Repair help; trying to identify hot and neutral - Page 1
Forum section: electronics / repair

author: jeremy
author_rank: Super Contributor
timestamp: September 29, 2013, 05:13:52 am
content: Hi all,Got a repair job here. A business throwing out a perfectly good amplifier, and they snipped the AC cable. The power supply has a 2 pin plug/connector with a purple and brown wire going into it. The purple is marked "AC-H" on the silkscreen and the brown marked "AC-G". While I'm pretty sure this means purple == hot and brown == neutral, it would be nice if someone could confirm this for me ?I've tried googling it to no avail.

author: kolbep
author_rank: Frequent Contributor
timestamp: September 29, 2013, 05:34:54 am
content: Open it up.The wire that goes through the fuse and powerswitch is usually the live.The neutral normally goes straight into the transformer, unless they use a double pole main switch.P

author: IanB
author_rank: Super Contributor
timestamp: September 29, 2013, 05:36:56 am
content: Where did he amplifier come from--anywhere in Europe perhaps? In Europe brown is live and blue is neutral. Usually that would be light brown and light blue, hard to mistake light blue for purple though.Why not trace the connections on the inside? Is there a chassis fuse for instance?It shouldn't matter a lot if you happen to reverse the connections as both sides should be insulated.

author: walshms
author_rank: Regular Contributor
timestamp: September 29, 2013, 05:37:17 am
content: Quote from: jeremy on September 29, 2013, 05:13:52 amProbably a safe bet. Can't speak for your systems there, but here in the US, neutral is usually tied to ground for our 120V systems.

author: jeremy
author_rank: Super Contributor
timestamp: September 29, 2013, 05:45:13 am
content: Hi guys,Sorry, it's an AU amp. I'm trying not to pull the whole thing apart because it is put together rather painfully. But I have probed the pins and the fuse, apparently AC-G is connected to the fuse? Now I'm confused.Just found the pcb layout of the PSU board, any repair dudes handy?

author: IanB
author_rank: Super Contributor
timestamp: September 29, 2013, 05:52:18 am
content: Clearly AC-H is live and AC-G is neutral.AC-H goes through the on/off switching relay, then through the fuse, then to the transformer.AC-G goes straight to the transformer.You measure continuity from AC-G to the fuse because the circuit goes through the transformer primary. There is no continuity between the fuse and AC-H because the on/off switch is off and breaks the circuit.

author: jeremy
author_rank: Super Contributor
timestamp: September 29, 2013, 05:57:15 am
content: Thanks heaps! Makes sense.

author: GEuser
author_rank: Frequent Contributor
timestamp: September 29, 2013, 06:24:48 am
content: See those big numbers in the picture, 125v , better check its ok for out here ...

author: ejeffrey
author_rank: Super Contributor
timestamp: September 29, 2013, 06:46:59 am
content: Quote from: jeremy on September 29, 2013, 05:13:52 amAre you sure it is perfectly good? Sometimes a cut power cable means that the equipment is damaged/defective in some way, and the cut power cord is to keep someone from accidentally using broken equipment. To be sure that isn't the only possibility (and depending on who did it, broken could mean a blown fuse...), but unless you have direct conformation that the amplifier is in working order you should be on the lookout for even non-obvious problems.

author: jeremy
author_rank: Super Contributor
timestamp: September 29, 2013, 06:58:57 am
content: Don't worry, not 120V, its definitely 230. That's just an international service manual.I can't be sure its "perfectly" good I suppose, but I just fixed it up and tested it, sounds beautifulNo blown fuse, caps look good, no dust or anything that looks burnt/blown. Speaker protection circuitry works too (you can hear the little relay click about 5 seconds after power on). It does have a ton of HDMI, optical, etc inputs so I wouldn't be surprised if one of those is faulty. Anything specific that I should be looking for?

author: David_AVD
author_rank: Super Contributor
timestamp: September 29, 2013, 07:17:02 am
content: No mention of brand or model yet?

author: jeremy
author_rank: Super Contributor
timestamp: September 29, 2013, 07:23:01 am
content: sorry, its a onkyo tx-sr578

author: SeanB
author_rank: Super Contributor
timestamp: September 29, 2013, 08:34:29 am
content: Probably the only fault is it is HDMI1.0 and they bought a new TV and BD player that needs a HDMI1.2 or higher switch. If you are going to use it for Audio only then there is no problem. They are pretty nice units, I have one that unfortunately has a dead amplifier on the one main channel, so it probably will soon be a teardown subject, as I have a few other audio amplifiers as well that are working perfectly.

author: jeremy
author_rank: Super Contributor
timestamp: September 29, 2013, 08:43:35 am
content: According to CNET though, it has HDMI 1.4a?http://www.cnet.com.au/onkyo-tx-sr578-339309196.htmNot really a big deal anyway as I just listen to music.

author: Monkeh
author_rank: Super Contributor
timestamp: September 29, 2013, 11:10:16 am
content: It's pretty standard practice to "Ensure that equipment being disposed of cannot be re-used", ie. cut the plug off.Doesn't stop anyone with more than three brain cells.

