Checking output of audio amplifier of Easy BITX and more!

I am checking the function of audio amplifier of Easy BITX in the method suggested by VU2TDD. After connecting the 12V power supply and keeping the enclosure open, I am touching the central pin of the volume control with a line tester. 50 Hz alternating current picked by the the line tester is transferred to the input of the audio amplifier, producing a loud 50 Hz hum in the loudspeaker. That established that the audio amplifier stage is functioning well. Volume control was kept in the mid position for this test. Further testing was done by touching the terminals of T1 trifilar transformer on the PCB. That produced a hum of lower intensity. A close look at the Raduino board can be seen at the end of the video clip.