Did I break my Oscilloscope? - Page 1
Forum section: electronics / projects

author: tayexdrums
author_rank: Contributor
timestamp: August 14, 2010, 03:41:53 pm
content: 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?

author: Simon
author_rank: Unknown
timestamp: August 14, 2010, 03:49:08 pm
content: 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 probecould 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 ?

author: tayexdrums
author_rank: Contributor
timestamp: August 14, 2010, 03:55:22 pm
content: 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.pdfIts on page 8

author: Simon
author_rank: Unknown
timestamp: August 14, 2010, 04:02:19 pm
content: you think? this is the mains your playing with ! treat it like a loaded machine gun !

author: tayexdrums
author_rank: Contributor
timestamp: August 14, 2010, 04:06:37 pm
content: 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?

author: Simon
author_rank: Unknown
timestamp: August 14, 2010, 04:18:53 pm
content: 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

author: Unknown
author_rank: Unknown
timestamp: August 14, 2010, 04:58:23 pm
content: 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 onwww.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.

author: tayexdrums
author_rank: Contributor
timestamp: August 14, 2010, 05:47:57 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: August 14, 2010, 05:55:07 pm
content: 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

author: tayexdrums
author_rank: Contributor
timestamp: August 14, 2010, 06:12:32 pm
content: This picture that I just took was the clearest way I can think of explaining it.lol

author: Simon
author_rank: Unknown
timestamp: August 14, 2010, 06:40:56 pm
content: 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)

author: tayexdrums
author_rank: Contributor
timestamp: August 14, 2010, 06:52:57 pm
content: 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, heheI connected the ground clip to the live(black) and the probe tip to the neutral(white)

author: jinuq
author_rank: Contributor
timestamp: August 14, 2010, 06:53:21 pm
content: 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!!

author: tayexdrums
author_rank: Contributor
timestamp: August 14, 2010, 07:03:10 pm
content: Okay, I'll do that now... Give me about 10 minutes

author: tayexdrums
author_rank: Contributor
timestamp: August 14, 2010, 07:23:25 pm
content: I looked carefully for blown caps and couldn't find any.I took a picture of it just for reference anyways

author: jinuq
author_rank: Contributor
timestamp: August 14, 2010, 07:41:31 pm
content: What make and model scope is it?

author: tayexdrums
author_rank: Contributor
timestamp: August 14, 2010, 07:44:35 pm
content: It is an EZ-Digital (formerly LG Precision) OS-5020 20 MHz Scope

author: tayexdrums
author_rank: Contributor
timestamp: August 14, 2010, 07:53:13 pm
content: More reference Pictures

author: jinuq
author_rank: Contributor
timestamp: August 14, 2010, 08:16:45 pm
content: 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.

author: tayexdrums
author_rank: Contributor
timestamp: August 14, 2010, 08:25:57 pm
content: Okay I will look carefully for any thing that looks blown or messed up. Do I need to take the front faceplate off?

author: jinuq
author_rank: Contributor
timestamp: August 14, 2010, 09:20:05 pm
content: You may or may not, I can't say for sure from the photos. You will need to take the knobs off.

author: tayexdrums
author_rank: Contributor
timestamp: August 14, 2010, 09:22:57 pm
content: 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?

author: RayJones
author_rank: Frequent Contributor
timestamp: August 14, 2010, 09:58:01 pm
content: Quote from: tayexdrums on August 14, 2010, 06:12:32 pmThis 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.

author: jinuq
author_rank: Contributor
timestamp: August 14, 2010, 10:21:26 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: August 14, 2010, 10:59:12 pm
content: Quote from: jinuq on August 14, 2010, 06:53:21 pmsounds like a plan but yes the CRT scopes will have 5000 - 15000 volts stored in them for a bit so be careful

author: jahonen
author_rank: Super Contributor
timestamp: August 15, 2010, 06:00:44 am
content: Quote from: tayexdrums on August 14, 2010, 03:41:53 pmTo me that sounds like you'll just need to adjust the trace rotation to put the trace straight again. Try that before trying to fix anything else. Could you please post a picture what the trace looks like now?BTW, mains should not be measured with an oscilloscope ever without using an appropriatedifferential probe. Reason for this is that scope chassis and input jacks are connected to the protective earth terminal on scope power plug. It is too easy to make short circuit.Regards,Janne

