High Voltage EHT Probe - is it safe to use on mains EHT? - UK Vintage Radio Repair and Restoration Discussion Forum

DoctorWho · 27th Feb 2008, 3:21 pm

Along with the recent collection of TV sets I also obtained a boxed "High Voltage EHT Probe" which seems to measure EHT voltages up to a very high voltage (much higher than any of my sets).

It is boxed and looks unused, it looks like you clip an earthing lead on to the chassis and then touch the EHT connection with the probe.

I just wanted to know if this is safe to use with Mains EHT (for that fact is it still safe for use on flyback and RF EHT?).

This would be a very helpful tool to me, but I forst wanted to make sure that it's safe to use on any of the sets I have.

Thank you.

Peter.

Steve_P · 27th Feb 2008, 3:46 pm

Well I wouldn't use it! Mains EHT gives a hell of a whack -and with mains sets its either there of it's not generally speaking.

Look here: http://www.oldtellys.co.uk/otindex.html under 'Line Timebase & EHT' - first item.

We want you here for many years to come!

Cheers,

Steve P.

Mike Phelan · 27th Feb 2008, 5:00 pm

I wouldn't go near EHT with that, Peter! Even less so if it's mains-derived.

I have never found a reason to measure EHT apart from seeing if there is some; the size of the picture and EHT regulation will tell you if it's OK or not, assuming all the other CRT voltages are OK.

DoctorWho · 27th Feb 2008, 5:03 pm

Good points Mike, I tend to use a screwdriver blade to check for flyback EHT, but I just wondered if this would be useful for the mains EHT sets, and of course the flyback sets, but I get the impression that going near mains EHT with this may perhaps not be the best idea...........

magnastat · 27th Feb 2008, 5:19 pm

I would have thought a modern Fluke probe would be a safer bet.
http://www.fluke.co.uk/comx/show_pro...product=VPROBE

ppppenguin · 27th Feb 2008, 5:27 pm

I'm sure that Fluke probe is a good bit of kit but they say:

Quote:

This means the probe can only be used to make measurements on energy limited circuits within equipment. Examples include high voltage within televisions or photo copy machines. DO NOT use this probe to measure high voltages on power distribution systems.

Does mains derived EHT count as energy limited? Yes when compared to 11kV mains but not when compared to flyback EHT.

DoctorWho · 27th Feb 2008, 5:35 pm

Interesting, the general opinion here seems to be not to use it. I guess that's why I asked. It's not the sort of thing you can play with, as it were.

ppppenguin · 27th Feb 2008, 5:45 pm

As a devout coward I would use any reasonable looking probe but connect it while the mains is off and not touch it until the mains is off again. Usual precautions about discharging caps apply too.

DoctorWho · 27th Feb 2008, 5:46 pm

I'd never use a screwdriver with mains EHT, only flyback.

Good point about connecting up the probe whist the set is off, may be worth a try just to see what happens.

I wonder what's inside the unit apart from the probe, meter and earthing lead?

Zelandeth · 27th Feb 2008, 7:33 pm

Generally these consist of a string of very high value high voltage resistors, the meter, and an earthing lead...that's usually about it.

Heatercathodeshort · 27th Feb 2008, 9:31 pm

I don't like the look of that Peter.....I would agree with everything said here! Flyback EHT used with monochrome sets will give you a nasty jolt but is unlikely to kill you.
Mains EHT will deliver considerable power and if arced or shorted can result in fire and destruction of the EHT rectifier and possibly the transformer. Its not usually necessary to actually measure the EHT on a mains derived system. Faults will usually present themselves as blown fuses, shorting, sparks, hissing and bubbling transformers.......A very rough test is to watch the directly heated EHT rectifier valve such as the U16 in a darkened room on switch on. A very faint blue glow will be seen around the anode that will last around half a second as the filament heats and the EHT condenser charges. If you observe this you can be pretty sure that the EHT is present and near enough of the correct voltage. Its a rough guide but effective. Take great care and I mean that.
Regards, John.

ppppenguin · 27th Feb 2008, 9:58 pm

Quote:

Originally Posted by Heatercathodeshort

Take great care and I mean that.

John has obviously lived according to his own advice for he is still with us. Far too many TV service guys died from mains EHT. Because it's not familiar it's perhaps even more dangerous now than when these sets were new. Despite being well aware of the hazards I came close to killing myself back in 1985. I thought the EHT of an HMV900 was turned off and I must have come within half an inch of death several times.

