measuring ac voltages on a tv switchmode power supply

[ant17]

hi guys i am still working on my bn-44-00516a out of a plasma tv but i am having the trouble of when i try to measure the voltages on the board with my oscilloscope it just seems to show the 50 hz 240v signal on most of the components even the smd parts is it normal to have these phantom like voltages

[S. Petrukhin]

Maybe you have a problem with connecting the ground of the probe?

[srb1954]

hi guys i am still working on my bn-44-00516a out of a plasma tv but i am having the trouble of when i try to measure the voltages on the board with my oscilloscope it just seems to show the 50 hz 240v signal on most of the components even the smd parts is it normal to have these phantom like voltages

If you are making measurements on the primary (mains) side of a SMPS then 240V 50Hz is exactly what you should expect to see because the whole circuit is live at mains potential!

To make measurements on a SMPS you need to either use a high-voltage differential probe or isolate the whole circuit from the mains using an isolation transformer.

[srb1954]

Maybe you have a problem with connecting the ground of the probe?

Don't connect the ground of the scope probe anywhere on the primary (mains) side of the SMPS unless you have unit under test powered from an isolation transformer.

Connecting a scope probe ground to a non-isolated circuit will result lots of sparks, a burnt out scope probe and possibly a damaged scope.

[ant17]

ok guys thank you for your reply which is most accurate way of measuring the little smd components and transformers on the primary side with you scope thanks