author: cybergibbons
author_rank: Frequent Contributor
timestamp: August 15, 2010, 06:00:54 am
content: There's a few lessons to be learnt here:1. You can't assume that line and neutral are black and white - two pin plugs, figure-of-eight connectors into AV equipment, wiring mistakes etc. mean that they can be swapped round.2. You always treat line and neutral with the same respect. In fact, until you know green is ground, treat that with respect too.3. Oscilloscopes are not normally isolated - when you connect the ground clip to line, the case and everything else would have likely been connected to line.

author: Simon
author_rank: Unknown
timestamp: August 15, 2010, 07:17:51 am
content: that's what I was just thinking, if the scopes chassis is earthed and you connected the earth pin to live your causing a direct short circuit ?

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 15, 2010, 12:23:31 pm
content: Quote from: jinuq on August 14, 2010, 06:53:21 pmwe use brown (life), blue (neutral), green (earth). but when its black and white... White is hot, Black is neutral.

author: safarir
author_rank: Regular Contributor
timestamp: August 15, 2010, 02:03:31 pm
content: Wikipedia have a great list of all the color code:http://en.wikipedia.org/wiki/Electrical_wiring#Color_code

author: saturation
author_rank: Super Contributor
timestamp: August 15, 2010, 04:11:16 pm
content: Sorry to read of this.Its not a good idea to measure line voltage with any oscilloscope, except via a HV probe. Its more than just the proper lines being used, good scope are very sensitive and most likely will not take transients on line voltage, transients that can be generated as simply as static atop the line voltage. I think the Rigol 1052E is only CAT I, not more than II rated.An HV probe will reduce line voltage 100:1 or more, providing a wide safety margin for delicate instruments. After all, what you want a scope to do is read the waveform, not the raw voltage, you can use a cheaper DMM for that.

author: jahonen
author_rank: Super Contributor
timestamp: August 15, 2010, 05:19:44 pm
content: Quote from: saturation on August 15, 2010, 04:11:16 pmEven HV probe is not sufficient, due to common ground. Thus if you accidentally connect the scope ground to live, short circuit results. Or if scope is itself floating, it creates a great shock hazard. There are scopes with isolated channels likeTektronix TPS2000-seriesto safely measure the mains directly. Most scopes require the use of the differential probe due to safety issues.Regards,Janne

author: tayexdrums
author_rank: Contributor
timestamp: August 15, 2010, 05:28:57 pm
content: I appreciate the feedback from everyone. I have spent at least a couple of hours looking carefully at the PCBs. The traces around the BNC connector all look good with no signs of charring. So none of the traces are bad. Now I'm thinking its either a bad component that didn't mess up the solder pad which would make it hard to detect or the CRT is screwed up.Jahonen: I have tried the trace rotation but the problem is that I need to bring the MIDDLE of the trace down, not rotate the trace altogether. Also, i've tried taking a picture of the trace but my camera won't pick up the minute detail.=/

author: RayJones
author_rank: Frequent Contributor
timestamp: August 15, 2010, 07:58:28 pm
content: A good degaussing comes to mind, can't hurt to try....

author: saturation
author_rank: Super Contributor
timestamp: August 15, 2010, 08:33:33 pm
content: Thanks jahonen, you are right. It can be done under certain circumstances but its best to get a scope designed for line use. An app note about line measurements with general DSO.http://www.tek.com/Measurement/App_Notes/tds220-p5100/eng/40W_11342_0.pdfQuote from: jahonen on August 15, 2010, 05:19:44 pm

author: tayexdrums
author_rank: Contributor
timestamp: August 15, 2010, 09:29:13 pm
content: Degaussing? Sounds good to me!How do you go about doing that to an oscilloscope CRT?

author: abbtech
author_rank: Contributor
timestamp: August 15, 2010, 09:38:11 pm
content: Quote from: shafri on August 15, 2010, 12:23:31 pmWow that would really mess me up. Reminds me of a job I was doing where the electricians wired our entire DC system in the plant backwards (red was negative and black was positive). That made troubleshooting a nightmare since you always needed to think backwards.The boss didn't force them to change it to save money...

