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MOtvGuy
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Nov 18, 2006, 2:50 AM
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Re: [rogersk8ter] Oscilloscope Ground Issue
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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.
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stevestar999
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Nov 19, 2006, 7:29 PM
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Re: [MOtvGuy] Oscilloscope Ground Issue
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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.
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MOtvGuy
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Nov 19, 2006, 10:55 PM
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Re: [stevestar999] Oscilloscope Ground Issue
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You should be working with both the TV and Scope on an isolation transformer.
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rrobor
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Nov 20, 2006, 1:03 AM
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Re: [MOtvGuy] Oscilloscope Ground Issue
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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.
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MOtvGuy
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Nov 20, 2006, 12:23 PM
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Re: [rrobor] Oscilloscope Ground Issue
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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.
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rogersk8ter
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Nov 20, 2006, 6:00 PM
Post #7 of 14
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Re: [MOtvGuy] Oscilloscope Ground Issue
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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.
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rrobor
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Nov 21, 2006, 7:22 AM
Post #8 of 14
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Re: [rogersk8ter] Oscilloscope Ground Issue
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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
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rogersk8ter
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Nov 21, 2006, 5:11 PM
Post #9 of 14
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Re: [rrobor] Oscilloscope Ground Issue
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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.
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rrobor
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Nov 22, 2006, 10:03 AM
Post #10 of 14
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Re: [rogersk8ter] Oscilloscope Ground Issue
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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..
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stevestar99
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Nov 22, 2006, 12:00 PM
Post #12 of 14
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Re: [rrobor] Oscilloscope Ground Issue
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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.
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rrobor
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Nov 24, 2006, 11:34 AM
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Re: [stevestar99] Oscilloscope Ground Issue
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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.
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rogersk8ter
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Nov 24, 2006, 3:58 PM
Post #14 of 14
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Re: [rrobor] Oscilloscope Ground Issue
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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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