Oscilloscope Fluke PM3082 powersupply problem?
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From: Jeff Liebermann on 14 Jun 2010 12:53 On Mon, 14 Jun 2010 09:20:10 -0700, "William Sommerwerck" <grizzledgeezer(a)comcast.net> wrote: >> However, this begs the question of what caused the flames and >> smoke? If the fuse was intact, it's unlikely. If it was an AC power >> line fault of some sort (lightning hit, lost neutral, hi-v distribution >> line falls on power line drop), it would have destroyed other pieces >> of test equipment in the lab. Pardon my suspicious nature, but >> methinks you should run it with the covers off and keep an eye >> on this scope for a while. > >Though Jeff is almost certainly correct, it's possible that the original >"blast" was caused by a screwed-up ground. There would have been some evidence of the fire and smoke if it were a bad ground. I've done it often enough. Fried scope probe leads were common. >I haven't used my scope in a long time because I'm paranoid (probably >without reason) about connecting it to a DUT that is also connected to the >power line. (Go ahead, start throwing garbage at me for my >ignorance/stupidity.) Ignorance is curable. Stupidity is not. Get an isolation xformer and GRI power jack. Use it on whatever you're testing. They have eliminated most of the pyrotechnics for me. Most of my scopes are dual trace analog scopes which offer A-B differential inputs. If you're not sure about the grounding, or working on the AC mains side of the PS, this can be useful. Do *NOT* connect the ground leads of the scope to the DUT. Instead, connect the B channel input to the DUT common, and the A channel input to whatever you're measuring. If you're not sure what you're dealing with, use an isolated DVM to measure the AC and DC potentials before applying the scope. If you have a large bank account, a battery operated Fluke ScopeMeter is isolated from AC power. Always remember that you have only one life to give to your chosen profession. -- Jeff Liebermann jeffl(a)cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
From: powerampfreak on 14 Jun 2010 13:08 On 14 Juni, 18:20, "William Sommerwerck" <grizzledgee...(a)comcast.net> wrote: > > However, this begs the question of what caused the flames and > > smoke? If the fuse was intact, it's unlikely. If it was an AC power > > line fault of some sort (lightning hit, lost neutral, hi-v distribution > > line falls on power line drop), it would have destroyed other pieces > > of test equipment in the lab. Pardon my suspicious nature, but > > methinks you should run it with the covers off and keep an eye > > on this scope for a while. > > Though Jeff is almost certainly correct, it's possible that the original > "blast" was caused by a screwed-up ground. > > I haven't used my scope in a long time because I'm paranoid (probably > without reason) about connecting it to a DUT that is also connected to the > power line. (Go ahead, start throwing garbage at me for my > ignorance/stupidity.) Took out the mains filter. It's a "konfektronics HGN365-2-F1". It measures 5mH through each line and about 100nF across. This seems ok, of course it can have some intermitent problem. It's a special type with 5x20 fuse holder and internal mains switch which the switch on the front of the scope alters, through a metal wire. I consider letting this to be unchanged for the moment. It's still a mystery what caused the original blast. I'll have the scope up for longtime testing and see if it will re- blast. :-O
From: powerampfreak on 14 Jun 2010 13:15 On 14 Juni, 19:08, powerampfreak <powerampfr...(a)hotmail.com> wrote: > On 14 Juni, 18:20, "William Sommerwerck" <grizzledgee...(a)comcast.net> > wrote: > > > > However, this begs the question of what caused the flames and > > > smoke? If the fuse was intact, it's unlikely. If it was an AC power > > > line fault of some sort (lightning hit, lost neutral, hi-v distribution > > > line falls on power line drop), it would have destroyed other pieces > > > of test equipment in the lab. Pardon my suspicious nature, but > > > methinks you should run it with the covers off and keep an eye > > > on this scope for a while. > > > Though Jeff is almost certainly correct, it's possible that the original > > "blast" was caused by a screwed-up ground. > > > I haven't used my scope in a long time because I'm paranoid (probably > > without reason) about connecting it to a DUT that is also connected to the > > power line. (Go ahead, start throwing garbage at me for my > > ignorance/stupidity.) > > Took out the mains filter. It's a "konfektronics HGN365-2-F1". It > measures 5mH through each line and about 100nF across. This seems ok, > of course it can have some intermitent problem. It's a special type > with 5x20 fuse holder and internal mains switch which the switch on > the front of the scope alters, through a metal wire. I consider > letting this to be unchanged for the moment. It's still a mystery what > caused the original blast. > I'll have the scope up for longtime testing and see if it will re- > blast. :-O The powersupply input has no MOV. I thought this from the begining until I realized it was a inrush limiter NTC. Actually, if looking very close to the NTC body, there are a small small mark in the middle, but not a hole. If this was the source of flame I would suspect more evidence.
