I want to use a toroidal transformer dual 35.5v AC currently inside an old power amp as an external power supply 24v AC single. There are no details on the trans but I include a pic and the voltage readings from the power switch module are: Transformer

White - I assume is minus, Orange - 117v AC, Yellow - 117v AC, Green - 0, Brown - 233v AC

Inputs on the PCB: Red - 35v AC, Black - Earth, Orange - 35v AC,

The trans is filled with epoxy resin which rules out adding or removing wire. Therefore, I am looking at modifying a step-down EI transformer as I cannot find one that does this voltage. eBay does a variable Power Transformer 110V/220V To 6V -380V single AC output here. eBay EI AC/AC transformer

Can I adjust this EI trans to my requirements by adding or subtracting wire? If so how? An idiot's guide would be helpful.

Also, on the eBay EI trans, there is a choice of watts, inputs, and outputs, which would be the best for a 35v AC input and 24v AC output requiring the least adjustment?

Which wires do I use on the toroidal trans to connect to the mains, and how do I convert from dual 35v to single 24v? Can I combine them or just use 1?

Or any suggestions how to achieve the above together with an idiots guide!

  • \$\begingroup\$ There's a hole through the centre I believe so, if you find a different way of mounting it, then you can open-up the hole (a drill might be needed) and, add a few turns that are wired anti-phase so that the present output voltage can be lowered. I'm choosing not to make a formal answer because I don't believe an idiot's guide is anything that is definable and, also calls for speculation. \$\endgroup\$
    – Andy aka
    Commented Dec 30, 2023 at 15:06
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    \$\begingroup\$ Read the sticker on the transformer. Or get a service manual with schematics to see what kind of transformer it is and how to wire it. Do not assume anything without measurements or data. \$\endgroup\$
    – Justme
    Commented Dec 30, 2023 at 15:10
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    \$\begingroup\$ What do you intend to use 24 V AC for? If it is to feed a bridge rectifier&reservoir capacitor, a contemporary SMPS may be a saner choice. \$\endgroup\$
    – greybeard
    Commented Dec 30, 2023 at 16:19
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    \$\begingroup\$ Please add to the question which of orange - white - yellow are "galvanically connected", and whether there is an interconnection to green - brown. \$\endgroup\$
    – greybeard
    Commented Dec 30, 2023 at 16:40
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    \$\begingroup\$ "35 V dual" suggests two independent secondary windings. You appear to have one centre-tapped secondary winding so we'd usually refer to that as 35-0-35 (red-black-orange). There won't be a "minus" wire on an AC supply. If you're going to purchase a second transformer then just get the one that will do the job in one transformation rather than have two. \$\endgroup\$
    – Transistor
    Commented Dec 31, 2023 at 11:36

1 Answer 1


What you do is get a die grinder and a carbide burr, or a diamond core drill and cut out all that sand filled epoxy and then start removing the tape,

The secondary winding will be outermost so you can get right down to business reducing the number of turns in the secondary. given that the original is 70V there'll be almost enough wire in the secondary to triple it (you'd get 23.3' volts) at the same VA as the original transformer)

Now get some new transformer tape and re-tape the transformer.

this transformer may not provide safety isolation so (so unless you can confirm that it does) ground the one and (or some other part) of the secondary

That's the hard way, the easy way is to use a different transformer.


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