Old Tektronix 465 with a non-polarized mains plug? - Page 1
Forum section: electronics / testgear

author: tobia
author_rank: Contributor
timestamp: August 02, 2014, 12:05:27 am
content: Hello! I'm a newbie electronics enthusiast. After watching many of Dave's excellent videos, I have acquired an old analog oscilloscope on eBay, a venerable Tektronix 465. (I already own a Rigol DS1052E.)The scope seems indeed to be working—I can see the two traces when fiddling with the controls—but everything is so noisy and uncalibrated I will have to put it through an anti-age treatment such as the one inModemHead's excellent thread.My main worry right now is a paragraph I found in the service manual:QuoteMy house's wiring is brand new and the earth connection is reliable. But I'm from Italy andboth plug types commonly used in my country are non-polarized:German SchukoandItalian type L.What are the dangers of randomly connecting this old piece of equipment, with such a peculiar requirement, with active and neutral swapped?(I apologize if this has already been covered, but I couldn't find it.)Would you always make sure to plug it the correct way around? I can certainly inspect and mark both the plug and socket I plan to use it with. I could even install some kind of polarized socket in my wall, to be extra-safe, just to use with this scope. Is it needed?

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 02, 2014, 12:16:00 am
content: Why wouldn't you simply install a modern power cord on it?See:https://www.eevblog.com/forum/projects/tektronix-465-repair-and-restoration/

author: tobia
author_rank: Contributor
timestamp: August 02, 2014, 12:34:27 am
content: Quote from: Richard Crowley on August 02, 2014, 12:16:00 amBecause, as I said, all power cords commonly used in my country have non-polarized plugs.Should I go the extra mile and make sure to always connect the neutral and active the correct way around? What could possibly go wrong if they are inverted?

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 02, 2014, 01:55:26 am
content: What country are you in? At least complete your user profile to reveal that much information.Exactly what main plug/socket are you talking about? Please identify it by name or find a photo of it and give us the link.The schematic diagram of the power input shows that it is completely symmetrical and should not be sensitive power mains "polarity".Is there a green-wire safety ground lead in your power cable? Does your unidentified power mains plug have a safety ground contact?

author: edpalmer42
author_rank: Super Contributor
timestamp: August 02, 2014, 02:43:43 am
content: Quote from: Richard Crowley on August 02, 2014, 01:55:26 amHis initial post and the references he included answered all of your questions. Reread his message.Ed

author: edpalmer42
author_rank: Super Contributor
timestamp: August 02, 2014, 03:12:25 am
content: Quote from: tobia on August 02, 2014, 12:05:27 amThe safety notice doesn't say which lead is hot and which is neutral, it just says that the neutral has to be near earth potential and there has to be a seperate ground connection.Both of the plug types that you listed have a ground connection so that's taken care of.I see in the schematic that the primary side of the power transformer is totally isolated from everything else - which is not surprising. So it doesn't really matter which side is hot or neutral. Just remember that since your plug isn't polarized you could have the situation where there's still power to the unit even though the fuse has blown. Notice that the power switch opens both sides of the line so they did cover that, but they couldn't open both sides when a fuse blows.Regarding the neutral being close to ground, I'm not familiar with the Italian electrical system. If one side of the line is grounded you're good to go. Even if it isn't, there are other notices in the manual that state that the unit is designed for power systems with no more than 250 V between any two conductors or the chassis. I'm sure your power system will meet that criteria.EdJust make sure that the line voltage switch is in the correct position for your location.

author: tobia
author_rank: Contributor
timestamp: August 02, 2014, 06:39:14 am
content: Quote from: edpalmer42 on August 02, 2014, 03:12:25 amThank you. I should have thought of looking at the schematic myself!Quote from: edpalmer42 on August 02, 2014, 03:12:25 amI just measured my system, the neutral is 1.5V~ from earth, and the active 215V~ from neutral, so it should be ok. I carefully checked the little line voltage switch before first connecting the appliance.

author: Lunasix
author_rank: Regular Contributor
timestamp: August 02, 2014, 07:18:49 am
content: I have had a Tek 465, and I don't remember of such problem, and I certainly connected it with neutral on the 2 sides. It would be a stupid design if neutral was possible only on 1 wire of the power cord. The best thing is to read schematic (I have sold it). I think (I'm sure..) that it only means that common mode must be limited, not floating at high voltage, for transformer isolation reasons (screen to gnd between windings).