Oscilloscope Safety

Found the problem. The negative probe clip had a small piece of film preventing good ground connection.
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Now I would like to get some mini-grabber or small alligator clips that would mate with this connector that is captive on the differential probe. Looked through the Pomona catalog without success.
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A very good reference on scope probes and probing techniques: http://eelabs.faculty.unlv.edu//docs/guides/ABC_of_Probes.pdf

Excellent advice on grounding starting on page 36.

Safety issues starting on page 44.

Speaking of safety concerns, I once say a guy using a Tek 475 dual channel 100Mhz scope which was expensive in those days, accidently drag a dangling probe ground lead across a 100kW power supply 300v rail. Needless to say, the scope was absolutely destroyed. Lucky he didn't get hurt.