| Don't isolate the scope from AC power ground. The scope frame and
ground
| points will go to the potential of the probe ground which could be
harmful
| to the health of you and your scope.

RIGHT, when you use an isolation transformerthen to isolate your
DUT!!! never the scope!!!

| Most 2 channel scopes have channel A+
| channel B (A+B) input selection. If you select that and invert A,
the scope
| will display the difference between the 2 channels. If there is a
large DC
| component, select AC inputs to measure low level ripple. Be sure to
connect
| the grounds of both probes to the frame or AC ground of the device
being
| measured.

The CMRR of such constructs is usually so bad that you see nothing
worthful. The only propper way is a fully differential probe like
these ones from tek
(http://www.tek.com/site/ps/0,,60-15865-INTRO_EN,00.html) or a
isolated oszi like Tektronix 720 series.

MArtin