Shorted mains through scope ground, now the y axis is slightly out of alignment - Page 1
Forum section: electronics / testgear

author: Njord
author_rank: Contributor
timestamp: June 12, 2020, 05:13:49 pm
content: Hello,I made the rookie mistake of shorting theneutralline voltage (120vac) through the probe ground. Not great.I now have an issue with my scope (unsurprisingly) and I'm hoping someone can give me an idea of what the issue is. I have an analog scope (Circuit test OS-2030) and the vertical lines seem to be out of alignment. The trace adjust only adjusts the horizontal. Otherwise, it seems okay... The picture shows what I'm talking about with the 2vp-p square wave. The measurements seem to be fine,it just needs to be tweaked back.Any thoughts?Thanks!

author: SilverSolder
author_rank: Super Contributor
timestamp: June 12, 2020, 05:23:32 pm
content: Wow, that looks amazing, like the scope is going backwards in time!What might have happened is that something inside the scope got magnetized by the high current flowing through the ground connection, causing the beam to distort.If you move your square wave pattern up to the top and then to the bottom of the screen, does the deflection / "bend" in the squares look the same?

author: Njord
author_rank: Contributor
timestamp: June 12, 2020, 05:34:09 pm
content: I think you're on to something! When I sweep up and down the scope line moves on a slight diagonal line. My little CRT needs a degauss function. The attached pictures is just moving the line up and down.I have the calibration documentation (Shout out to @Specmaster. THank you!) which shows all the pots, but reading through it nothing jumps out as the adjustment for this.

author: Njord
author_rank: Contributor
timestamp: June 12, 2020, 05:36:03 pm
content: Calibration documentation from Specmaster

author: Njord
author_rank: Contributor
timestamp: June 12, 2020, 05:48:10 pm
content: I think because of this, the triggering is a bit messed up. Measuring the same crazy waveform from last night from an LED driver, it shows two lines. Which I imagine should be overlayed on top of each other. This looked better before my mistake of shorting mains through the ground but I dont have a picture. Adjusting the trigger, this was teh best picture I could get.Maybe its worse than I thought.Sorry for the multiple posts. The files were too big for 1.

author: SilverSolder
author_rank: Super Contributor
timestamp: June 12, 2020, 07:13:19 pm
content: You might need to degauss the actual steel chassis, LOL! :-)Looks like the effect is strongest at the bottom right (is that where the BNC probe socket is, by any chance?)But it looks fixable with a thorough degaussing.Regarding the triggering, maybe play with that on some simpler waveforms from your function generator to build confidence?

author: floobydust
author_rank: Super Contributor
timestamp: June 12, 2020, 07:24:22 pm
content: Also take an ohmmeter and confirm the scope ground is still intact. There should be continuity from the input channel BNC to mains power cord ground, and all other BNC's. Sometimes a trace or connection ends up being the fuse, inside the scope if there was a ground fault.

author: bdunham7
author_rank: Super Contributor
timestamp: June 12, 2020, 08:29:43 pm
content: What happened during your 'event'. The neutral line ought not have had that much potential!I would suggest looking for an adjustment called 'trace rotation'. I don't see in on the outside anywhere, perhaps if you pop the case off you'll see it.

author: Njord
author_rank: Contributor
timestamp: June 12, 2020, 09:39:28 pm
content: Thank you for all the replies!The ground is still functional. There is continuity between the exterior of the BNC to the ground plug.It turns out it was actually the line voltage (not neutral) that I shorted to ground. the power supply I was measuring had an ambidextrous (not keyed) plug. Where I assumed it was neutral, it wasn't.Somethings I noticed;If I use the trace adjust to line up the vertical axis, it looks like the attached. Left side is higher, and the right side is lower. Adjusting the trace to make the trace horizontal makes the vertical axis not straight (like the first pictures I posted).One picture shows the square wave test waveform. the other is the "empty" one channel.

author: Njord
author_rank: Contributor
timestamp: June 12, 2020, 09:43:51 pm
content: The picture was too big for one post. Both channels are exactly the same.

author: SilverSolder
author_rank: Super Contributor
timestamp: June 12, 2020, 09:45:26 pm
content: And is the amount of "bend" the same at the top and the bottom of the screen, if you move the trace up and down?

author: Njord
author_rank: Contributor
timestamp: June 12, 2020, 09:58:10 pm
content: Interesting! it does seem to be worse at the bottom.I guess I need to figure out how to degauss the case/monitor.Maybe with time, it will sort itself out. Looking at the pictures of when it first happened, it does seem to be a little better.