Step Down Transformer connections

[condense]

I have recently bought a 60s Wurlitzer 2510 jukebox. It has an old step down transformer with it which after testing the continuity of both windings I am planning to wire in.
The UK end is already wired (neutral to 'com' and live to '220')

I can't seem to work out which way around the US end (live/neutral) should go. The RHS (in this photo) is labelled '1' and the LHS labelled '2' so I assumed that 1 would be neutral - does anyone know if that makes sense?

I've tried looking up the company and I think they still exist but I suspect my inexperience is showing and so I hope that someone on here will know and be kind enough to help me

thanks

[McMurdo]

if it's an isolating transformer then I dont suppose it matters, if it's an auto then the easiest way is to power it up and measure each terminal voltage to earth.

[Terry_VK5TM]

Wire it so that the secondary N is on the same side as the incoming mains N.

Theoretically, it shouldn't matter which way round it is wired if it is a fully isolating transformer.

If it is an auto-transformer, then it matters.

[Rubberfingers]

First thing is to ascertain whether this is an autotransformer or an isolating transformer. With the transformer disconnected from the mains, use the lowest ohms range on a multimeter to check whether all of the terminals, including 1 and 2 show continuity to the com terminal, or whether there is continuity between 1 and 2 but not to the com terminal

[vinrads]

The juke box will have an isolating transformer usually with wire coded black and white , for 115 volt mains so it matters not which way round you connect your auto transformer for 240 mains . better still get yourself a fully isolated step down . Mick.

[condense]

Thank you for your posts
There is continuity across the 2 sides. From the looks of it I am wondering whether it may just be better to buy a new fully isolated one

Thanks you

[usradcoll1]

Originally Posted by condense

I have recently bought a 60s Wurlitzer 2510 jukebox. It has an old step down transformer with it which after testing the continuity of both windings I am planning to wire in.
The UK end is already wired (neutral to 'com' and live to '220')

I can't seem to work out which way around the US end (live/neutral) should go. The RHS (in this photo) is labelled '1' and the LHS labelled '2' so I assumed that 1 would be neutral - does anyone know if that makes sense?

I've tried looking up the company and I think they still exist but I suspect my inexperience is showing and so I hope that someone on here will know and be kind enough to help me

thanks

Looking at the picture, it seems that there's small bits of wire still connected to the secondary terminals, with a black wire on terminal #2, so that's the way I would connect it. Your observation is correct.
On the primary side, reconnect the live side to the 240v terminal, as your mains voltage is nearer to 240v or more.
Dave, USradcoll1

[condense]

They seem to be a bit pricey so think I may be resurrecting this one after all…
So in terms of Kevin’s suggestion - seems like the only way to be sure. I usually avoid doing tests on AC mains like this for safety reasons.
Presumably both prongs in one hand and one on the mains earth wire and the other on 1 and 2. The one with a result should be live?

[condense]

Originally Posted by usradcoll1

Looking at the picture, it seems that there's small bits of wire still connected to the secondary terminals, with a black wire on terminal #2, so that's the way I would connect it. Your observation is correct.
On the primary side, reconnect the live side to the 240v terminal, as your mains voltage is nearer to 240v or more.
Dave, USradcoll1

I’m not sure I trust the black wire - I suspect both would have been black somehow and I want to be sure. The step down transformer is to 115v so I was guessing that they may have gone to 220 the other side to compensate as ideally it would be no more than 110v

[Herald1360]

FWIW single phase plug wiring in the US is coloured black (and/or bare copper core) for line and white (and/or tinned copper core) for neutral.

If the TX is an auto type, one or other of the output terminals will be short to COM and therefore neutral provided that COM is neutral as described. The other output terminal will then be the 120V line.

[condense]

Thanks Chris - that’s very helpful.
I get 0.4 R between com and point 2. This is the same as I get when I touch my prongs together. I get 1.8 R between com and point 1.
Between com and the 220v I get 3.8 R so am now fairly confident which way around it is.

Thank you all so much -

[GrimJosef]

Originally Posted by condense

... The step down transformer is to 115v so I was guessing that they may have gone to 220 the other side to compensate as ideally it would be no more than 110v

If they have then they've gone the wrong way. Transformer voltages are proportional to one another. So if you apply more than the nominal volts to one winding you'll get more than the nominal volts out of the other. If you put 240V into a 220V winding (about 9% more than it's designed for) then you'll get 125V out of the 115V winding (again 9% more than it's designed to give).

EDIT: Crossed with Graham.

Cheers,

GJ

[Graham G3ZVT]

Originally Posted by GrimJosef

If they have then they've gone the wrong way. Transformer voltages are proportional to one another. So if you apply more than the nominal volts to one winding you'll get more than the nominal volts out of the other. If you put 240V into a 220V winding (about 9% more than it's designed for) then you'll get 125V out of the 115V winding (again 9% more than it's designed to give).

EDIT: Crossed with Graham.

Cheers,

GJ

Originally Posted by usradcoll1

On the primary side, reconnect the live side to the 240v terminal, as your mains voltage is nearer to 240v or more.
Dave, USradcoll1

Originally Posted by condense

The step down transformer is to 115v so I was guessing that they may have gone to 220 the other side to compensate as ideally it would be no more than 110v

Using the 220v tap instead of 240 would increase the secondary voltage.

GJ, I deleted my post for a moment while I thought it through one last time

[Silicon]

What supply voltage was your 1960s Wurlitzer designed to work on?

Apparently American mains supply voltages have increased slightly over the years.

When you first power up the amplifier, check the heater voltage getting to the valves.
This is usually more important than the HT voltage.

Adjust the transformer connections to get the best heater voltage.

[condense]

Thank you

I really appreciate that - I was thinking I needed to work it through as I may have it the wrong way around with regards to the input voltage.

Thank you - very helpful

I think the lower end of the voltage is generally preferred in order to take the pressure off of the fluorescent lighting as it is designed for 60hz. I will change to 240v and see how things go.

[Herald1360]

Interesting about the fluoros. Makes sense if they're using traditional inductor ballast gear.

[McMurdo]

the reason I suggest measuring the secondary voltages to earth is because it's easy doing it the resistance way and get it wrong, the right and wrong way only being an ohm or so apart. Quite easy on old dirty terminals to make a mistake, whereas measuring the voltage to earth will leave no doubt.

I'm just speaking from experience with autotransformers where I've had to recently repair a batch bought from China which had miswired primaries, meaning the 115V-supplied equipment had 115V across them but 240V to earth