Post by thread:[EE]: Re: Question for an Electrical engineer

>Why does my monitor go "Boink....Crackle..crackle" when I push the >little button with the green light next to it? I know a bit about crt's >and high voltage, but what physically happens that causes the noise? Bbob, Mega-ish energy and strong magnetic fields. This is (presumably) the "degaussing" button. Mains is applied to a coil around the CRT rim which generates a very healthy 50 Hz (possibly 60 Hz if you don't live in God's own country) magnetic field. A positive temperature coefficient resistor is in series with the coil and as it heats up with the large current flow it increases in resistance thereby decreasing the current and thereby also the field. The field therefore starts at a very high value and decays away nicely to almost nothing in a small time. The initial current is largish. Why do this?, you may well ask. If the screen is subject to stray magnetic fields it may acquire random localised magnetisations which will deflect the electron beam in an equally random manner giving an interesting picture. The strong field overrides these small error magnetisations and force everything that is easily magnetised to march in step with the changing field.As the field decays all the camp followers follow less and less until the field gently falls below the level at which they will continue to be moved. Ultimately they all settle gracefully into place in a randomish pattern thereby removing the stray magnetisation. Modern TVs and monitors often do this automatically at turn on. In the good-old-days you had to use a "degaussing wand" which was a portable AC magnet which you waved at offending spots while slowly pulling it away to reduce the field. TVs could not be moved without risking needing degaussing. Woe betide the person who eg turned off a vacuum cleaner adjacent to the screen :-). A permanent magnet brought near a modern screen will demonstrate the effect. This will not usually do any damage. BUT Murphy says YMMV. Russell -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Just something that others may not be aware of:- Some TV's do not in fact do a degauss at turn-on. They only do it when initially plugged into a wall socket. We had the ocurrance of gradually diminishing colour purity despite many turn off / turn on cycles. (Front Panel Switch - not just remote standby cycles either). Eventually I asked a repair guy and he advised unplugging the set completely for 20 mins or so (to allow the PTC to cool). Problem solved after 2 cycles. I guess the mains switches aren't rated for the degaussing current. I would guess the crackle is the increase in EHT capacitively coupling to dust etc in the back of the set and causing mini-arcs until it stabilises. Never really thought too much about that one! Richard P >Why does my monitor go "Boink....Crackle..crackle" when I push the >little button with the green light next to it? I know a bit about crt's >and high voltage, but what physically happens that causes the noise? Bbob, Mega-ish energy and strong magnetic fields. This is (presumably) the "degaussing" button. Mains is applied to a coil around the CRT rim which generates a very healthy 50 Hz (possibly 60 Hz if you don't live in God's own country) magnetic field. A positive temperature coefficient resistor is in series with the coil and as it heats up with the large current flow it increases in resistance thereby decreasing the current and thereby also the field. The field therefore starts at a very high value and decays away nicely to almost nothing in a small time. The initial current is largish. Why do this?, you may well ask. If the screen is subject to stray magnetic fields it may acquire random localised magnetisations which will deflect the electron beam in an equally random manner giving an interesting picture. The strong field overrides these small error magnetisations and force everything that is easily magnetised to march in step with the changing field.As the field decays all the camp followers follow less and less until the field gently falls below the level at which they will continue to be moved. Ultimately they all settle gracefully into place in a randomish pattern thereby removing the stray magnetisation. Modern TVs and monitors often do this automatically at turn on. In the good-old-days you had to use a "degaussing wand" which was a portable AC magnet which you waved at offending spots while slowly pulling it away to reduce the field. TVs could not be moved without risking needing degaussing. Woe betide the person who eg turned off a vacuum cleaner adjacent to the screen :-). A permanent magnet brought near a modern screen will demonstrate the effect. This will not usually do any damage. BUT Murphy says YMMV. Russell -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> A permanent magnet brought near a modern screen will demonstrate the > effect. This will not usually do any damage. > BUT > Murphy says YMMV. > > Russell I recently stripped down monitors of various make and age. I'm keeping the very fine mesh masks for some reason that will become apparent to me one day. They are magnetic and so I guess could be permanently distorted by a strong magnetic field. My speakers are next to the monitor and seem to do no harm there, but I wouldn't dream of putting them in front where they could. My mother's TV has a pronounced and very long- lived pink area in the top corner which I've tried to remove. And bless the flatmate who once plonked a bare big speaker on top of my TV once. It was unwatchable after that. Admittedly it was a 15" 200W speaker. More than adequate a magnet to crush a flatmate with. Getting back to the short exchange about parallel importing, one of the comments made by Philips NZ was that the TVs they sold were "set up" for Southern Hemisphere magnetic fields and that the cheaper discount store Philips brand weren't, and as such could be liable to bad pictures and/or expensive "tuning". Is that hooey ? -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Jinx, > And bless > the flatmate who once plonked a bare big speaker on top of my TV once. > It was unwatchable after that. A pass or two of a TV serviceman's degausing wand should have fixed that.... > one of the > comments made by Philips NZ was that the TVs they sold were "set up" > for Southern Hemisphere magnetic fields and that the cheaper discount > store Philips brand weren't, and as such could be liable to bad pictures > and/or expensive "tuning". Is that hooey ? Nope. Many monitors imported to the Southern hemisphere have a decided keystone tilt unless they are re-aligned..... ....................... Zim -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

