Did I break my Oscilloscope?

[tayexdrums]

So, I get an oscilloscope in and was measuring all types of things. After measuring audio circuits and other minuscule things I decided I would measure the wall voltage. Now on the case of the scope it clearly says "400V pk-pk", so it made sense that I could put 120 volts AC into it. Hooked the probes up and heard a small "click" sound from the oscilloscope. Now, every time I turn my scope and set it on the GND coupling the trace is slightly bent. Its like I need to turn a knob that pulls the center of the trace down because the trace goes up slightly and goes back down. It is small but noticeable enough to cause an inaccurate measurement. Did I break my oscilloscope?

[Simon]

hm and apart from that it all works fine ? I'm not really qualified to answer your question but I would say check that your specs do not include a particular multiplier value, for example my Rigol digital scope cannot take more than 30 in it, but can measure hundred if not thousands of volts with a the right probe, many of the specs and examples are given for use of the included 10X probe

could be you blew a cap, have you tried a DC voltage with AC component to see if the blocking cap on the input is ok, does it do this on both channels assuming you did the measurement on one channel only ?

[tayexdrums]

Yes, everything else works fine. Let me check the specs and do that. It does it on both channels and I only measured it on channel 1.

I just read something on the internet and I think I connected the probe incorrectly. I'll post a link to it and let you read it yourself.
http://mas.lvc.edu/~walck/phy112/labs/oscilloscope.pdf

Its on page 8

[Simon]

you think ? this is the mains your playing with ! treat it like a loaded machine gun !

[tayexdrums]

I know, I know...I'm so mad at myself for being so stupid!
I hooked the black alligator clip and the probe to both of the leads coming from the wall so yeah according to what I just read on the PDF I hooked it up wrong and broke something.

What exactly did I break?

[Simon]

well your connection is correct, that's really the only way you can connect it but you must make sure you don't go beyond the scopes specs

[alm]

You did connect the ground clip to wires coming from one of the flat blades (assuming US plug)? If you clipped it to phase, lots of current went through the ground lead, through the scope, and something might have been vaporized before the breaker was activated. If there was a GFCI on that circuit, I'd expect that to trip. If you connected it to neutral, there could still be a voltage between the neutral and ground, so some current might still have gone through the ground lead. The ground is not really supposed to carry any significant amount of current.

I would verify continuity between the ground clip and the ground pin from the scope's power plug. Tektronix has a helpful document on troubleshooting scopes (I think it's somewhere on www.sphere.bc.ca), and the service manual of you scope might be useful. But given your apparent lack of experience, it would probably be a bad idea to operate the scope with the case off, or come anywhere near the CRT/EHT unit.

[tayexdrums]

I connected the ground clip and probe across both flat connections. Not the neutral. I'll try the continuity test. My question is though..what specifically did I break when I did that and how can I go about fixing it.

[Simon]

ok so you have a socket with Live, Neutral and Earth, what did you connect to the Ground and Input of the scope ? either way if you did not go over your scopes maximum input voltage either way of connecting the scope should not damage it, unless of course it was weak due to age but then you just unlucky

[tayexdrums]

This picture that I just took was the clearest way I can think of explaining it.lol

[Simon]

you mean you don't know which wire is which and you can't explain such a simple connection ? PLEASE DO NOT touch the mains until you can !, I'm not in america so those wires mean nothing to me (that's why we call them live neutral and earth)

[tayexdrums]

I can! But I just thought that a picture would be the clearest way of explaining it!
I didn't know you weren't in America, hehe

I connected the ground clip to the live(black) and the probe tip to the neutral(white)

[jinuq]

Black is Hot.
White is Neutral.
Green is Ground(Earth) could also have a yellow stripe.

I would say unplug it from everything take the cover off(don't take any of the pcbs off) provided it is not under warranty and look for anything that looks burnt, broken or down right blown up. Take some picture if you find anything like the above. Also watch the HIGH VOLTAGE CAPS etc, if it is a crt style unit. They can KILL!!

[tayexdrums]

Okay, I'll do that now... Give me about 10 minutes

[tayexdrums]

I looked carefully for blown caps and couldn't find any.
I took a picture of it just for reference anyways

[jinuq]

What make and model scope is it?

[tayexdrums]

It is an EZ-Digital (formerly LG Precision) OS-5020 20 MHz Scope

[tayexdrums]

More reference Pictures

[jinuq]

I would render a guess that if you took the top PCB, where the adjustment knobs are, off you would find something damaged. Either trace or some actual component. All the manual has is a block diagram, so I am unable to validate my assumptions.

When you hooked the ground up to the hot you dropped full voltage on a place that it shouldn't have gone.

[tayexdrums]

Okay I will look carefully for any thing that looks blown or messed up. Do I need to take the front faceplate off?

[jinuq]

You may or may not, I can't say for sure from the photos. You will need to take the knobs off.

[tayexdrums]

I can see the back of the front PCB ( the one that has all the knobs on it) and I don't see any problems. So do I still need to take the faceplate off? How do I take the knobs off because mine will not come off just by pulling?

[RayJones]

This picture that I just took was the clearest way I can think of explaining it.lol

This picture tells it all.

The black mains lead in the US is ACTIVE.

Your CRO's alligator clip is EARTH.

You have most likely fused a part of the earth system within your CRO, most likely candidate is a PCB trace, perhaps close to the BNC connections given you now see a distorted trace (lack of proper earth when measuring a signal).

With everything off the mains, including the CRO itself, check for continuity from the mains lead's earth to the BNC outer.
Also check the probe is OK with your multimeter.

I think you must have learnt by now to treat the mains with respect.
You do realise that 120VAC is actually root 2 higher peak to peak?
This is getting damn close to the max spec for your CRO's input.

Also standard CRO leads on mains - ummm - I wouldn't do it.

[jinuq]

Like RayJones said it may be trace. Which mean you will have to check all the the PCB trace along the input path for defects.

[Simon]

Black is Hot.
White is Neutral.
Green is Ground(Earth) could also have a yellow stripe.

I would say unplug it from everything take the cover off(don't take any of the pcbs off) provided it is not under warranty and look for anything that looks burnt, broken or down right blown up. Take some picture if you find anything like the above. Also watch the HIGH VOLTAGE CAPS etc, if it is a crt style unit. They can KILL!!

sounds like a plan but yes the CRT scopes will have 5000 - 15000 volts stored in them for a bit so be careful