There is no such thing as a 'daft' or totally lame question in electronics. Become an expert by asking, asking, asking everything that comes into your mind. Especially when a topic that was believed to be mastered turns out to have 'shockingly' unintended consequences.
The standard practice for oscilloscope use is to put the negative probe on ground-earth-zero volts and to check for the signal with the positive probe. Sometimes with older equipment, the wall plug is not polarized and can be inserted with either metal tong on the plug going to the 'hot' side of the electrical outlet. And often the chassis or ground point is connected to the metal frame chassis of the device under test. This will have the metal frame enclosure be at mains 120VAC voltage. This might be what happened in this case.
When I was a child I often would get throbbing, pulsing, and not pleasant shocks when I touched the metal plate of my child's record player that was behind the volume control. I realize now that the non-polarized plug was delivering 120VAC to this plate. Showing my age maybe, but am now glad that it didn't seriously shock me senseless.
Was the test point supposed to have high voltage? I'm not sure, but doesn't the voltage have to be several hundred volts before an arc occurs? Was the vertical volts/division setting on the scope set for 10 volts/division or millivolts per division? Scope still work? Device under test DUV still work at all?
The problem needs to be isolated to be either in the scope or the DUV. Check the DUV chassis with a voltmeter DMM set at 200 volts both AC and DC. Check the pin that arced with the DMM without the scope attached to the DUV. Check the positive probe tip of the scope with the DMM (with the DMM black wire attached to the scope ground). Maybe the voltage that created the arc came from the scope.
Keep asking questions here. You might draw out a response from a reader that we all can learn from.
Thank you.
--- On Sat, 5/2/09, r_j_d_2.phila wrote:
From: r_j_d_2.phila
Subject: [vintagesynthrepair] totally lame question about probing technique w/ scope....
To: vintagesynthrepair@yahoogroups.com
Date: Saturday, May 2, 2009, 8:15 PMhi folks, i just want to double check something really quick....
i was probing recently with my scope, and got a spark as i made contact w/ a leg. i want to check my "scope technique" before a new project.
when i have had to use my scope(not too often), i generally connect the NEG probe to ground, and check w/ the POS probe. is this generally suggested? or, say for instance, if i were checking a waveshape output of a chip, should i be picking a ground point closer to where im trying to read the signal?
i know this is really daft. i just want to definitively know im going about this properly. thanks-rj
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