Power transformer

[dan stanton]

I found an old tube stereo (Hi-Fi). How do I check the PT? I can tell the heater line, A/C in and out lines, but would like to know what typical ohms readings I should get across the leads. Looks like a 10 -15 watt system. Tubes have no numbers on them.

[Enzo]

There are not any real typical resistances. Two transformers that are otherwise identical, one might use heavier wire for the windings, and so the resistance will be much lower, but the output voltage will be the same.

One thing you don;t want to see is shorts betwen the windings, and between windings and the frame. And of course you don;t want to see an open winding.

But if the heater winding is not shorted to the B+ winding - for example - and none of the windings are shorted to the frame, then I'd just appply mains power to it, and see what happens.

For safety, I'd bring it up on a variac and watch the current, or at least connect a fuse in series, or everyone's favorite, use a light bulb limiter. If you don;t know about light bulb limiter, do a search for it, make one, and use it.

Find the primary leads, connect mains power to it. Now see what comes out the other windings. Do not connect the part to anything else for this test.

While there are certain problems you can find, most things that go wrong with a power transformer are not the sort of thing you can discover with an ohm meter. For example a winding arcing to the frame or other winding, or a winding with a couple turns shorted together.

[zero cool]

What Enzo said plus this. If you have a variac you can bring the transformer up say 25% and check with a meter to see that the windings are at least generating some voltage. With unknown transformers i will connect my meter and bring them up a bit on a variac and then quickly check the other windings. then proceed to bring the variac up the rest of the way. while watching the current draw and keeping a keen ear, nose and eye open for arc's, sparks, odd smells etc. I also check from the transformer to AC ground to make sure the core isn't somehow live.

I have heard many stories from the antique radio guys about old transformers going bad. apparently the enamel or coating on the wire used for the windings can shrink, crack or otherwise go bad and can cause the windings then to touch. now the transformer may still work, but may overheat or may have offset voltages. IE if you had say for example a transformer with a 500Vct secondary and you measured from the CT to each end winding. one may be 250 the other 240ish and that can cause problems.

other problems can happen as well. trafo runs fine no load. but once under load and it warms up, then it can arc, spark or go poof!

So use caution.

The vintage guitar amp guys tell me you can check windings for output transformers buy connecting a 1v ac signal to the 4 or 8 ohm winding and check the primary side to see if you have a reasonable signal there etc.

[dan stanton]

Thanks guys!