Scope ground causing interference - Page 1
Forum section: electronics / beginners

author: jmurphy
author_rank: Contributor
timestamp: September 22, 2018, 07:48:28 pm
content: 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?

author: bson
author_rank: Unknown
timestamp: September 22, 2018, 10:50:56 pm
content: 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...

author: ArthurDent
author_rank: Super Contributor
timestamp: September 22, 2018, 10:58:04 pm
content: 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.

author: jmurphy
author_rank: Contributor
timestamp: September 23, 2018, 12:48:13 am
content: 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.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 23, 2018, 02:02:27 am
content: 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.

author: ocset
author_rank: Super Contributor
timestamp: September 23, 2018, 08:25:40 am
content: So yes, as others have implied, you may have a ground loop going on, because even when you fed your scope without an earth connection….dont forget, the neutral is still connected to earth back at the installation point, and the neutral is connected to the earth node in your scope via the Y2 capacitors.But anyway, are you connecting your scope probe ground “tail” to the secondary “ground” of your video game thing?Maybe you are unknowingly connecting the scope “tail” to a switching node of some local switcher in there?But anyway….as others have alluded, why not get a piece of wire say 1 metre long, and solder that to the rail that you connected to before, and see if the noise is there then (it’ll be just a flying lead antenna kind of thing)……..also, if no noise seen like that , then connect the other end of that wire to earth ground and see again if the noise is there.Also, why not connect the scope probe across the secondary side capacitors of the transformer secondary, is the noise there then?