author: Simon
author_rank: Unknown
timestamp: August 16, 2010, 06:49:32 am
content: Quote from: abbtech on August 15, 2010, 09:38:11 pmWHAT !!!!!!! Just tell me, roughly where do you live - I'll make sure i never set foot there

author: shodan
author_rank: Contributor
timestamp: August 18, 2010, 07:40:17 am
content: oh please, there is not enough energy to kill in any of the caps inside a scopeunless you count having a cardiac arrest for the surprise of being shocked

author: Jamesthresher
author_rank: Newbie
timestamp: August 20, 2010, 01:59:00 pm
content: Quote from: shodan on August 18, 2010, 07:40:17 amI wouldn't risk it, the heart can be stopped with only a small amount of current (in the milliamp's), It isn't worth the risk. I always employ the one hand in the pocket strategy when poking around inside anything like this (I occasionally work with large HF amplifiers where the risk is much higher). At least then if you do get a shock it won't be directly across the chest.

author: Unknown
author_rank: Unknown
timestamp: August 20, 2010, 02:55:26 pm
content: I don't think there's enough charge in the CRT/HV circuit, the main danger there is injuries from the reaction to the shock, like getting the scope on your toes (and hurting your head from banging against the wall that you just destroyed an expensive piece of equipment). I wouldn't be so sure about the mains voltage power supply filter caps (if it's a switcher). But I would just stay clear of both to be safe, no point in taking unnecessary risks.

author: tayexdrums
author_rank: Contributor
timestamp: August 21, 2010, 03:43:25 pm
content: Okay guys, so I crucially analyzed all traces and components and found nothing that is broken. I turned the scope on with the case off of it and voila! the trace was back to normal so I thought I had fixed it somehow. So I everything back together, put the case back on it, turned it on and the trace was once again slightly bent. I am so confused!!

author: jinuq
author_rank: Contributor
timestamp: August 21, 2010, 05:05:29 pm
content: Sounds like the case is connecting two point that shouldn't be. Is there any discoloration on the inside of the case?

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 21, 2010, 08:37:44 pm
content: or any lose solder joint.

author: RayJones
author_rank: Frequent Contributor
timestamp: August 21, 2010, 08:43:10 pm
content: I'll say it again, degauss the CRO.Your lid is probably magnetised.

author: allanw
author_rank: Frequent Contributor
timestamp: August 21, 2010, 09:52:52 pm
content: Take a picture.

author: jinuq
author_rank: Contributor
timestamp: August 22, 2010, 12:37:51 am
content: @RayJones, Makes perfect sense. Completely slipped my mind.

author: RayJones
author_rank: Frequent Contributor
timestamp: August 22, 2010, 12:57:23 am
content: Actually, magnetisation (is that a word?) can be very insidious.I know of a colour TV set that was being used for a long period of time beneath a low metal roof.One day the set was moved, and rotated 90deg to it's usual position and the colour purity went for a row.It ended up being the roof had become magnetised, and over time the set established a normal balance in the environment.Once moved, the magnetic environment was disturbed, and the electron beam in the CRT behaved slightly differently and thus distorted colours.A once over with a de-gaussing wand seen the situation corrected.Back to the CRO, are you certain the curved trace was not always there and the BANG mde you more observant?Or does it change if you rotate the CRO relative to the earth's magnetic field?

author: tayexdrums
author_rank: Contributor
timestamp: August 22, 2010, 10:16:59 pm
content: I checked the case for any signs of discoloration and found none. The scope was sold to me as "brand new and never been used" so how could it have magnetized the case already?Okay, so you're telling me to degauss it. My dad says he has a degaussing gun but its probably from 1980 when he owned a rental business. There is no specific type of degaussing gun/wand that I should use is there?Also, I just bought a canon power shot SX20 IS, so I took a video of the trace and as I type this I am uploading it to youtube. So when its uploaded you can look at it for yourself.

