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Post by mattmayfield on Aug 2, 2006 17:14:42 GMT -5
Hi everyone, My name is Matt and I've been tinkering with guitar wiring for some 10 years or so. I just have a wiring theory question. It's in reference to this on the GuitarNuts site, in the article about shielding a Strat: If the grounding for the entire guitar guts is all connected to the sleeve of the 1/4" jack, how can it be possible for there to be a ground loop *inside the guitar*? Why would it matter that these pots are connected with extra wires, when they'll be connected through a potentially infinite number of different paths via the shielding? Isn't the final connection to the 1/4" sleeve, by definition, "automatic" star-grounding? Thanks in advance for information that can clear up my idea about this. Matt
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Post by vonFrenchie on Aug 2, 2006 19:37:55 GMT -5
Thats the thing. You dont need the ground loops because the pots are touching the shielding. All you have to do is solder the lug to the pot's case. Unshielded guitars have connected pots to keep the output jack free from extra wires.
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Post by mattmayfield on Aug 2, 2006 22:34:02 GMT -5
Thanks for your reply.
That makes sense - they don't want to star ground everything on the 1/4 jack itself.
I don't understand why leaving them connected after shielding would matter, though. People with more experience and EE degrees and such are saying that it does, so I believe that it's true, but I don't understand why it would be.
Thanks again,
Matt
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Post by CheshireCat on Aug 3, 2006 1:02:17 GMT -5
All you have to do is solder the lug to the pot's case. Hello! Reality check! You don't solder the log from the pot to the pot's case. Go back and review JohnnyA's stargrounding methodology. Also, note that all external grounding is connected to the starground via one solitary connection that passes thru a 400v .33uf isolation cap, so as to avoid any potential electric shock hazard. Basically, the wiring that most electric guitars have is an electro-shock induced cardiac arrest just waiting to happen. And several musicians have died that way. For the thread's OP, Matt, go look at JohnnyA's diagrams again. He is creating two different grounding paths: the circuit oriented ground points, all of which go to a starground, and an outer chassis, comprised of the shielding from aluminum foil or copper paint (or whatever). These two grouding paths connect thru the aforementioned isolation cap. Review the Quieting the Beast Mod a few times, and you'll see what I mean. Chesh
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Post by mattmayfield on Aug 3, 2006 12:04:43 GMT -5
I think I get it. With the isolation cap, it's necessary to break the connections between the backs of the pots because otherwise current could cross through the "signal ground" into the string ground via those wires. Yes?
To check my understanding, would this be true: If you *wanted* to wire a guitar so that it could potentially kill someone, but also wanted to shield it as well as possible, then it wouldn't matter whether the wires between the pot backs were there or not. Is that correct?
Thanks,
Matt
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Post by CheshireCat on Aug 3, 2006 12:28:57 GMT -5
I think I get it. With the isolation cap, it's necessary to break the connections between the backs of the pots because otherwise current could cross through the "signal ground" into the string ground via those wires. Yes? Precisely. Basically, the isolation cap is a firewall to keep you from getting electrocuted in case some external pieces of equipment weren't properly grounded. Remember, you don't ground the strings . . . they ground you. You are a huge RF antennea, and you are not grounding the system; the system is grounding you. So, because of that, you can become part of the circuit, and if any of the elements of that circuit aren't properly grounded, then you go from being an antennea that will have a few volts of RF drained from you to being a jumper for over 100 volts of ungrounded juice to pass thru . . . specifically across your heart if your one hand is holding the ungrounded casing of a microphone and your other hand is touching the strings. So, to that end, JohnnyA created the QTB Mod, with the isolation cap, which won't completely eliminate the potential for getting shocked, but will radically ramp down the possibility for a lethal volt, so iow instead of getting killed, you'll get a pretty good bite, which isn't any kind of fun (unless you're into that kind of thing) but at least you'll live. Now, as far as the impact of ground loops on your sound, I think John covers that as well. I'll see if I can find the link to the article, tho I think it's all in the same article on how to do the QTB Mod. To check my understanding, would this be true: If you *wanted* to wire a guitar so that it could potentially kill someone, but also wanted to shield it as well as possible, then it wouldn't matter whether the wires between the pot backs were there or not. Is that correct? Precisely. Basically, guitar wiring as we know it is pretty shoddy. It's very rudimentary, and as John also pointed out, no manufacturer of even cheapo $10 transistor radios or cheap toasters would ever settle for the kind of wiring that we often find in guitars that run as much as $3K or more. Now, fortunately, that's changing. However, ultimately this wiring is the prevailing MO and standard for most manufacturers. See, the standard grounding convention makes the assumption that the guitarist will be mature enough (and have a clue) to know the risks and will take care not to handle ungrounded equipment and so on. Simply put, that's not a risk that we need to have on the table, especially for want of a $0.50 cap and a bit of tin foil. Does that make sense? Chesh
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Post by mattmayfield on Aug 3, 2006 12:32:58 GMT -5
Thanks for your detailed reply. That makes perfect sense to me and answers my questions.
