how does this circuit work?

[crazyhog]

Hi guys,

I came across the attached circuit diagram in the battery charging / power supply circuit of a Tek 212 oscilloscope.  I don't understand this circuit. 

1) Normally, the primary and secondary of a transformer are separated.  However, for the Tek 212, one prong of the AC inlet is connect to primary of T207 and the other prong is connected to the secondary (see green oval in pic)!  The parts list doesn't have any detail on the spec of the T207 transformer.  What is the purpose of T207 and how does it work in the circuit?  What should I google to read more about this transformer circuit layout?

2) What does R210 and C210 do in the circuit?  How does it work?  The manual suggest this is used for adopting to different line voltages.  I don't understand how that is achieved.

The attached circuit snappet is taken from the tek 212 manual.  The original is here http://w140.com/tekwiki/wiki/212

3) There is no filter cap to smooth out the voltage output of the rectifier before charging the nicad batteries BT216 and BT217 (6v each).  Isn't that a problem?

Please note that the s215 switch turns on the oscilloscope.  Even when s215 is off, the battery charging circuit continues to charge the nicad battery (each 6v).

[Andy Watson]

T207 is a common-mode choke. Note the polarity of its windings.

C210 acts as a potential dropper - limiting the current due to its impedance at mains frequency. R210 is there to discharge C210 such that there is no remaining potential when the unit is unplugged.

I assume that the soothing is performed by the battery.

Also note that if you are dabbling inside the unit, or with the battery, it is not isolated from the mains supply! Always unplug it before dabbling.