways to vary the output voltage. As long as you do not have a short(less than 1/2 ohm), it should be good. Also use your meteron the highest scale and check from the primary to the secondary-- it should NOT read any resistance except infinity.
To test the transformer after the resistance check, I use a special extension cord that I built with an on/off switch and an inline fuse holder, with a 1 amp fuse. Plug the transformer into the fused cord, then flip the switch.
If the fuse doesn't blow, then i'd check the output voltage. If voltage is present, I'd then put a load(engine, accessory, etc.) on the output and adjust the voltage to a normal operating level. After a few minutes, check the transformer for heat. Should not be anything but slightly warm. If it is too hot, then it's a bad unit.
HTH

[fwright]

willpick

Very nice progression of tests that are worthwhile.

Replacing the cord - always do if there are nicks, cuts, crumbling, etc of the cord insulation. If you possibly can - for postwar and prewar transformers only, not modern - replace with non-polarized plug (both prongs are same size). The polarization means nothing to the older transformers, and will make it more difficult to phase multiple transformers.

yours in transforming
Fred Wright

[lionelsoni]

Replace a cracked or brittle cord. But don't assume that the cord is bad just because the transformer is old. I recently bought a prewar type-T with a perfect, supple cord.

Small transformers (like train transformers) are normally designed to be a little over the edge of saturation, which wastes energy even when the transformer is unloaded but makes it smaller and cheaper. Saturation increases with voltage; so, depending on age, many old transformers will be operating as much as 10 or 15 volts above the voltage they were designed for and getting hotter than they should, since voltages have risen over the years. Even if an old transformer is perfectly okay, it might get too hot. There are ways to get around this, but nothing I know that I would recommend to the public.

[Anonymous]

I would advise taking the better-safe-than-sorry approach, and replace all of the power cords. Sometimes the problems or potential problems are obvious (cracked or brittle cord, for example), and sometimes they are not. If you have no experience with such things, I would further recommend that you have a competent toy train repairman make the changes for you. For one thing, they'll know how to take the transformer apart without risk of damaging it. Some require special tools to disassemble.

[Anonymous]

I have a trick I use for the old tube audio equipment I sometimes work on. I have a test set up with an extension cord with a standard light bulb wired in series with the load, plugged into a variac. I plug the unit under test, then bring the voltage up slowly. If the series lamp glows brightly, there's a short - but the bulb is absorbing much of the energy. The light might glow just a little if there's no short. Beats the "smoke test" every time.

Oh, yeah. Replace the cord. Just do it.

Old 2037

[lionelsoni]

I wouldn't say, "If it ain't broke, don't fix it," since I have "fixed" my share of unbroken stuff just for fun. But I will say, If it ain't broke, you don't have to fix it. However, it seems that c50truck's cord is in fact broke by virtue of its being brittle; so he should fix that.