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Determining polarity of A/C line plugs

Posted: 14 Mar 2019 14:13
by Jim Leach
Hi guys.

Quick (maybe) question. For those using vintage gear, with non-polarized two-prong A/C plugs, do you have a procedure for determining the best way to plug it in? I.e. which line is "hot" and which is "neutral"? And taking it further, have you updated to a polarized plug, or a grounded three-prong plug to keep polarity as it should be?

As I'm amassing a bunch of 70's era Kenwood gear now ('table and cassette deck, with tuner and integrated amp to follow) they all have spindly, non-polarized plugs. I know that ground loop issues and/or hum can be induced with varied polarity and since I'm going to be cleaning everything up, I'll probably upgrade the power cords as well- no reason not to, and it might avoid a fire...

So, any advice?

Re: Determining polarity of A/C line plugs

Posted: 14 Mar 2019 17:38
by bernard1
I determine the best way by ear : one way gives less hum or no hum at all. Then I mark the plug, and I repeat for each element.

Re: Determining polarity of A/C line plugs

Posted: 14 Mar 2019 20:06
by Alec124c41
I plug the component in, then lightly run a finger across the faceplate. Ine way gives a slight tingling sensation, the other none. Go with none.

Cheers,
Alec

Re: Determining polarity of A/C line plugs

Posted: 14 Mar 2019 20:23
by zlartibartfast
Gee - I kinda liked that tingly feeling I got from my old amp....;-}

Re: Determining polarity of A/C line plugs

Posted: 15 Mar 2019 00:24
by lenjack
You can also check for AC between components with a meter.

Re: Determining polarity of A/C line plugs

Posted: 15 Mar 2019 02:11
by cary114
If the cord has one side ribbed that should be the neutral the plain was the hot lead. I have been restoring vintage lamps and the ones with vinyl wire from the 50's on have had this standard. that is also the current standard for rewiring.

Re: Determining polarity of A/C line plugs

Posted: 15 Mar 2019 10:53
by vanakaru
In EU there is no difference at the socket end of the cable plug. One day I put it in one way then another - but I feel nothing.

Re: Determining polarity of A/C line plugs

Posted: 15 Mar 2019 15:52
by Jim Leach
Alec124c41 wrote:
14 Mar 2019 20:06
I plug the component in, then lightly run a finger across the faceplate. Ine way gives a slight tingling sensation, the other none. Go with none.

Cheers,
Alec
Alec, that made me laugh.

reminds me of a time when my Bass player was over, with his quite vintage Ampeg amp. He pluged in and we were playing as usual. Then he decided to place a hand on the steel column holding the house up (while muting his strings with the other)... THAT was a wake-up call as 110AC went across his body from one hand to the other.

The good news is that he didn't die.

Re: Determining polarity of A/C line plugs

Posted: 15 Mar 2019 15:55
by Jim Leach
cary114 wrote:
15 Mar 2019 02:11
If the cord has one side ribbed that should be the neutral the plain was the hot lead. I have been restoring vintage lamps and the ones with vinyl wire from the 50's on have had this standard. that is also the current standard for rewiring.
I believe they do, but I wasn't positive that manufacturers abided by any hard n fast assembly rules... I guess they would have to in order to secure UL approval back then.

I have the tape deck open right now, so I'll take a look at which way it appears to be polarized, and note the same when I open up the turntable.

I still feel more comfortable going with a modern three-prong grounded cord, once I determine the proper polarity.

Re: Determining polarity of A/C line plugs

Posted: 15 Mar 2019 16:25
by dysmike
vanakaru wrote:
15 Mar 2019 10:53
In EU there is no difference at the socket end of the cable plug. One day I put it in one way then another - but I feel nothing.
There is, it's just not visibly obvious. One pin will be live (brown), the other will be return (blue) (that's it if you have a two pin type C plug) and if you have there are earth (green/yellow) paths built into the sides for type F (and the center for type E).

Not feeling a 'tingle' means the ground is good (generally).

Re: Determining polarity of A/C line plugs

Posted: 15 Mar 2019 20:33
by Alec124c41
The new 2-prong plugs have one prong widened at the end. They only fit into sockets one way. The wide one is the return/neutral.
You can put one of those on, instead of a 3-prong plug.

Cheers,
Alec

Re: Determining polarity of A/C line plugs

Posted: 19 Mar 2019 02:25
by ChrisfromRI
In theory you should disconnect all interconnecting cables from the piece of electronics and then using your VOM set to a low AC Voltage scale plug the two prong plug (or adapted two or three prong plug) in both ways while checking the AC Voltage between the chassis of the piece of electronics and the ground terminal or central screw in the electrical outlet. Whichever way yields the lowest voltage reading is the best way for that piece of electronics. Mark its AC cable and move on to the next piece of electronics. When you're done with all of the pieces of electronics the interconnecting cables can be reinstalled.

The newer polarized plugs with a fatter blade require a non-polarized three prong to two prong adapter to be able to flip the plug both ways in the outlet while making this check. If your adapter also has a fat blade it will need to be filed down for this purpose.

Often this process results in a lower noise floor in the quiet parts of the music.

Please be careful.

Re: Determining polarity of A/C line plugs

Posted: 19 Mar 2019 02:52
by lenjack
Too many plugs in my system to take it apart, behind too many pieces of furniture, to make it worthwhile, since I get no tingling, nor do I hear any hum.

Re: Determining polarity of A/C line plugs

Posted: 21 Apr 2019 23:30
by ChrisfromRI
lenjack wrote:
19 Mar 2019 02:52
Too many plugs in my system to take it apart, behind too many pieces of furniture, to make it worthwhile, since I get no tingling, nor do I hear any hum.
This could be one of the contributors to why you're unable to hear differences between turntables, in the other thread.

Re: Determining polarity of A/C line plugs

Posted: 22 Apr 2019 16:59
by Jim Leach
@chrisfromri this is exactly where I was going.

It seems they take the AC straight to the transformer and the output is then rectified. So in theory, it doesn’t matter, but I may swap to modern (polarized) 2-prong or three prong cords once I do your test.

Based on how it’s wired, unless there is internal leakage on the transformer, the metal chassis is “floating”. But I can do the testing easily enough.