From: Jeff Liebermann on 14 Jun 2010 13:48 On Mon, 14 Jun 2010 10:15:57 -0700 (PDT), powerampfreak <powerampfreak(a)hotmail.com> wrote: >The powersupply input has no MOV. It's usually on the fuse holder or input filter. The idea is that the MOV should be the first device the power sees after the fuse, but before any circuitry. For power supplies with AC connectors mounted on the PCB, the MOV will of course be mounted on the PCB. >I thought this from the begining >until I realized it was a inrush limiter NTC. >Actually, if looking very close to the NTC body, there are a small >small mark in the middle, but not a hole. >If this was the source of flame I would suspect more evidence. It's probably not the NTC thermistor. It looks very much like an MOV, but goes between the input bridge and the input DC filter caps. <http://www.gesensing.com/downloads/datasheets/920_325a.pdf> If it were smoked, the resistance would be quite high and the scope would not work. However, you might consider scraping the hole with something sharp to see if it can be enlarged. If the outer coating on the TNC themistor collapses into an internal cavity, then there's a problem. Got an IR thermometer? Wave it around the power supply and input section to see if anything is hot and ready to blow. -- Jeff Liebermann jeffl(a)cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
From: powerampfreak on 14 Jun 2010 15:25 On 14 Juni, 19:48, Jeff Liebermann <je...(a)cruzio.com> wrote: > On Mon, 14 Jun 2010 10:15:57 -0700 (PDT), powerampfreak > > <powerampfr...(a)hotmail.com> wrote: > >The powersupply input has no MOV. > > It's usually on the fuse holder or input filter. The idea is that the > MOV should be the first device the power sees after the fuse, but > before any circuitry. For power supplies with AC connectors mounted > on the PCB, the MOV will of course be mounted on the PCB. > > >I thought this from the begining > >until I realized it was a inrush limiter NTC. > >Actually, if looking very close to the NTC body, there are a small > >small mark in the middle, but not a hole. > >If this was the source of flame I would suspect more evidence. > > It's probably not the NTC thermistor. It looks very much like an MOV, > but goes between the input bridge and the input DC filter caps. > <http://www.gesensing.com/downloads/datasheets/920_325a.pdf> > If it were smoked, the resistance would be quite high and the scope > would not work. However, you might consider scraping the hole with > something sharp to see if it can be enlarged. If the outer coating on > the TNC themistor collapses into an internal cavity, then there's a > problem. > > Got an IR thermometer? Wave it around the power supply and input > section to see if anything is hot and ready to blow. > > -- > Jeff Liebermann je...(a)cruzio.com > 150 Felker St #D http://www.LearnByDestroying.com > Santa Cruz CA 95060http://802.11junk.com > Skype: JeffLiebermann AE6KS 831-336-2558 I actually got in touch with the industrial engineer who operated the scope when it failed. He said the scope was NOT under any measurements when the flame came out of the rear! It was just standing switched on and suddenly bang and flame...... It sure looks more and more to be the mains filter, though measures fine.
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