Nothing else has changed, just the diode .

Okay, you convinced me.

 

Okay, you convinced me.

Just to convey the importance of the flyback (flywheel) diode, I've just done a few seconds with it disconnected ..... Bear in mind this is a single 15V (ish) supply !! Just look at that Pk-Pk figure !!

 

I promised to show the oscilloscope trace after I replaced the 1N4004 diode with an SB260 Schottky diode.

For completeness, here are both traces. I think you will agree the Schottky diode produces a much cleaner trace, and gone is that huge negative spike. Nothing else has changed, just the diode ...

View attachment 178306 View attachment 178307

Thanks for posting that, it looks like it cleaned up that spike and the overall trace like you said. Very good information. Thanks!