Years ago an engineer at work floated one of our then brand new HP Jedi
scopes (HP 56605A I think) to capture the peak current through the
crowbar device in a 30 kV 125 uF capacitor vault. They connected it
across a low value shunt resistor at the ground side. Needless to say,
when the test was done, the crowbar trigger fine, but the scope was
hammered and had black arc marks internally from ground side to AC
neutral. Sure glad that no one was touching the scope at the time.
Back in the 1970s when I was CE at an FM station in Virginia, I had a
troubled Collins FM rig (2.5 kW) that had been struck by lightning. The
SCR regulator card was in the primary line to the plate transformer and
screen supply. It was misfiring often, after I had cobbled wires back on
the board where the traces evaporated. Boss man didn't want to spend
anything on repair, "just fix it in place". At one point, I had to look
at the trigger card output, which is driving the gates of the SCRs. This
meant the low side of the output (after a small transformer I think) was
floated at one of the line potentials (220 VAC single phase primary
power). Had to ground the scope to this, and set it up on a block of
wood, didn't touch it! Needless to say, I got my results and had to
order some part that was partially failing.
I wish (in both of these cases) I had known about the scope isolators
that Tektronix sells. They are right on the inputs, and can float at 1
kV above ground I think, and pass pulses to the scope with reasonable
fidelity.
73
John
K5PRO
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