Time to shield and star-ground my other guitars...
Matt
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Post by CheshireCat on Aug 3, 2006 12:36:45 GMT -5
Thanks for your detailed reply. That makes perfect sense to me and answers my questions. Time to shield and star-ground my other guitars... Matt Indeed. Incidentally, the improper bit of grounding may not even be in the equipment, but in the house. You might find yourself playing a gig in an rather old, rustic venue, with an antiquated and potentially dangerous electrical system. In fact, many musicians carry various testers with them and check the house's or venue's wiring before they plug a single piece of equipment in. Chesh
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Post by UnklMickey on Aug 3, 2006 14:21:53 GMT -5
...To check my understanding, would this be true: If you *wanted* to wire a guitar so that it could potentially kill someone, but also wanted to shield it as well as possible, then it wouldn't matter whether the wires between the pot backs were there or not. Is that correct?... not exactly. if the wires interconnecting the pots, are also used to make the connections for the signal ground, that would allow a bit more noise from the shielding to enter the signal path. i don't have the time or interest to do a complete tutorial on ground loops, but here's the gist of it: the later the shielding is connected to the signal ground, the better.connecting at the output jack is a little better than at the controls. (and still easily accomplished) running a separate wire that connects the shielding only at the amp's sleeve of the input is better yet. (but unwieldy) even better, would be to connect the separate wire to the same "true earth" (multiple metal rods literally driven into the earth) connection that the 3rd wire of the amp's power cable is connected to. (now we've gone a bit over-the-top.) all other things being equal, will you be able to hear the difference, between a wiring scheme with the shielding and signal grounds connected at the controls, vs one with the connection at the output jack? maybe, maybe not. but if you CAN, it will be quieter with the connection at the output jack. cheers, unk
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Post by sumgai on Aug 3, 2006 15:36:48 GMT -5
Point of Order, Mr. Chairman. No, that's not the whole story either.
In fact, the large isolation cap is there ...."in case of a catastropic failure in the amp's power supply". (Quoted from JohnA, on the original GN site.) That cap is supposed to do only one job, and that is to prevent a large surge of DC from getting to you, in the event that you might be even near another large conductor (a chassis, an equipment rack, a metal window frame, etc.), let alone touching it. Such a surge will kill you as surely as if it were AC, but the cap itself will not prevent an AC current loop (commonly called a ground loop) from shocking or killing you.1 That must be handled by properly wiring all grounds to one point, and I'm not speaking of those inside the guitar, that has to do with the signal and hum. I'm speaking of power, the 110/220 volt variety.
