Dual Voltage without a transformer with a center tap?

[Yamin]

Hi guys,
I have acquired a theremin which someone had thrown away. The power adaptor is missing. Before investing to get a new supply I wanted to check whether the unit works. The original power adaptor has got three pins and its states that its 14Vac - I assumed that it would be a center tap transformer and it was confirmed when I looked at the power adaptors pin configuration. 
When I had a look at the schematic, +/-12V is derived, but the 'center-tap' seems to be connected to one end  (CN5 on schematic). I am having a hard time understanding how the power rails are derived or how shorting the center tap to one end works.
14Vac with a center tap transformer is very hard to source locally. If the center tap is shorted at one end can I use my variac to get the 14Vac? If this is not possible do you have any idea how I can test the unit. I have got DC bench power supply. Local shops carry 12Vac transformer with center tap but thats 24Vac from end to end.

Hope you could shed some light and hope someone could explain the power supply circuitry.
Thanks in advance.

[TimFox]

The rectifier could be considered to be a voltage doubler, but it is simpler to think of it as two half-wave rectifiers, one producing a positive voltage output and one a negative voltage.  Full-wave rectification is more popular now (since diodes are cheap), but half-wave rectification was common in line-operated transformer-less vacuum-tube equipment (where there was no center-tap available.)

[Andy Watson]

It is not a centre tapped transformer - only a single 14V AC output. They have doubled-up on using pins 1 and 3. If you look at the schematic these pins are connected together - this would cause a short-circuit if it were centre tapped.

Actually, according to the label on the transformer, the  output is between  pins 1 and 3, pin 2 is a ground terminal.

[TimFox]

Yes.  The single non-CT output drives two half-wave rectifiers with the same common node.

[Yamin]

Could I ask what the purpose of a grounding pin2? It would be connected to mains earth right? Some of the power adaptors sold online for this particular theremin does not have the ground pin which would connect to mains earth.
So the good news is I could use my variac or a non center tap 12Vac transformer right for testing purpose. Anything I should be aware of?
I still can't use my DC bench supply right?
Thanks for the help.

[Ian.M]

No you can *NOT* use your Variac (on its own) as 99% of all Variacs are simply autotransformers so don't isolate the load from the mains supply.    We'd rather not have your relatives posting how you electrocuted yourself . . .

However if you have any normal isolated secondary transformer with an output voltage greater than 14V RMS at  200mA RMS rated for that or a greater current, you can feed it from your variac and dial its output voltage down to the required 14V.  Even a >1A 12V transformer may be possible as its output voltage will be higher with only a 200mA load, and for short term testing most transformers will tolerate boosting their supply voltage by 10%.  Also I doubt the regulators require all the headroom a 14V transformer supplies - if its competently designed there should be 10% margin there to cope with low mains supply conditions.

[TimFox]

The label shows 14 VAC between pins 1 and 3, no CT, and pin 2 as “Gnd”, presumably the ground/PE pin of the line cord.  Within reason, you can connect an ungrounded secondary winding as you wish.  If you used a full-wave bridge rectifier, you coul connect one side of the bridge output to ground for a single-ended voltage supply.

[Yamin]

No you can *NOT* use your Variac (on its own) as 99% of all Variacs are simply autotransformers so don't isolate the load from the mains supply.    We'd rather not have your relatives posting how you electrocuted yourself . . .

Yes very good point  . I run my variac off an isolated output from an isolation transformer.

[Yamin]

Within reason, you can connect an ungrounded secondary winding as you wish.  If you used a full-wave bridge rectifier, you coul connect one side of the bridge output to ground for a single-ended voltage supply.

Thanks  Tim, could I please ask from you to explain these two points a bit further. What could be the potential issues caused by an ungrounded secondary winding?
And with regards to the bridge rectifier topology if I understood correctly the negative side would be grounded right?

Thanks again

[TimFox]

There should be a very high resistance (but some capacitance) between the secondary winding, the primary winding, the iron core, and any metal container.  However, there is a maximum voltage rating for all of those insulation resistances.  Vacuum-tube era “filament transformers” had explicit insulation ratings (sometimes in the kilovolts for tube rectifier circuits).  In your voltage range, where the secondary insulation (except to the primary) will see < 50 V, they don’t bother to post the specification, but the UL specification for power adapters does call out voltage test values.
With a bridge rectifier and a non-CT secondary, you can choose to ground either the positive or negative node of the bridge, but not both, to obtain a negative or positive voltage, respectively.