On Wed, 1 Nov 2000, Graeme Zimmer wrote: > Jinx, > > > And bless > > the flatmate who once plonked a bare big speaker on top of my TV once. > > It was unwatchable after that. > > A pass or two of a TV serviceman's degausing wand should have fixed that.... I use a cheap Radio Shack mag tape degausser -- 110V electromagnet with a handle and momentary switch, basically. Start close to the tube, wave it around while slowly backing off. Works every time. Dale --- The most exciting phrase to hear in science, the one that heralds new discoveries, is not "Eureka!" (I found it!) but "That's funny ..." -- Isaac Asimov -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use spam_OUTlistservTakeThisOuTmitvma.mit.edu?body=SET%20PICList%20DIGEST

I know a guy who can (roughly) degauss monitors with a _permanent_ magnet. We waves it around in circles of varying radius and varying distance from the monitor. He also varies the aspect angle (so he may wave it in a circle around the side of the monitor). He is able to take the magnet, severely magnetize the monitor, and then get it almost back to normal. Sean At 11:45 PM 10/31/00 -0600, you wrote: {Quote hidden}>I use a cheap Radio Shack mag tape degausser -- 110V electromagnet with a >handle and momentary switch, basically. Start close to the tube, wave it >around while slowly backing off. Works every time. > >Dale >--- >The most exciting phrase to hear in science, the one that heralds new >discoveries, is not "Eureka!" (I found it!) but "That's funny ..." > -- Isaac Asimov > >-- >http://www.piclist.com#nomail Going offline? Don't AutoReply us! >use .....listservKILLspam@spam@mitvma.mit.edu?body=SET%20PICList%20DIGEST -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use listservKILLspammitvma.mit.edu?body=SET%20PICList%20DIGEST

In message <000e01c04397$73915180$65c8a7cb@joe>, Jinx <.....joecolquittKILLspam.....CLEAR.NET.NZ> writes >> A permanent magnet brought near a modern screen will demonstrate the >> effect. This will not usually do any damage. >> BUT >> Murphy says YMMV. >> >> Russell > >I recently stripped down monitors of various make and age. I'm keeping >the very fine mesh masks for some reason that will become apparent to >me one day. They are magnetic and so I guess could be permanently >distorted by a strong magnetic field. My speakers are next to the monitor >and seem to do no harm there, but I wouldn't dream of putting them in >front where they could. My mother's TV has a pronounced and very long- >lived pink area in the top corner which I've tried to remove. And bless >the flatmate who once plonked a bare big speaker on top of my TV once. >It was unwatchable after that. Admittedly it was a 15" 200W speaker. More >than adequate a magnet to crush a flatmate with. I don't think you can permanently damage a CRT with a magnet, everytime a set is switched on from cold degaussing coils around the CRT de- magnetise it. However, if the shadow mask has been subjected to an extremely high field the inbuilt coils might not have sufficient power to correct it. In this case a TV engineer would be able to effect a cure using an external degaussing coil, it's a common procedure. Speakers for use with a TV usually have shielded magnets, which is why special speakers are used for the centre channel in Pro-Logic systems. >Getting back to the short exchange about parallel importing, one of the >comments made by Philips NZ was that the TVs they sold were "set up" >for Southern Hemisphere magnetic fields and that the cheaper discount >store Philips brand weren't, and as such could be liable to bad pictures >and/or expensive "tuning". Is that hooey ? Yes, it's a load of crap!. -- Nigel. /--------------------------------------------------------------\ | Nigel Goodwin | Internet : EraseMEnigelgspam_OUTTakeThisOuTlpilsley.co.uk | | Lower Pilsley | Web Page : http://www.lpilsley.co.uk | | Chesterfield | Official site for Shin Ki and New Spirit | | England | Ju Jitsu | \--------------------------------------------------------------/ -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use listservspam_OUTmitvma.mit.edu?body=SET%20PICList%20DIGEST