As has been noted, older venues might present unsafe conditions, and the best way to prevent problems there is with an isolation device of some sort. Hum-X comes to mind, but there are plenty of others. The simple way to understand a potential threat is, if you hear hum, worse than usual, then you're in a killing zone, and you need to take self-defense measures, pronto. Better yet is to be anally paranoid, and to treat every venue as if it were out to kill you. I say this because if you get lazy and comfortable about a familiar venue, and the owner has some re-wiring done without telling you about it, then the next time you come in, you're gonna be in for a real treat.
Bottom line is, electricity is like a gun - always treat it as if it were loaded and pointed at you, or turned on and seeking to touch you like a thing alive. Trust me on this one.
sumgai
1. What's a capacitor's purpose? To pass AC and block DC. That's it in a nutshell. Thus it does no good in a ground loop situation.
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Post by ccoleman on Aug 4, 2006 8:44:45 GMT -5
This post is great: from a licenced master electrician he says to buy an extension cord with an inline GFCI, use this at gigs, to protect your life !!!!!!! 20 dollars. Required because even with perfect wiring at the live venue, you can get shocked and killed when an Air Conditioner kicks in and causes a voltage difference / sag / brownout on the venue's power lines. Referred from this excellent article on protecting your life, it shows you how to test grounds for safety before playing a gig: Personally I like to use either WIRELESS on both the guitars and vocal mics, or DIRECT BOX WITH ISOLATION TRANSFORMER AND GROUND LIFT on all mics and direct inputs to the FOH and backline amps/monitor desk, to isolate and protect as much as possible...
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Post by ccoleman on Aug 4, 2006 9:50:48 GMT -5
the cap itself will not prevent an AC current loop (commonly called a ground loop) from shocking or killing you. Sumgai.... In fact.. the cap acts as a HIGH-PASS FILTER at 6db per octave. The cutoff point is determined by the value of the cap. So yes it will pass 60Hz power line AC, but the voltage and power levels will be much lower than without the cap. So I do believe it has a protective effect on saving your life. This has to be verified.. there is a math forumla that tells you the cutoff frequency based on capacitor value and resistance.. anyone want to run the numbers?? The question now is... how much 60Hz AC voltage does it take to cause the heart to stop ?? Because if that cap cuts it down from 120V to 10V.. well then there probably is not much of a lethal risk.. I may be off here, someone please post a correction if that is so.
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Post by sumgai on Aug 4, 2006 10:47:09 GMT -5
cc,
It isn't a question of whether or not your figures are off, it's only a question of "who wants to try it out". Believe me when I say to you that I have already had one episode of heart stoppage in my life, and I am [glow=red,2,300]NOT[/glow] gonna volunteer to be your guinea pig here.
Not that I don't believe you, all of your assertions are correct so far. But unless you are willing to personally try this (preferably not at home, but near to, or directly inside, an Emergency Room), then I can't get excited about you (or anyone else) telling folks that a capacitor will give some kind of positive protection against electrical shock. After all, it takes time for that capacitor to charge up, and start delivering that so-called "reduced level" of AC. In that first TC period, all the damage can be done before your head has even registerd any pain.
Look at it this way...... if capacitors were valid shock protection devices, don't you suppose that the Electrical Code would require them on all outlets? And that manufacturers would have to include them on nearly every electrical device ever made? I rest my case.
Call me a nazi about this particular topic if you wish, I won't mind. After all, I'm still here to talk about it - I've got some credentials. But please don't take me as honking at you personally on this, it truly isn't personal. You just happened to start that particular ball rolling this morning. ;D
sumgai
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Post by ccoleman on Aug 4, 2006 14:45:38 GMT -5
OK I agree this is not a risk that anyone should take.
What we should do however is look into simple (or not so simple) ways of protecting the guitarist, bassist, vocalist from electrocution induced heart attack.
The $20 GFCI extension cord is one.
Is there anything else we can do to the wiring inside a guitar to provide a redundant protection ??