I'll disagree with John on one thing. I like to measure what I'm working on. Maybe it's because my background is design rather than service. If I had a HV scope probe and current probe I'd be looking at the details of line output stages too. I believe that Darius has done this. John, with his vast experience of servicing sets has a very keen appreciation of all the signs that say "EHT is OK" or whatever. The rest of us, mostly less experienced, may be better off with a bit more measurement.

DoctorWho · 27th Feb 2008, 10:47 pm

Thanks for the advice here guys, it's very much appreciated. I think it is categorically stated here that it's not for use.

That having been said, it is no use to me, it's too modern for me personally, so does anyone else here want it, completely free of charge? I don;t want it, so would be happy to pass it on ot anyone else who would like it, drop me a PM if you want it and it's yours free of charge, along with the warnings given in this thread of course.

Thanks again for the info.

Peter.

dominicbeesley · 29th Feb 2008, 12:39 pm

Just out of ignorance, why is mains EHT so much more dangerous than flyback. Is it the increased current that's available?

With my homemade EHT probe I connect to the set and then don't touch again until, after switching off, the EHT has fallen back to less than 500V...Only dared use it on a TV24 so far though. Not sure what the breakdown voltage of a drinking straw is....

Dom

ppppenguin · 29th Feb 2008, 12:52 pm

Quote:

Originally Posted by dominicbeesley

Just out of ignorance, why is mains EHT so much more dangerous than flyback. Is it the increased current that's available?

Absolutely. Old saying: It's the volts that jolts, the mils that kills. It's the current flowing through your body that's dangerous. A flyback EHT system, even on a first generation colour TV can only deliver 1mA or so before the voltage starts to collapse. More or less harmless. Rather less on a monochrome set.

Mains derived EHT will deliver a lot more than that. Maybe 100mA+ continuously, at least for short periods before the rectifer and transformer give up. Except that you're likely to be dead or badly injured first. This is well into the danger zone.

Ordinary mains or HT is actually more dangerous than flyback EHT because of the high current available.

Heatercathodeshort · 29th Feb 2008, 11:03 pm

One of the problems with mains EHT is that it is very easy to come to grief by the power supply chassis actually toppling onto you when your working on the set. The Pye D16T is a nasty example of this due to the fact that the HVR2 EHT rectifier valve stands proud on its extension base and the uninsulated top cap is exposed to catch you. Its all a matter of taking care and keeping one hand behind your back when working on the power chassis. That way you can avoid the nasty 'across the heart' path that would probably be curtains with a potential of several thousand volts.
Another precaution is NEVER have metal angle pieces screwed to the edge of your workbench to protect it......I'll leave you to work that one out. If in doubt, don't do it!
Plenty of advice available from us all here, you only have to ask. Regards, John.

27th Feb 2008, 3:21 pm	#1
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	High Voltage EHT Probe - is it safe to use on mains EHT? Along with the recent collection of TV sets I also obtained a boxed "High Voltage EHT Probe" which seems to measure EHT voltages up to a very high voltage (much higher than any of my sets). It is boxed and looks unused, it looks like you clip an earthing lead on to the chassis and then touch the EHT connection with the probe. I just wanted to know if this is safe to use with Mains EHT (for that fact is it still safe for use on flyback and RF EHT?). This would be a very helpful tool to me, but I forst wanted to make sure that it's safe to use on any of the sets I have. Thank you. Peter. Attached Thumbnails

27th Feb 2008, 3:46 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: High Voltage EHT Probe - is it safe to use on mains EHT? Well I wouldn't use it! Mains EHT gives a hell of a whack -and with mains sets its either there of it's not generally speaking. Look here: http://www.oldtellys.co.uk/otindex.html under 'Line Timebase & EHT' - first item. We want you here for many years to come! Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

27th Feb 2008, 5:00 pm	#3
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: High Voltage EHT Probe - is it safe to use on mains EHT? I wouldn't go near EHT with that, Peter! Even less so if it's mains-derived. I have never found a reason to measure EHT apart from seeing if there is some; the size of the picture and EHT regulation will tell you if it's OK or not, assuming all the other CRT voltages are OK. __________________ Mike.

27th Feb 2008, 5:03 pm	#4
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: High Voltage EHT Probe - is it safe to use on mains EHT? Good points Mike, I tend to use a screwdriver blade to check for flyback EHT, but I just wondered if this would be useful for the mains EHT sets, and of course the flyback sets, but I get the impression that going near mains EHT with this may perhaps not be the best idea...........

