Hummm .... good educational SNAFU this .... trap for young players, posting so we all learn.
So it seems a transformer is just a simple inductor where the input and output voltages are determined by the differential windings of a pair of coils.
the ratio of windings between the input and output determines if it is a step up or step down transformer.
Most transformers are step down transformers in domestic power supply's .
So ... working this out in my head
Step Down Transformer: The input coil has more windings than the output coil => Voltage Decreases (steps down) from the Src and current increases.
Step Up Transformer: The input coil has less windings than the output coil => Voltage Increases (steps up) from the Src and current decreases.
It looks like its no coincidence that US Power supplies are 110/120v which is half the rest of the world 230v/240v.
So manufactures can ship the same transformer in all their products that is a 230/240v version designed to create the voltage/current needed for their PCB's.
And if it happens to ship to the US ... they simply "wire in" a terminal instead of half way down the output coil that drops the output voltage by 50% on a 230V src supply .... to the full extent of the coil on a Src voltage of 110V to achieve the same effect,
This is what those switches on the back of Commercial appliances do when they give the consumer the option to select either select 110V or 230V by moving a switch on the back of the unit.
... they simply flip the output coils location from the middle (230v Src Voltage) on a to the end of the coil (110v Src Voltage)
... while it looks like its not exactly half'ing documented output voltage or doubling the output current
=> the PCB has tolerances to be able to cope with deviations in the range +/- 10V and +/- 10Hz (50Hz vs 60Hz differences between "Rest Of the world" and US Power sources respectively) as well as deviations in Current and Voltage in a countries regional power system.
So .... I am thinking ,,,, until I find better data sheets of the "Ei 66*28 power transformer" it seems to be rated to 230V
... which suggest that they have simply saved costs (Munced) the back of the unit by not putting a switch that flips from the units half and full coil position.
All they have done is use the same transformer for both US and Rest of the world units ....
US Version: Transformer Output Wire to the PCB Bridge rectifier is the full extent of the coil.
Rest of the world version : Transformer Output to the PCB Bridge rectifier is only half way of the output coil.
So tongue at the side of the mouth ..... technically I am wondering if all I have to do is de-solder the output wire on this US Version of transformer from full coil output terminal position to the half way position.
I need to inspect the transformer in the unit to see if it has 2 out put terminals for the output coil position.
It may not make economic sense to source 2 different transformers for US (10% market) and Rest of the World (90%) for their units so this has a high probability of being a successful assumption.
### Now ... next problem .... Current ###
If the US Transformer is using the full coil to get the desired output voltage (which I think is 9-12V AC to the PCB's Bridge rectifier) ... and then I move it to the half way output terminal.
I may get the desired output voltage ..... but the current output is inversely proportional to the voltage decrease. So by using half the coil I may be doubling the current supplied to the PCB by making this change.
... so I am thinking that there maybe a resister on the PCB before the bridge rectifier that also needs to be replaced to reduce the current supply
... or its been hard wired into the different output terminals of the transformer to provide a consistent output voltage and current for US vs rest of the world input supplies to make it easier for the customers to implement.
Wow if I am close to understanding the situation ..... its down to 5 years of watching Mr Chicken Dinner himself on you tube !
#needmoredata on the Transformers specifications
Update: If V = IR => the resistance of the Bridge recitifer +PCB will always be the same ... if I get the same voltage to the PCB ... there will be no need to swap out any resistor as the circuit will pull as much current as it needs.
While the available current may be double on a UK Power supply using the same transformer at the half way position ..... its just that "available"
... wont be a problem in the UK as what is available current is always grater that what the PCB will draw as as the PCB has to be rated to draw no more than 50% the current in the rest of the world provides when it is used in the US market!