Electronics > Beginners
Scope ground causing interference
jmurphy:
I bought my first oscilloscope 6 months ago and so far it has been very helpful and worked as expected. Today I’m trying to troubleshoot an old home video game. As soon as I connect the scope ground to the board, I get terrible interference on the display.
Just the ground and it occurs even if the scope is turned off. I tried powering the scope without the earth ground and I still get interference.
The video game is powered by an AC transformer which goes through a filter and then feeds a full wave rectifier which supplies DC to a few linear voltage regulators.
What am I doing wrong?
bson:
Clip the probe ground to a 1k resistor and then hook that up to the ground. Measure the potential across the resistor with a DMM. If it's not very close to zero, then what you think is ground or floating on the board isn't...
ArthurDent:
Are you sure that the point you chose is ground? Is the A.C. cord 2 or 3 wire? if 3 have you checked to see if you have continuity between the cord ground pin and where the scope ground is connected? Can you just connect a piece of wire to this spot and get the same interference?
There is not enough information to make any guesses other than it's probably not what you think it is and it's definitely dangerous to have an ungrounded scope.
jmurphy:
The voltage across the 1k resistor was zero on my DMM. The AC transformer is a 2 prong 12V. I only connected the scope without an earth ground for a second to determine if there was any difference and there was not.
I'm sure sure of my ground spot. I tried several spots and checked continuity and resistance using the ground post of the linear voltage regulator.
ArthurDent:
If the A.C. cord on the video game is only 2 prong, what you connected the scope ground to on the board can not be ground because there is no ground brought into the video game. What you refer to as 'ground' could be the power supply common in the video game, but not ground.
Your scope has nothing to do with what you are seeing. If you connect a wire from the same point to a known ground it should give the same results. Even with the scope ground disconnected there is an input filter on the scope that capacitively couples the scope to the line so it isn't totally disconnected. The video game may have a similar filter on on the transformer primary and if its plug isn't polarized you might see a difference by reversing the plug, maybe not.
If connecting something to that point in the video game causes 'interference' it may be that is actually a point that is carrying a signal and you are disturbing that signal but you wouldn't necessarily see that on a DMM.
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