In message <@spam@4.3.2.7.2.20001101005209.00ac9b60KILLspampostoffice2.mail.cornell.e> du>, Sean H. Breheny <KILLspamshb7KILLspamCORNELL.EDU> writes >I know a guy who can (roughly) degauss monitors with a _permanent_ magnet. >We waves it around in circles of varying radius and varying distance from >the monitor. He also varies the aspect angle (so he may wave it in a circle >around the side of the monitor). He is able to take the magnet, severely >magnetize the monitor, and then get it almost back to normal. 'Almost' is the key word, to de-magnetise a CRT (or anything else really) you need a slowly decreasing AC magnetic field. This is achieved inside the set by feeding the degaussing coils from mains via a posistor, which is a combination of negative and positive temperature coefficient thermistors. In the handheld degaussing coils it's done my slowly moving the coil (fed from AC mains) away from the tube. -- Nigel. /--------------------------------------------------------------\ | Nigel Goodwin | Internet : RemoveMEnigelgTakeThisOuTlpilsley.co.uk | | Lower Pilsley | Web Page : http://www.lpilsley.co.uk | | Chesterfield | Official site for Shin Ki and New Spirit | | England | Ju Jitsu | \--------------------------------------------------------------/ -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use spamBeGonelistservspamBeGonemitvma.mit.edu?body=SET%20PICList%20DIGEST

Dale Botkin wrote: > > On Wed, 1 Nov 2000, Graeme Zimmer wrote: > > > Jinx, > > > > > And bless > > > the flatmate who once plonked a bare big speaker on top of my TV once. > > > It was unwatchable after that. > > > > A pass or two of a TV serviceman's degausing wand should have fixed that.... > > I use a cheap Radio Shack mag tape degausser -- 110V electromagnet with a > handle and momentary switch, basically. Start close to the tube, wave it > around while slowly backing off. Works every time. Yeah, I have one too. I also have an expensive "professional" type degauss wand, the cheap radio shack one is MUCH more powerful. Have to be a bit careful you dont get too close to the picture tube or you can wreck the shadow mask. Very nasty - like buy a new picture tube! They are superb for bulk erasing floppy disks too, I have even had floppies that had bad sectors for months that could not be fixed, actually come good perfectly after a severe magnetic erasing! :o) -Roman -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use TakeThisOuTlistservEraseMEspam_OUTmitvma.mit.edu?body=SET%20PICList%20DIGEST

Sean H. Breheny wrote: > > I know a guy who can (roughly) degauss monitors with a _permanent_ magnet. > We waves it around in circles of varying radius and varying distance from > the monitor. He also varies the aspect angle (so he may wave it in a circle > around the side of the monitor). He is able to take the magnet, severely > magnetize the monitor, and then get it almost back to normal. > > Sean Yeah, that's a party trick used to amuse TV apprentices. The "small magnet on a stick" things do it better, the ones you use to get a screw out of the tight spot it fell into. -Roman -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use RemoveMElistservTakeThisOuTmitvma.mit.edu?body=SET%20PICList%20DIGEST

From: "Dale Botkin" <EraseMEdaleBOTKIN.ORG> To: <RemoveMEPICLISTEraseMEEraseMEMITVMA.MIT.EDU> Sent: Wednesday, November 01, 2000 5:45 AM Subject: Re: [EE]: Re: Question for an Electrical engineer > On Wed, 1 Nov 2000, Graeme Zimmer wrote: > > > Jinx, > > > > > And bless > > > the flatmate who once plonked a bare big speaker on top of my TV once. > > > It was unwatchable after that. > > > > A pass or two of a TV serviceman's degausing wand should have fixed that.... > > I use a cheap Radio Shack mag tape degausser -- 110V electromagnet with a > handle and momentary switch, basically. Start close to the tube, wave it > around while slowly backing off. Works every time. This works very well, but don't get *too* close to the screen. A friend trashed his TV by playing around "making pretty colours" on his screen with a permanent magnet. No amount of degaussing restored the correct display so presumably he managed to distort the shadowmask with his antics. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use RemoveMElistservspam_OUTKILLspammitvma.mit.edu?body=SET%20PICList%20DIGEST

