Oscilloscope Ground Issue


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Oscilloscope Ground Issue

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rogersk8ter
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Nov 18, 2006, 1:22 AM

Post #1 of 14 (1943 views)
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Oscilloscope Ground Issue	Can't Post

Im trying to scope a TV signal. Where should I ground the probe's
ground? Over the common ground on the PCB board? Isn't that a floating
ground?

There seems to be 30V ac with reference to the scope ground when I use
a VOM to measure voltage. Will it blow or damage the scope? Anyway to
prevent sending live current down the scope ground and blowing it up?
Thanks.

MOtvGuy
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Nov 18, 2006, 2:50 AM

Post #2 of 14 (1937 views)
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Re: [rogersk8ter] Oscilloscope Ground Issue [In reply to]	Can't Post

Most TV sets have both a hot and cold ground. You have to determine the correct ground by looking at the service manual. Generally, I attach my ground cable to the tuner shield or one of the dag springs on the CRT if I'm measuring a cold ground source. Those are most normally on the secondary side of a switching power supply. The primary side of a switching power supply is a hot ground.

Note: Some chassis, Panasonic and RCA are a couple examples, have hot grounds used in other parts of the set. Check your service manual if you're unsure.

stevestar999
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Nov 19, 2006, 7:29 PM

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Re: [MOtvGuy] Oscilloscope Ground Issue [In reply to]	Can't Post

You might want to cut off the ground plug on the AC plug of your scope.
I blew the hell out of a set with a floating ground when I connected my B&K's ground lead to the HOT side of the set. How I just ground to the Hot or Cold side depending what my working on. Works great, no more blown diodes and fried print.

MOtvGuy
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Nov 19, 2006, 10:55 PM

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Re: [stevestar999] Oscilloscope Ground Issue [In reply to]	Can't Post

You should be working with both the TV and Scope on an isolation transformer.

rrobor
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Nov 20, 2006, 1:03 AM

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Re: [MOtvGuy] Oscilloscope Ground Issue [In reply to]	Can't Post

No this is incorrect. If the phase of your isolating transformers are in reverse, the potential difference can be such that you explode things. So do not use 2 separate isolating transformers. If you have a powerful enough one to run both units OK or if you can be sure you have the phase the same in both secondaries OK. If not TV only on isolating transformer.

MOtvGuy
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Nov 20, 2006, 12:23 PM

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Re: [rrobor] Oscilloscope Ground Issue [In reply to]	Can't Post

I don't use seperate Isolation transformers, one for each. I use a single isolation transformer and have the set and scope hooked up to it. If you've got a scope that's incorrectly wired internally you need to correct that.

rogersk8ter
New User

Nov 20, 2006, 6:00 PM

Post #7 of 14 (1905 views)
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Re: [MOtvGuy] Oscilloscope Ground Issue [In reply to]	Can't Post

Thanks for the replies. Many would advise me to use an isolation trans. As what Steve mentioned, cutting the ground of the scope may be a good idea. I just have to make sure I do not touch the scope metal body just in case.

rrobor
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Nov 21, 2006, 7:22 AM

Post #8 of 14 (1898 views)
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Re: [rogersk8ter] Oscilloscope Ground Issue [In reply to]	Can't Post

Roger my advice is get an isolating transformer. I use one that is out of an old TV. If you do this you can connect a loop from the power supply earth to the chassis earth without any problem as to where actual earth is. Your life is worth more than a transformer. OK, You only may be fixing your own. Then why have a CRO. I work alone in my workshop, I have an earth leak trip, I use 2 transformers and yes this is against my own advice, but they are in phase as they are looped in a common config. If you fix for a living, to kill yourself on a job is stupid, add a few buck per job and figure out your method of being safe. We all make mistakes, to allow those to be lethal is to deprive your family of a future with you

rogersk8ter
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Nov 21, 2006, 5:11 PM

Post #9 of 14 (1895 views)
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Re: [rrobor] Oscilloscope Ground Issue [In reply to]	Can't Post

Thanks robor and all for the kind advise. Will surely respect that. Does it mean that if I used an iso trans for the Tv, I can connect my CRO probe ground lead to the TV circuit without worrying damaging the CRO. Any other precaution? Thanks.

rrobor
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Nov 22, 2006, 10:03 AM

Post #10 of 14 (1889 views)
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Re: [rogersk8ter] Oscilloscope Ground Issue [In reply to]	Can't Post

No there is no common path as such all is safe. I avoid the power supply and advise you to do the same. On the primary there are few issues, all best solved without being powered up. The new sets using fet choppers, any messing around in the gate circuit can blow the lot up. I have yet to find a faulty Fet that bled the mains filter so check without power the DC on the mains filter, fully charged Fet OK look for other issues. Never check the FET without checking for DC on the filter. If you do the fet is faulty and needs replacing. IE check for DC on the filter, if charged DISCHARGE IT before you check elsewhere..

MOtvGuy
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Nov 22, 2006, 11:49 AM

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Re: [rogersk8ter] Oscilloscope Ground Issue [In reply to]	Can't Post

In Reply To

You're really making this into something bigger than what it is. I've been a bench tech in the TV service industry for 32 years. When I started I was working on primarily vacuum tube TV sets. In all those years I've never had an issue of exploding TV sets or electric shock due to bad grounding. If such an event were to happen it would be due to a incorrectly wired scope or TV set on the AC side of the equation. I have seen instances of customers hooking incorrectly phased equipment up to the inputs of a TV and blowing ground foil off boards but again, something in the chain of equipment was incorrectly wired.

stevestar99
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Nov 22, 2006, 12:00 PM

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Re: [rrobor] Oscilloscope Ground Issue [In reply to]	Can't Post

Yes, SAFETY FIRST. That's why I don't want anything around me that has a earth ground on it. I've been the service "biz" for over 30 years and I can't count how many times I've been "lit up" from reverse polarity and my test equipment being grounded. If you eliminate everything at could possible be earth ground, then you are as safe as a bird sitting on a 440 line. The best safety rule is the "one hand rule". Never stick your two hands in side a set. Never stand in puddle of water when your working. NEVER TRUST CABLE COMPANIES TO HAVE THEIR GROUNDS AT THE SAME GROUND POTENTIAL AS THE POWER COMPANY! That might be why you've got that 30 volt ripple.

rrobor
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Nov 24, 2006, 11:34 AM

Post #13 of 14 (1867 views)
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Re: [stevestar99] Oscilloscope Ground Issue [In reply to]	Can't Post

I have to agree that its harder today. I worked with 30 or so others and we all knew how to pull someone off the mains if need be. Now Im a one man band, I use earth leak detectors and isolating transformers. If you fix for a dollar, sooner or later you will make a mistake. You have to work on the knowledge that it will happen and make that mistake not lethal. I have to go into my house tell my wife Im OK and reset all the clocks each time I do.

rogersk8ter
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Nov 24, 2006, 3:58 PM

Post #14 of 14 (1862 views)
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Re: [rrobor] Oscilloscope Ground Issue [In reply to]	Can't Post

Nice to have all the senior techs here giving advise. Will get a Iso Trans soon. Happy trouble shootings guys. Good luck.

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