Power Transformer confusion

[jaunty]

I'm posting this under 'newbies' because it's kind of a 'newbish' question - insofar as I've never really had to deal with AC transformers before...
but i certainly understand the principles of how they work - coils and taps and all that - and i'm pretty sure I could build one - but i'm trying to troubleshoot some power supplies at the moment and am getting a BIT confused about what I'm finding.

in particular - one out of an old yamaha synth... the PS is blowing fuses. So i thought i would check the secondary winding continuity - just in case.

there are THREE wires only coming out of the thing on the secondary side. I'm thinking it's a single coil with a centre tap as a result. But I get continuity (7 ohms) between one pair only - and open circuit between any other combination. Is it POSSIBLE it's still good? I'm thinking that it wouldnt' be even POSSIBLE for it to blow a fuse DOWNSTREAM if the secondary coil is open ...

I would hook it up to the wall and take direct measurements but the PS requires sense wires from downstream circuits to run balanced and I KNOW that if i hooked these up I would blow a fuse (and possible components) if i did. SO I'm just wondering about general topology and why i might be getting the readings i am (could it just be capacitance somehow?)

[WaveyDipole]

If its a centre tapped secondary then its likely that the centre wire will be connected to GND. The outer two wires will often be of the same colour. If the third (open) wire is of a different colour to the other two then it is possible that it connects to a screen between primary and secondary windings. Where the wires connect to the circuit might provide a clue.

It is also quite possible that you have a shorted capacitor or rectifier that is causing the fuse on the primary to blow, whether that's across a two wire secondary, or the remaining half of a centre tapped secondary. Does the fuse still blow with the secondary wires disconnected from the circuit?

[IanB]

in particular - one out of an old yamaha synth... the PS is blowing fuses.
...
I'm thinking that it wouldnt' be even POSSIBLE for it to blow a fuse DOWNSTREAM if the secondary coil is open

This is confusing. If a fuse is blowing on the mains input side of the transformer, then it is possible the transformer has a fault. But if a fuse if blowing on the secondary side after the transformer, then it is unlikely the transformer is the cause. A fault causing a fuse to blow is usually located downstream of the fuse, not upstream. Look for faults/short circuits in components after the transformer.

[technogeeky]

I'm posting this under 'newbies' because it's kind of a 'newbish' question - insofar as I've never really had to deal with AC transformers before...
but i certainly understand the principles of how they work - coils and taps and all that - and i'm pretty sure I could build one - but i'm trying to troubleshoot some power supplies at the moment and am getting a BIT confused about what I'm finding.

in particular - one out of an old yamaha synth... the PS is blowing fuses. So i thought i would check the secondary winding continuity - just in case.

there are THREE wires only coming out of the thing on the secondary side. I'm thinking it's a single coil with a centre tap as a result. But I get continuity (7 ohms) between one pair only - and open circuit between any other combination. Is it POSSIBLE it's still good? I'm thinking that it wouldnt' be even POSSIBLE for it to blow a fuse DOWNSTREAM if the secondary coil is open ...

I would hook it up to the wall and take direct measurements but the PS requires sense wires from downstream circuits to run balanced and I KNOW that if i hooked these up I would blow a fuse (and possible components) if i did. SO I'm just wondering about general topology and why i might be getting the readings i am (could it just be capacitance somehow?)

If a wire emerging from a transformer has no connection to any other wire, then it's not connected. It's just not connected. Many transformers have extra pins which are there just to provide mechanical support.

I think we'll need to see pictures in order to determine if the transformer and/or the associated circuitry is in a center-tapped configuration.

Failing that, I think you need to be very specific and draw/label every circuit of the transformer and its respective resistance.

[SeanB]

A lot of transformers, especially in audio equipment, have a screen between primary and secondary, which is just a winding with one end in many cases, or a sheet of copper foil with an insulated overlap ( so as not to make a shorted turn) with a grounding wire. This is there to reduce any noise from the mains from appearing on the power supply rails, reducing the noise in the system.