One approach could be to find the primary following the first link in @Markw4's post. The power that with, say, 12 VAC from another transformer. Then measure the secondary voltages. Unless the transformer is intended to provide kV, which seems unlikely given the insulation on the wires, this will result in safe voltages at the secondary.
Tom
Tom
Determine which are higher voltage windings first. Ohm everything out to separate the windings. Relative resistances will tell you which is likely physically longer.
There are many overall possibilities. When you list it all out we can get clues as to whether it’s step up, step down, combination thereof, or what’s likely to be a primary. THEN apply your 12V test.
There are many overall possibilities. When you list it all out we can get clues as to whether it’s step up, step down, combination thereof, or what’s likely to be a primary. THEN apply your 12V test.
The black wires are often the input wires. That’s where the wall voltage goes. But, as was suggested, use another transformer with 12 volts out to connect to the black wires. Before this, use a meter to see which wires go together and their impedance. Colors usually go together. Lower impedance will most likely be the 6.3 or 12 volt windings. If they follow convention they will be greens or yellows. A higher impedance with slightly thinner wire will be the high voltage. Most likely the HV will have a third center tap wire. If you put 12v in the out will be one tenth of the real voltage when you use 120v. Be careful to make sure the stripped leads don’t touch when you’re testing. It would be bad!
Not just 4 wires. Sometimes 6 or 8, if there is 100, 115 and 120. Plus the 2x Euro versions. And the way they parallel may or may not be obvious. You need to measure the resistances AND the turns ratios (by applying proper low voltage somewhere) to figure which sections need to be paralleled if needed.
Actually the numbers from the 12 V test are helpful. With them you can determine the transformer winding ratios. This allows you to make an educated guess as to the primary voltage and resulting secondary voltages. As with any educated guess, your guess will depend on some assumptions. One assumption being that the transformers are designed for 50/60 Hz operation and not, say, 440 Hz of aircraft. You're also crossing your fingers (and taking your life into your own hands) when you assume that the original designer and manufacturer have ensured sufficient insulation between primary and secondary. You further assume that this insulation is still intact. As you point out, the voltages could be dangerously high.
Once you've determined the voltages you need to make another educated guess about the VA rating of the transformer (which you can guess from its core area) and the current rating of each of the windings (which you can guess from the cross-sectional area of the windings, assuming you can reach them well enough to measure).
And, of course, if you guess wrong and, say, mistake a 10 V primary winding intended for international voltage options for a 12.6 V filament winding you could end up making a death trap. After all, the insulation between primary windings would likely have a much lower dielectric breakdown strength than that between primary and secondary.
Once you're done guessing you may end up concluding that forking over $60 for a custom wound transformer from Toroidy (www.toroidy.pl) is a pretty good deal after all.
Tom
Plain corrugated cardboard box from New Jersey! I got excited when I saw Jersey thinking it might have been a builders name. I have 2 brand new in their boxes... Thanks!
You apply the 12 volts to something that is likely to be the PRIMARY, not the lowest voltage winding there is. Then all the voltages you get should be safe. An HV tube winding might reach 50-60 volts. And they’ll go only down from there.
