First short with an Fluke Scopemeter

[ElecT-Ron]

So, i have recently acquired the 123b Scopemeter from Fluke, and I was under the impression that this scope was channel isolated

Well, I was probing two MOSFETs in a charger circuit to check if they were gone. The control circuit is modulated by one of these transistor. I first probed the gate, and the incoming signal was about 0.2V on both.

Probing the drain, with common ground on both channels caused a short circuit. I havent measured the resistance of the ground probes before, again, because i was under the impression the unit was isolated. I sent a test signal into both channels and they seem good. But the thing worrying me now is that i only measure 160 ohm over channel grounds and 80nF. Are they isolated.

I now realize that the high frequency over a cap can cause the short and should have used a differential probe on the second channel.

Anyways. On my 124 Scopemeter the ground channel impedance resistance is 0.8kohm.

Can somebody please explain why i am seeing so low values on isolated scopes, or are these not isolated?

[bdunham7]

AFAIK, they're not isolated.  There's no indication on them that the channels are isolated from each other.  I don't have either here to test, but many older 2-Ch Scopemeters were not isolated between the channels and you're clearly measuring that they are not.

[wasedadoc]

See the block diagram on page 3-2 of the Service Manual or the schematics in Figs 9.1 and 9.2, https://assets.fluke.com/manuals/123_124_smeng0000.pdf.  The input "grounds" have a connection to each other through two PTC resistors.