Identifying windings of a power transformer

[shepdog]

I have almost completed rebuilding my amp, but I am stuck at wiring the primary winding to the mains. The transformers primary side has got four wires: black, black/white, brown, brown/white. The schematic shows that there is a center tap on the primary side which goes to ground. I would assume that two of the wires go together for the center tap, and the other two go to the mains. Is there some way to identify which go where with a multimeter?

Thanks

[martin manning]

I've never seen a grounded CT on a PT primary. What amp is this? What voltage system are you on? Do you have a schematic you can post or link to?

[Phil_S]

The primary does not have a center tap that goes to ground. That is a dead short for the transformer and you will likely let the smoke out when you do that. Think about this. You have an earth ground from the mains bolted to the chassis. You attach the line and neutral to the primary. That so called CT goes right back to the earth ground and POP!

What I think you have is perhaps a dual primary, where you will need to join the two windings in series for 220/240 line voltage. If this is the case, then you run them in parallel for 120 line voltage.

If you have a dual voltage primary, usually the two windings are separate internally, so your ohm meter should tell you they are isolated from each other. If the two windings are the same reading (or very close) for ohms, that's a good sign it's what I'm guessing.

You don't say where you are, so I don't know what mains voltage to set up for.

I'm going to guess that for series wiring (220-240) you join the two with the white stripe and use the brown and black as the outer legs to connect to the mains. You just cover the joint with heat shrink so it is insulated or make the connection at a terminal strip.

For 120 mains, wire in parallel. This is probably brown and black together, and the two white striped wires together.

What's important is getting the wires in phase. If they are out of phase, you won't get any output on the secondary because they are bucking the voltage. You'll know right away, and be prepared to shut down the transformer when this occurs so you don't let the smoke out.

Wire it up with a screw type terminal/barrier strip so you can test it and modify it. I wouldn't want to solder anything until I was sure.

[shepdog]

Thank you very much. I really need to write down all the errors with the schematic for future reference.

[Phil_S]

So, the suspense is killing me. Are you at 240 or 120? How good was my guess?

[shepdog]

Hahah. I'm in the states, 120 it was. Stripes together and solids together like you said.