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Post by sumgai on Aug 4, 2006 18:19:56 GMT -5
cc, I'd kind of like to devise a test to see if the GFCI can properly interpret and take action in the presense of that DC spike we're worried about. If not, or not very well, then I think both the GFCI and the capacitor are pretty much de rigueur. If the GFCI does do a more than adequate job, then I'd be tempted to just rely on it alone. Redundancy is good, but unless one has a thorough grounding in all aspects of what an item can do, and what it can't or won't do, then one should not place too much faith in having too many "protectors" in the chain. (Insert plug for wireless systems here. BTW, FWIW, I didn't know about the GFCI-built-into-extension-cord trick, but I do have, and use, a regular 4-outlet square box (the metal kind) with a standard GFCI outlet mounted in it (and protecting the other outlet too). I used a 15' extension cord for power, you can get those from Home Despot or otherwise for less than US$9. I simply cut off the "outlet" end, and the rest of the assembly procedure should be obvious. Oh, and I used a standard 'electrical panel' cable clamp, too. sumgai
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Post by vonFrenchie on Aug 6, 2006 19:36:04 GMT -5
I guess computer's count as the heavy machinery stated on medication bottles. Sorry about the false info. I've had a rough week (surgery ugh). So dont take my advice for a while (or ever for that matter). I've been in many accidents and done alot of stupid stuff. But I would never want some one else to get hurt on my account. Sorry... once again.
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Post by ccoleman on Aug 7, 2006 17:36:00 GMT -5
here is some ideas.
How about building a GFCI into the body of the guitar, next to the quarter inch socket. Any wiring fault will simply throw the breaker and protect the player's life.
Same thing with the bassist.
And while we're at it, build a GFCI into the handle of the microphone for the vocalist.
Do you know... Can these GFCI's be made small enough to fit in there ??
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Post by UnklMickey on Aug 7, 2006 18:31:27 GMT -5
cc,
nice idea in principle, but it's just not gonna happen.
the way a GFCI supposedly operates, is to insure a balance between the current going "out" on the hot, and back "in" on the neutral.
i'm not certain, but i also suspect that it uses the ground of the outlet to insure that the neutral isn't connect to ground "downstream" and/or to sense for current.
bottom line:
it requires power from the A.C. input to "unlatch" the mechanism and allow it to spring open disconnecting the circuit.
we certainly don't want to have 120vac any where near our guitar or signal cables.
it might be possible to design something similar to a GFCI and build it into an amplifier, to "disconnect" the cable in the event an appreciable current is seen there.
however, a standard GFCI as we know it, won't translate.
unk
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Post by ccoleman on Aug 7, 2006 19:46:34 GMT -5
It would have to be a modified GFCI. It would have to detect a current (greater than 1A) flowing in the shield to ground. That would be a dead giveaway that there is an electric shock underway.
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Post by ccoleman on Aug 7, 2006 21:37:03 GMT -5
Scratch that...!! The guitar GFCI has to protect against currents greater than 25-30 milliamps (at 120-240Volts) !!!! Proof here: www.chess.cornell.edu/safety/manual/chptr7/lctrcshk.htmI hope this would be somewhat cheap and easy to design... in today's era of massive electronics knowledge and cheap mass manufacturing. Comments ?!
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Post by ccoleman on Aug 7, 2006 23:13:15 GMT -5
A technique I just realized might work:
use a built-in direct box... with a 1:1 isolation transformer of 10K impedence on both sides of the transformer. At 120V passing through 10K resistane, the current would be limited to 12 milliAmps.. annoying but too low to cause a heart stoppage.
Your opinons ??
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Post by CheshireCat on Aug 8, 2006 0:01:02 GMT -5
Scratch that...!! The guitar GFCI has to protect against currents greater than 25-30 milliamps (at 120-240Volts) !!!! Proof here: www.chess.cornell.edu/safety/manual/chptr7/lctrcshk.htmI hope this would be somewhat cheap and easy to design... in today's era of massive electronics knowledge and cheap mass manufacturing. Comments ?! I'm not sure what I'm looking at. What is that a link too? All I saw was info on electroshock hazard. Was there a circuit or something to protect against that? Link? Pic?