author: DJPhil
author_rank: Frequent Contributor
timestamp: August 22, 2010, 10:51:23 pm
content: Quote from: tayexdrums on August 22, 2010, 10:16:59 pmThere's not much difference between degaussing tools except for size and power. They're basically a coil of wire powered with an AC signal (often line voltage through a transformer) to set up an oscillating magnetic field. You can check the case with a compass to see if it's magnetized, any needle deflection will show what areas are magnetized and how strongly. It's the alignment of the magnetic domains in the metal that constitutes magnetization.To use a degausser just turn it on while it's a few feet away from the item in question and move it close, then away again before turning off the power. This basically scrambles up and randomizes the domains in the magnetic material, which is what you're after. Check with a compass after a pass, and repeat as necessary. Be sure to do this well away from sensitive magnetic media!For a poor man's version you can use the coil that's built in to most CRTs (monitor or TV). Each time they're turned on they degauss with a coil that's wrapped around the big end of the tube. You'd have to hold the case near the face of the monitor, turn it on, and move the case away a couple of feet in the few seconds the coil's active. It works surprisingly well, and was often used by the folks in my father's shop in the military to demagnetize their wristwatches on occasion. Keep in mind that the degaussing coil will have diminished effect if used again quickly, best to wait five or ten minutes. It's often current limited by a thermistor, and letting the thermistor completely cool off will ensure that you get the full effect each time.Hope that helps.

author: tayexdrums
author_rank: Contributor
timestamp: August 23, 2010, 12:05:43 am
content: Here is a video of it...

author: allanw
author_rank: Frequent Contributor
timestamp: August 23, 2010, 12:08:50 am
content: Oh cmon that's barely perceptible! Haha. Nice crisp trace you got there though.

author: DJPhil
author_rank: Frequent Contributor
timestamp: August 23, 2010, 12:44:33 am
content: Quote from: allanw on August 23, 2010, 12:08:50 amI agree, my old 465B is twice that bad, but it's a lot better when it warms up for ten minutes.I can see how it'd be alarming if it wasn't like that before measuring mains, but I don't think you've got much to worry about really.

author: tayexdrums
author_rank: Contributor
timestamp: August 23, 2010, 01:02:30 am
content: Well I think its quite perceptible.lolI'll do the compass test and everything.thanks=]

author: allanw
author_rank: Frequent Contributor
timestamp: August 23, 2010, 03:02:16 am
content: Also try asking here:http://tech.groups.yahoo.com/group/TekScopes/and post a good picture. These guys really know their stuff, even if your scope isn't actually a Tektronix.

author: DJPhil
author_rank: Frequent Contributor
timestamp: August 23, 2010, 05:04:35 am
content: Quote from: tayexdrums on August 23, 2010, 01:02:30 amNo worries, I was just surprised. I expected worse, so it was reassuring to see that worst case you're still doing better than my scope!

author: RayJones
author_rank: Frequent Contributor
timestamp: August 23, 2010, 07:54:55 am
content: LOL, the ubiquitous Rigol 1052E shows more noise than the amount that trace is deflected.I'm trying to understand the problem now.In terms of magnetism effects, you could try the trace at the top and bottom of the screen. There is more chance for magnetic field to impart nonsense with the trace up high.As for degaussing wands, yep anything from the early 19th century would be as good as anything you'd buy these days, the technology is not that complex, just a winding activated by the mains to do as alluded to before, scramble the magnetic field.It is important however to only release power from the wand when it is quite some distance from the CRO.A you tube video of the CRO being degaussed whilst turned on would be much more worthy of distorted traces!

author: nickmorano777
author_rank: Newbie
timestamp: August 25, 2010, 01:02:25 am
content: The way that you connected the probes is adead short!Never connect the ground to a hot of any kind! The ground probe always goes to ground, and never connect the ground to the hot or the neutral.

author: nyo
author_rank: Contributor
timestamp: September 05, 2010, 01:50:12 pm
content: I've done the same thing before while in school. I had a plc training board that I decided to probe the 120v to 24v transformer with a oscilloscope. I did the same thing, I connected the probe ground to hot and the probe tip to ground by mistake, fortunately the plc board had a 5A circuit breaker that tripped before any damage was done, the oscilloscope seemed fine after.