Quote from: Loudthud on May 12, 2025, 10:34:01 PMWhat I find curious about the scope photo in post #9 is that the little peaks are 10mS apart but of alternating polarity. The only thing I can think of is that one or two of the diodes have failed open in the bridge rectifier effectively making it two half wave rectifiers. The plus peak of the 50Hz makes a little positive bump in the ground, then on the next half cycle the negative peak makes a little bump of opposite polarity.
Quick edit: Could be that one side of the transformer winding has failed open.
I may be wrong, but what I see there is:
1) waveforms are measured from one point until it repeats "x" time later, always the same..
So a 50 Hz sinewave has positive peaks separated by 1 second / 50 = 1000 milliseconds / 50=20 milliseconds.
Or alternatively, 20 milliseconds separating negative peaks, same thing.
We find both in the picture so I *guess* they are artifacts or parts-of a 50 Hz wave.
50Hz AC because I see no rectification there, both polarities are present, same amplitude,and in the proper sequence: + ... - ... + ... - ... and so on.
I can also guess *where* are they coming from.
Will try to find a drawing, too sleepy to draw anything from scratch now, 02:30 AM here, to show where that waveform is *guaranteed*

I have found and solved this problem many times, but it plagued me in my early days ... some 40-50 years ago.
Grounding is walking a minefield.
You ground *here* and you have hum.
Screening is useless because ground itself, screen too, BOTH are Hummy



Now you ground *there*, one inch away, same chassis, same ground bus bar, whatever, and it doe not hum


You measure ZERO ohm between both points, of course