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Post by ccoleman on Aug 8, 2006 9:17:54 GMT -5
It was just a spec on how much current it takes to cause a heart attack and kill ya while you're playing guitar. 25-30 mA, at standard wall voltage, 60Hz.
So if we invent a GFCI built into the guitar... it will have to trip at around 20 mA to protect the player.
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Post by sumgai on Aug 8, 2006 12:04:52 GMT -5
cc, That chart is a good start, but it is not the be-all, end-all for every player. It is based on a "reasonably healthy individual that is otherewise dressed properly, and in a controlled environment". IOW, the person isn't sweaty, nor standing in a puddle (more conductive), nor are they wearing insulative boots (less conductive), etc. It's that part about the health that you have to deal with. I have seen at least two guys who can literally grab wires coming right out of a live wall socket, and they don't feel a thing! OTOH, heart patients such as myself are much more suseptible (have a lower resistance) to shocks of this nature. For us, it is estimated that we might withstand only 20% of what a normal person can take. Let me re-phrase that - we might be able to handle 5 or 6 or 7 mA of current (120v, 60Hz), but much more than that, and we may not be around to tell the doctor how it felt. (These figures are, of course, a guestimate based on very few real world examples.) Besides which, the NEC specifies that GFCI devices must operate (trip) at less than 4 to 6mA of current, not 20. And unk is right, why bring 120 volts near our intruments on purpose, when it's not needed for anything meaningful. Better to let a floor device (a central power controller of some sort) do the job. And as far as protecting microphones (and their stands) goes, you're a step ahead of most folks, but not everyone. I was at an outdoor concert once where I saw these jumper cable-sized clamps attached to a leg of each mic stand. I traced the wiring back to a central box, which handled all of the mic stands, as well as all of the rack gear and all of the guitar/bass amps. Each item had a wire and a clamp, and they all ended back at this one ground point. Crude, but for certain, there were no little blue sparks jumping to any lips, and there were no loud exclamations of "WTF?" I talked to one of the stage techs (he was from a "sound reinforcement" rental company), and he said that the large clamps were meant to accomodate any size device or stand. They dealt with both their own rented-out gear, and whatever belonged to the bands. The clamps let them set up quickly without any tools, and they didn't have to hassle with any wires attached to the stands (which also meant that they didn't have any problems if they had to replace a mic stand in a hurry, like if it broke or something). All in all, a pretty neat idea that most players/bands could stand to benefit from. sumgai
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Post by sumgai on Aug 8, 2006 12:27:35 GMT -5
cc, I have several things to say about this, but first, I need to know how you intend to wire this thing up. When you referred to 'direct box', I assumed you meant as in "a device for connecting a mic or guitar to an amplifier input". But then you said 120 volts, and my mind went haywire. Are you perchance referring to a situation where the 120 volts appeared on a signal line accidentally (such as a ground loop), it wasn't supposed to be there in the first place? But then in that case, how is the transformer hooked up so as to couple one device to another? Or is it that you are using the transformer as a sensing device? That's an idea that has merit, we could explore that avenue, if that's your intention. But first, I gotta know, how is the transformer to be employed? sumgai
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Post by ccoleman on Aug 8, 2006 13:42:47 GMT -5
the transfo would be simply isolating the guitar signal from making a direct connection with quarter inch cord.
the idea is identical to a direct box (device for connecting mic or guitar to snake which connects to FOH mixer..) built directly into the guitar itself.
the protection comes in when you look at it and notice, the transformer electrically isolates the "ground" on the guitar side from the "ground" on the quarter inch cord.
So even if there is a wiring fault and 120V AC appears on the instrument cord, 120V should NOT appear on the guitar strings in lethal shock amounts.