On Wed, 1 Nov 2000, Sean H. Breheny wrote: > I know a guy who can (roughly) degauss monitors with a _permanent_ magnet. > We waves it around in circles of varying radius and varying distance from > the monitor. He also varies the aspect angle (so he may wave it in a circle > around the side of the monitor). He is able to take the magnet, severely > magnetize the monitor, and then get it almost back to normal. I've done that too, with good results, but the degausser is a lot faster and easier. {Quote hidden}> > Sean > > At 11:45 PM 10/31/00 -0600, you wrote: > > >I use a cheap Radio Shack mag tape degausser -- 110V electromagnet with a > >handle and momentary switch, basically. Start close to the tube, wave it > >around while slowly backing off. Works every time. > > > >Dale > >--- > >The most exciting phrase to hear in science, the one that heralds new > >discoveries, is not "Eureka!" (I found it!) but "That's funny ..." > > -- Isaac Asimov > > > >-- > >http://www.piclist.com#nomail Going offline? Don't AutoReply us! > >use RemoveMElistservTakeThisOuTspammitvma.mit.edu?body=SET%20PICList%20DIGEST > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > use EraseMElistservspamspamBeGonemitvma.mit.edu?body=SET%20PICList%20DIGEST > > > > --- The most exciting phrase to hear in science, the one that heralds new discoveries, is not "Eureka!" (I found it!) but "That's funny ..." -- Isaac Asimov -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use RemoveMElistservKILLspammitvma.mit.edu?body=SET%20PICList%20DIGEST

And here's the 'your mileage may vary" confirmation :-) !!!!!!!! -----Original Message----- From: Andy Howard <musicaSTOPspamspam_OUTUKONLINE.CO.UK> To: spamBeGonePICLISTSTOPspamEraseMEMITVMA.MIT.EDU <KILLspamPICLISTspamBeGoneMITVMA.MIT.EDU> Date: Wednesday, 1 November 2000 23:24 Subject: Re: [EE]: Re: Question for an Electrical engineer {Quote hidden}>From: "Dale Botkin" <EraseMEdaleEraseMEBOTKIN.ORG> >To: <@spam@PICLIST@spam@spam_OUTMITVMA.MIT.EDU> >Sent: Wednesday, November 01, 2000 5:45 AM >Subject: Re: [EE]: Re: Question for an Electrical engineer > > >> On Wed, 1 Nov 2000, Graeme Zimmer wrote: >> >> > Jinx, >> > >> > > And bless >> > > the flatmate who once plonked a bare big speaker on top of my TV once. {Quote hidden}>> > > It was unwatchable after that. >> > >> > A pass or two of a TV serviceman's degausing wand should have fixed >that.... >> >> I use a cheap Radio Shack mag tape degausser -- 110V electromagnet with a >> handle and momentary switch, basically. Start close to the tube, wave it >> around while slowly backing off. Works every time. > > >This works very well, but don't get *too* close to the screen. A friend >trashed his TV by playing around "making pretty colours" on his screen with >a permanent magnet. No amount of degaussing restored the correct display so >presumably he managed to distort the shadowmask with his antics. > >-- >http://www.piclist.com#nomail Going offline? Don't AutoReply us! >use spamBeGonelistservKILLspammitvma.mit.edu?body=SET%20PICList%20DIGEST > > > > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use .....listservspam_OUTmitvma.mit.edu?body=SET%20PICList%20DIGEST

>I know a guy who can (roughly) degauss monitors with a _permanent_ >magnet. We waves it around in circles of varying radius and varying >distance from the monitor. He also varies the aspect angle (so he may >wave it in a circle around the side of the monitor). He is able to take >the magnet, severely magnetize the monitor, and then get it almost back >to normal. > >Sean A trick: place a silicon diode in series with the power lead of a monitor or TV with switching power supply. The device will magnetize its screen in an unthinkable way at the first turn-on. This was discovered when someone decided that SMPSU powered monitors and computers could be run safely on 240V DC ;-) He forgot about the degaussing problem (which needs ac). I have seen such a monitor and I can assure you that it is VERY HARD to remove the magnetization and restore the screen to normal. Peter -- http://www.piclist.com hint: PICList Posts must start with ONE topic: "[PIC]:","[SX]:","[AVR]:" =uP ONLY! "[EE]:","[OT]:" =Other "[BUY]:","[AD]:" =Ads