Tracking Down the Surprise Hum and Ground Loop in a Vintage Telefunken Andante Tube Radio from 1963.

After restoring a Telefunken Andante Tube radio from 1963 it had an annoying hum with the volume turned down. I discuss the diagnostic path used to track down and eliminate the hum. I suggest adding another cause of hum to the usual causes (filter caps and rectifier). I was aided in the task by evaluating the signal at the speaker terminals on an oscilloscope using the FFT (Fast Fourier Transform) math function. A Ground Loop was found and eliminated. Correction: at 7:30 the chart showing that a square wave produces even and odd harmonics is NOT correct. A square only produces odd harmonics. Correction: at 10:24 I said "second harmonic 220 Hz" . It should by 120 Hz. Correction: Silicon not the spoken "silicone". Link to silicon rectifier with mosfet voltage dropper    • Replace Selenium Rectifier with Silicon Di...