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Post by sumgai on Aug 8, 2006 15:15:58 GMT -5
cc, This is wrong in so many ways, I don't know where to start. cc, I'm not trying to get down on you, but here's the lay of the land from my perview...... Experimentation is not only great, but it's required! It's the very reason we're here!! Ditto for asking questions, it's the reason for this board's existance. And no topic is taboo, particularly not safety. But now, the topic suddenly shifts focus. Instead of saying "let's see what happens if we do this....", we, or at least I, say "don't do that!". The difference is, if someone takes away from here a tidbit that he thinks will save him a buck, or a minute, or whatever, and then he goes and kills himself, who's to blame? You can be sure that if his widow has a good weasel lawyer, then ProBoards, and RandomHero, and anyone else they can identify, will all be sued pantsless. I've seen it done, day after day. Hastings (jhng) is an attorney, ask him, he'll tell you the risks of discussing safety practices. It ain't pretty, and it can get mighty ugly. So, let's continue, shall we? First, if you wish to try inserting a DI box between your guitar and 120 volts, and depending on it to isolate you from shock, be my guest. But before you do that, tell us where to send the flowers and condolence cards. I'm an old EE, and I got that way (old) because I don't do things like that! Remember, the only old guitar players are the ones who didn't use their own lips to personally test for electical shock hazards. ;D Question: How many DI boxes do you see advertised that state explicitely "will prevent or eliminate all electrical shock hazards"? A DI box is intended to match impedances, or possibly to lift the ground on a mis-wired XLR connector or cable. Anything beyond that is taking risks, in my book. A transformer does not in itself draw current - it is not a load. Also, according to Ohm's Law, a transformer must change any current being drawn through it by the same amoung as the voltage is changed, but in the opposite direction. If the voltage is stepped up, the current available is stepped down by the same amount. So, given your original hypothesis of a 1:1 (isolation) transformer, we see that if 120 volts is present on the primary, it will also be available on the secondary. And if a load is placed across the secondary (a human body) that draws perhaps 20 ma, then that same 20 ma will be drawn from the main power source (the 120 volt line). Net result? No protection. "But wait", you say, "I'm talking about ground connections here, and stuff like that." Sure, and what's that got to do with 120 volts across the primary? You said it: There is only one solution, and that is to send all ground points to one central location. This means every piece of equipment that the musician can possibly touch, bar none. Failing that (and in most large venues, it would be difficult), the next best thing is to use a GFCI in the 120 volt supply of each major equipment group. Each method has an advantage, and the disadvantages are so small that I won't even mention them, considering the alternative. Oh, and one should still use an outlet tester before plugging in any gear at all, no matter what else he does to prevent shocks or other problems. sumgai
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Post by UnklMickey on Aug 8, 2006 17:02:31 GMT -5
...Oh, and one should still use an outlet tester before plugging in any gear at all, no matter what else he does to prevent shocks or other problems.... okay so we've strayed a bit from the original post, but statements like that are useful no matter what the orignal topic. so i'm gonna add another.if anyone thinks a GFCI is a licence to act carelessly, think again.there are some things that it WON'T protect against. if one grabs the Neutral (not the ground) and then the Hot, they are in for one wicked ride. the current through the Neutral and the Hot will be equal, and the GFCI won't sense a failure. GFCIs are a wonderful invention, but they ain't so perfect that they can always trump stupidity. (should i Chris?.................of course i should) Stupid IS.
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Post by sumgai on Aug 9, 2006 1:24:26 GMT -5
Dammit unk, you always get to say the best lines!If anyone thinks a GFCI is a licence to act carelessly, think again. Instant classic right there. Should be required reading for anyone who has ever said the word "electricity". Well, to that I can only add "Stupid is a self-fulfilling prophesy". sumgai
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Post by CheshireCat on Aug 9, 2006 2:41:26 GMT -5
Do I sense a new gadget coming together here?
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