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I am currently rebuilding a toroidial transformer from an old Cambridge audio amplifier/receiver. This toroid supplies the +5 V and +/-15 V rectifiers for op-amps etc. The fault was the typical blown thermal fuse on one one of the split primary legs for the 100 V/115 V input.

So far I have installed the new fuse and rewound the 1st secondary (single wind for 12 VAC which runs the 5 V DC PS). I was about to start the 2nd split secondary when I came across a YouTube video on the phase issue. Now I think I would be correct in assuming that the phase is not an issue when it is just feeding rectifiers for DC PSs... anyone correct me here if I am mistaken.

Now here's my conundrum: I didn't bother to pay attention to the split secondary windings direction as I removed it. I started with one bobbin on the 1st winding end that seemed to want to come off 1st and easy... until I got a ways into it and found the wire from the other end was overlapped/crossed over, so I had to start a second bobbin and proceed in the other direction.

Long story short... question: can I wind the 1st leg around until it's done, then join the 2nd leg to the 1st for the CT and wind in the same direction on top of the 1st OR?
Should I make the CT and go in the opposite direction again over top of the 1st?
Or should I start the 1st leg and go say halfway round, stop and start at the CT jion with my 2nd bobbing going in opposite direction?

And one more issue: phase. I'm pretty sure I have to wind both legs in the same orientation ie. if I start 1st leg with the end pointing up and go under come up through and over and so the 2nd leg would have to match that same direction. Correct?

Otherwise, in my understanding, if I get it mixed up my phase will be opposite on each leg with reference to the CT. Am I correct?

Any help would be appreciated.

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  • \$\begingroup\$ Before rewinding the secondary you should make sure that the primaries are OK. Connect them to a current limited and fused AC source and measure the no load current, which should be only a small fraction of rated current based on VA. You don't need to be concerned about secondary phase for a rectifier supply. And make sure you find out what caused the overload and blown fuse. \$\endgroup\$
    – PStechPaul
    Commented Jul 24, 2022 at 22:12
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    \$\begingroup\$ Of course I checked the primary for issues and found it fine. After I wound the 1st secondary (single wind) I tested it again to check secondary voltage for enough to run the 5V recifier/regulator ... all good. As for the cause that blew the thermal fuse, I did a cursory check and found nothing obvious ... this is a common failure for these units and in almost every case, it's the thermal fuse which just simply wears out over time. The thing is quite old, 20 yrs at least. I have fixed many a device that had a thermal somewhere and every time ... it was just wear. \$\endgroup\$
    – Journey
    Commented Jul 25, 2022 at 18:29

1 Answer 1

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Keep the winding direction as if there were no CT. If you change the direction in the middle of the primary winding the resulting inductance is far too low and the transformer blows a fuse in the best case.

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  • \$\begingroup\$ Thank you for your input mate. This is what I figured. What I am curious about now is ... I have started the 1st leg of the 2nd secondary ... there is quite a lot more thinner wire and this 1st leg may go around the core twice. So my plan and or question is ... after this leg is done, join the next leg there and that is my CT ... so then continue with leg 2 around in the same direction OVER TOP of the 1st. I'm thinking this shouldn't be a problem. As I mentioned above, it appeared while unwinding, it looked like they started at the ct and started winding one leg one way and 2nd the other \$\endgroup\$
    – Journey
    Commented Jul 25, 2022 at 18:40
  • \$\begingroup\$ It may be better to make a bifilar winding, using two wires as a pair. Of course for a center tap you would need to connect the end of one winding to the start of the other. I have also seen a statement that it is not good to continue a toroidal winding past 360 degrees, because multiple windings essentially create one turn per layer, unless direction is reversed. I can't find the reference, so I might pose it as a new question. \$\endgroup\$
    – PStechPaul
    Commented Jul 25, 2022 at 22:29
  • \$\begingroup\$ Hi All ... Success ! Cambridge Amp lives again ! \$\endgroup\$
    – Journey
    Commented Jul 28, 2022 at 23:09

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