27th Feb 2008, 5:19 pm	#5
magnastat Retired Dormant Member Join Date: May 2007 Location: East Sussex, UK. Posts: 40	Re: High Voltage EHT Probe - is it safe to use on mains EHT? I would have thought a modern Fluke probe would be a safer bet. http://www.fluke.co.uk/comx/show_pro...product=VPROBE

27th Feb 2008, 5:35 pm	#7
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: High Voltage EHT Probe - is it safe to use on mains EHT? Interesting, the general opinion here seems to be not to use it. I guess that's why I asked. It's not the sort of thing you can play with, as it were.

27th Feb 2008, 5:45 pm	#8
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: High Voltage EHT Probe - is it safe to use on mains EHT? As a devout coward I would use any reasonable looking probe but connect it while the mains is off and not touch it until the mains is off again. Usual precautions about discharging caps apply too.

27th Feb 2008, 5:46 pm	#9
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: High Voltage EHT Probe - is it safe to use on mains EHT? I'd never use a screwdriver with mains EHT, only flyback. Good point about connecting up the probe whist the set is off, may be worth a try just to see what happens. I wonder what's inside the unit apart from the probe, meter and earthing lead?

27th Feb 2008, 7:33 pm	#10
Zelandeth Heptode Join Date: Jun 2006 Location: Milton Keynes, Buckinghamshire, UK. Posts: 931	Re: High Voltage EHT Probe - is it safe to use on mains EHT? Generally these consist of a string of very high value high voltage resistors, the meter, and an earthing lead...that's usually about it. __________________ Zel's Lair - A random bag of vintage tech, lighting information and other assorted geekery

27th Feb 2008, 9:31 pm	#11
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: High Voltage EHT Probe - is it safe to use on mains EHT? I don't like the look of that Peter.....I would agree with everything said here! Flyback EHT used with monochrome sets will give you a nasty jolt but is unlikely to kill you. Mains EHT will deliver considerable power and if arced or shorted can result in fire and destruction of the EHT rectifier and possibly the transformer. Its not usually necessary to actually measure the EHT on a mains derived system. Faults will usually present themselves as blown fuses, shorting, sparks, hissing and bubbling transformers.......A very rough test is to watch the directly heated EHT rectifier valve such as the U16 in a darkened room on switch on. A very faint blue glow will be seen around the anode that will last around half a second as the filament heats and the EHT condenser charges. If you observe this you can be pretty sure that the EHT is present and near enough of the correct voltage. Its a rough guide but effective. Take great care and I mean that. Regards, John.

27th Feb 2008, 10:47 pm	#13
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: High Voltage EHT Probe - is it safe to use on mains EHT? Thanks for the advice here guys, it's very much appreciated. I think it is categorically stated here that it's not for use. That having been said, it is no use to me, it's too modern for me personally, so does anyone else here want it, completely free of charge? I don;t want it, so would be happy to pass it on ot anyone else who would like it, drop me a PM if you want it and it's yours free of charge, along with the warnings given in this thread of course. Thanks again for the info. Peter.

29th Feb 2008, 12:39 pm	#14
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	Re: High Voltage EHT Probe - is it safe to use on mains EHT? Just out of ignorance, why is mains EHT so much more dangerous than flyback. Is it the increased current that's available? With my homemade EHT probe I connect to the set and then don't touch again until, after switching off, the EHT has fallen back to less than 500V...Only dared use it on a TV24 so far though. Not sure what the breakdown voltage of a drinking straw is.... Dom

29th Feb 2008, 11:03 pm	#16
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: High Voltage EHT Probe - is it safe to use on mains EHT? One of the problems with mains EHT is that it is very easy to come to grief by the power supply chassis actually toppling onto you when your working on the set. The Pye D16T is a nasty example of this due to the fact that the HVR2 EHT rectifier valve stands proud on its extension base and the uninsulated top cap is exposed to catch you. Its all a matter of taking care and keeping one hand behind your back when working on the power chassis. That way you can avoid the nasty 'across the heart' path that would probably be curtains with a potential of several thousand volts. Another precaution is NEVER have metal angle pieces screwed to the edge of your workbench to protect it......I'll leave you to work that one out. If in doubt, don't do it! Plenty of advice available from us all here, you only have to ask. Regards, John.