sorry about the late response. Your plan B with the old oscilloscope should be done with a capacitive voltage divider probe to keep from zapping the oscilloscope. It is like a resistor voltage divider, but uses capacitors instead.
this is a link to a pdf document showing you how to make one. see page #5
https://www.princeton.edu/ssp/tiger_..._waveforms.pdf
oscilloscope probe diagram.png
The clamping ends of the probes are clothespins hot-melt glued to split pieces of tubing. You probably could use foil also, but is less consistent.
Hope this sheds some light on secondary ignition probes. The capacitors are 1000pF.
Don't forget that the signal will be inverted as it is a measurement of negative voltage.
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