How do I tell the different primary wires in a power tranny?

[slidincharlie (Carlo P)]

G'day all!
The amp: my '79 Fender Vibrolux Reverb with international power tranny (100 to 260 VAC selector).
Foreword:
a few years ago I disconnected all primary wires from the selector in order to avoid any damage due to accidental rotation of the switch. I left only the 240 VAC lug connected because the mains voltage in Italy is supposed to be 230 VAC, and in an old thread that I had in this forum I was advised to do so for reasons that we do not need to discuss here now.
Now I have one primary wire connected to the 240 VAC lug on the back of the switch, another primary wire alone and disconnected (which I suspect is the 220 VAC wire), and all other primary wires disconnected and tied together.
Now I would like to reverse back to the original 220 VAC connection from the switch, but I don't want to make a big error using the wrong wire.
My question:
how do I tell which primary wire is which?
Thanks in advance.
--Carlo

[R.G.]

Now I have one primary wire connected to the 240 VAC lug on the back of the switch, another primary wire alone and disconnected (which I suspect is the 220 VAC wire), and all other primary wires disconnected and tied together.
Now I would like to reverse back to the original 220 VAC connection from the switch, but I don't want to make a big error using the wrong wire.
My question:
how do I tell which primary wire is which?

With a voltmeter.

You completely unload the secondaries and then make sure all of the primary wires except the ones you are powering it with are both disconnected from any load and accessible with your voltmeter. . Then you apply the proper voltage to one set of primary wires. The other wires will then automagically have the proper voltages appear on them.

You have to do this carefully, as every single wire you're working with will have some thing like AC mains voltage on it, and be as dangerous as a live poisonous snake.

But done properly, it's as simple as powering the transformer with only the reference primary pair (i.e. neutral and 240Vac) with power, then measuring all the others from neutral to the tap.

[J M Fahey]

Agree and add: since you will be working with exposed/untaped wire ends, all as dangerous as RG states, for safety tape them to a piece of wood so they are not dancing around.

Then you have stable points to apply your test leads.

If nothing better, spread them fan like on a wooden table and tape them down ... leaving ends exposed of course.

[olddawg]

Corrected if I'm wrong but it's just a transformer. You are looking for a turns ratio. There is a much safer way. Put a low current low voltage ac source on the secondary side as described earlier and measure the primary taps. You should then be able to work out what is what with simple arithmetic. I would use a 100hz tone. But anything low will work. You are looking for a ratio.

[eschertron]

agree and add
this is assuming that the reference wires are connected to the voltage they're supposed to. If, on the other hand, a 240VAC wire is connected to 220VAC mains line (for example) then all the other wires will be a ratio off from their intended voltage.
220vac actual/240vac tap = 92%
so a 120vac tap would read 92% low also.
Not serious if there are a few taps easily identified, but with more closely-spaced taps, you may need to do some mathin' to get everything lined up and making sense.

[slidincharlie (Carlo P)]

I clearly did something wrong... Please correct my steps:
1) all tubes plus pilot lamp taken out (to have zero load, right?). All secondaries are in their place
2) 240 Vac primary connected to its own lug on the back of the switch
3) switch to 240 Vac
4) all other primaries in the air
5) turn the amp on.
Measuring Vac at each disconnected primary with reference to chassis ground I get the following Vac readings:
18 19 107 113 133.
Mains supply is 225 Vac.
Selections available at the switch: 100 120 127 220 240 260.
Where is the mistake?

[eschertron]

very interesting.
This does not match up with either the expected tap or one of the lower-voltage taps.
I'd find a lower voltage source and do what olddawg suggests, since you'll now have to check every pair of wires. the wire you thought was ground reference does not appear to be so.

edit: If I had to hazard a guess, I'd say that the ground ref wire is actually the 240vac wire, so one wire is 20 higher (or 18..19), another is 20v lower (or 18..19), and the others are 100 to 140-ish lower. Can you disconnect the landed wires and try again using the other as ground ref?

[Enzo]

Why are you measuring voltage to chassis ground? The mains wires are not grounded. That is an invalid reference point.

The amp works wired for 240v? So you have 240v across the primary now? So you have the 240v tap wire, and the other wire now in use would be the zero end of the primary, right? SO connect the ground probe of the meter to that zero end, verify 240v to the 240v wire. Now probe that loose wire, is ther 220v on it? The primary will act as an autoformer for this. The 220v wire will have about 20 volts less on it than the 240v wire when 240 if put on the 240 tap. And if the loose wire is not 220, then simply probe each reclaiming wire looking for one with 220v on it with respect to the zero end. Then connect that wire to the mains instead of the 240 wire.

[slidincharlie (Carlo P)]

I'm really puzzled.
Please refer to the attached image.
The reading between the central lug and the wire on the 240V lug is 0 Vac when the amp is on and connected to the mains supply (228 Vac right now).

[Enzo]

It is a switch, presumably connecting those two wires in the position selected. When two wires are connected together, how much voltage would you expect between them? Zero sounds right to me.

The mains voltage comes in on brown and blue wires. They run to the power switch, the blue side through the fuse, yes? When the switch is on, it connects the mains to the transformer on one side and to the selector on the other side. The selector decided which transformer primary tap was to get the voltage.

[eschertron]

Enzo's right of course
I was even talking about zero reference point and did not see that. There must be two wires on the primary side of the transformer that are hooked up. one comes from the voltage selector (by whatever path) and the other comes from neutral, the wire in the power cord that doesn't go to the 'hot' selector and doesn't go to ground. measure across the transformer wires so that the reference point is accurate.

[Enzo]

Hard to see in the photos, but I THINK the brown comes into the switch, while the blue goes to the end of the fuse holder. Then a short wire from the side of the holder to the other half of the switch. Then the other two wires from the switch, I THINK, go to the tranny and the selector.

[slidincharlie (Carlo P)]

Please bear with me a little more, guys...
The actual hookup is as Enzo and eschertron described: see image below. The six purple wires coming out of the PT are the primaries. I left all secondaries out of the sketch.
Now between WHICH wires should I read AC voltages to tell which of them corresponds to each of the selector lugs?
I am looking for the 220V wire.
EDIT: The purple colour is obviously not true: each primary wire has a different colour

[eschertron]

So 1 black wire and 6 purple from the PT?
Measure from where the black wire is landed to each of the purple wires. Take care to not let any high voltage wires get loose and 'fly'.

[slidincharlie (Carlo P)]

So 1 black wire and 6 purple from the PT?
Measure from where the black wire is landed to each of the purple wires. Take care to not let any high voltage wires get loose and 'fly'.

Do you mean from the power switch lug where the black wire is hooked? It carries the mains voltage (around 225 Vac). Should I read between it and each of the 'purple' wires?