author: David_AVD
author_rank: Super Contributor
timestamp: September 29, 2013, 11:52:59 am
content: Damaged HDMI sockets (customer forcing on an angle as they plug in around the back) and blown HDMI ports (never sure how) are reasonable common.

author: mcinque
author_rank: Unknown
timestamp: September 29, 2013, 03:30:32 pm
content: Forgive me for my OT, but I am really curious about why you need to know wich wire is the live and wich is the neutral: in many european countries (expecially in Italy) main sockets can be plugged without worrying about the position of the plug (you can insert the plug also upsidedown, so with the live of the device in the neutral of the socket), while in USA or other countries sockets has an unique insertion way.Of course the device can work without caring about the live and the neutral position (many devices are the same sold in USA and Europe), but since you're asking this I guess there is an important reason behind. What's this reason? Maybe this is important because you're referring to an AUDIO amplifier?

author: GEuser
author_rank: Frequent Contributor
timestamp: September 29, 2013, 03:47:33 pm
content: http://www.docstoc.com/docs/110243318/The-MEN-System-of-Earthing

author: Burnerjack
author_rank: Newbie
timestamp: September 29, 2013, 04:58:27 pm
content: You may be able to "Ohm out" the wiring to chassis. The "hot" side should have infinite or nearly so resistance to chassis.I have found two issues if this polarity is reversed. 1 is an annoying 50/60 Hz hum at the output, 2 if an electric guitar is plugged into it a noticeable electric shock will be felt at the strings (if steel).

author: mcinque
author_rank: Unknown
timestamp: September 29, 2013, 05:59:37 pm
content: Quote from: GEuser on September 29, 2013, 03:47:33 pmVery interesting. I wonder why we don't use that system here.

author: Monkeh
author_rank: Super Contributor
timestamp: September 29, 2013, 07:30:33 pm
content: Quote from: Burnerjack on September 29, 2013, 04:58:27 pmBoth should be insulated from the chassis.

author: GEuser
author_rank: Frequent Contributor
timestamp: September 30, 2013, 03:42:17 am
content: Quote from: mcinque on September 29, 2013, 05:59:37 pmI dunno? , maybe because your county has been around longer?So as you see , "if" a fuse was in the neutral leg in the equipment (some do put it in there) it effectively would be useless as it is also tied to earth therefore the only fuse available will be the 10 or 15 or 20 amp mains fuse , or the 2" nail

author: ejeffrey
author_rank: Super Contributor
timestamp: September 30, 2013, 06:29:44 am
content: Quote from: mcinque on September 29, 2013, 03:30:32 pmA couple of reasons. In some situations there is one contact that is more exposed than the other. The most prominent case still around are edison lamp sockets. The shell should be 'neutral' and the center contact hot to reduce the risk of inadvertent contact e.g., while changing a bulb. I think even in italy permanently wired light fixtures should be arranged this way. Presumably for plug in lamps you are supposed to unplug it before changing the bulb.The other reason is that at least in the US it is common to only switch and fuse the live conductor inside a device. If you use a device like this and the fuse blows or the switch is off while it is plugged in backwards, you disconnect only the neutral and leave the bulk of the circuit at mains voltage. In principle this shouldn't be a problem if the circuit is properly isolated and chassis grounded, but it is still undesirable. More troubling, if there is a live-chassis fault you actually have no fuse to blow except the main circuit breaker. The way around this is to use a double-pole switch and two fuses to protect both live and neutral.

author: mcinque
author_rank: Unknown
timestamp: September 30, 2013, 11:49:29 am
content: Quote from: GEuser on September 30, 2013, 03:42:17 amThat's exactly what I've always thought since I start to understand some basic electrical laws.Quote from: ejeffrey on September 30, 2013, 06:29:44 amThis is true, and also here smart electricians apply this security rule to their installations.Quote from: ejeffrey on September 30, 2013, 06:29:44 amAmerica and Europe share many devices with the same circuitry and only a different transformer: I've seen only fused and switched L cable (at least for color) inside power amplifiers, decoders, vcr etc. but with our simmetrical sockets, it's useless: I can easily reverse L and N.Quote from: ejeffrey on September 30, 2013, 06:29:44 amExactly. That's why I think some european (at least Italy, Spain, Finland and Germany) main sockets system is not completely safe.We should do like US, Australia, Switzerland and UK: a one way insertion main socket as standard and MEN system.Thank you all for answering my question.

author: GEuser
author_rank: Frequent Contributor
timestamp: September 30, 2013, 01:04:28 pm
content: It might not help the US, 1980 through 1992, 5,348 workers died which is 411 a year and that's not including the average joe blow at home , and that was only until 1992!Over there it seems either things are very faulty or no-one dies and goes somewhere else?Also a complete opposite for instance is New Zealand which is just across the pond from here , all basic at home work is fully allowed , the government even promotes it , there are also basic course's to take if one wants and over 20 years or so a at home death is still Zero or abouts ..http://www.oriongroup.co.nz/your-network/safety/your-safety/home-owners-diy.aspx"One has to be CAREFUL where one pokes the Finger"