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Replacing old dual channel AC transformer with new dual channel AC wire + negative wire


New Member

I am posting in this forum after a long time. Previously I posted my issue with the IC on a cheap PCB built for a 2.1 Speaker. You can read the post here.

Anyway, I figured that my old transformer started to create some buzzing noises which directly came out of the speaker boxes (from both left and right). Whenever I touched and did shake the transformer the noise would stop for sometime and then start buzzing again.


Now I want to replace the faulty transformer with the new one. But there is this issue with the new transformer. It has 3 wires for AC low voltage output. In the picture, you can see that two blue ones provides AC voltage and the yellow one in the middle has no voltage flow. That is the negative wire.

As I attached it with the AC 240v on the primary Red wires (on the opposite side), I started measuring the voltage output for the AC on the secondary wires (blue and yellow). Thing is, whenever I attach my multimeter points, one on the blue and the other on the yellow, it shows 14.4 AC output (from both the blue wires, connecting with the yellow). But when I attach both of my multimeter points on both the blue wires (not touching the yellow wire), it doesn't show any AC output voltage. Meaning, the only way I get voltage output is by attaching one of the blue wires and yellow wire on my multimeter points.

As you can see that my PCB has two AC voltage inputs. I do not know how to use this transformer now.

Should I attach the yellow and blue (one of the wires) with these two AC wires of my PCB? Or should I attach the blue wires only? Or there is something else that I am missing?

I am totally confused here now. When I attached both the blue wires on the AC voltage, there was no output voltage.

Please help me out here. I am waiting for your kind reply.

Thanks in advanced.


Well-Known Member
Most Helpful Member
The new transformer is made to look like a "centre tapped" style.
If it was that, you would get the same voltage from yellow to either blue - and double that between the two blue wires.

As there is no voltage between the two blue wires, it seems they are connected together inside the transformer and it's just built to look like a more complex type...

Use one blue and the yellow to feed the AC in on the PCB.

(For info, there is no negative or positive in an AC part of a circuit).

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