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Old 02-14-2021, 06:17 AM   #1
EasyGoing1
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Default Question on AC grounding - found an old device that has me confused

So I needed to find something that I don't use so I could cut the power cord off of it and use it in a project.

So I found this ... Radio Shack looking device that apparently takes an external TV antenna as an input as well as RCA connections as inputs Then on the front you can choose to have it send out to the TV on channel 3 or 4 - though I have no idea what it does with the two different inputs...

But anyway, I figured before I cut the cord and toss the device in the trash, I should bust it open and see if there's anything useful inside to add to the spare parts bins... and when I pulled the cover, I saw something that seemed really odd to me.

It has a two-wire electrical cord on it, yet one of the wires, it connects to the metal portion of the chassis.

Now I'm aware that the third lead on an electrical outlet is earth ground, and that it's common to use that third prong in an appliance so that if somehow the unit malfunctions and one of the other two leads gets loose or something, that it will divert current to earth ground so that if a person touches it, it won't prefer channeling current through the person.

But to connect the return line from an AC outlet to the chassis ... isn't that potentially dangerous? I mean if someone was barefoot, standing on wet carpet and just picked up the unit, couldn't it electrocute them?

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Old 02-14-2021, 06:37 AM   #2
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Default Re: Question on AC grounding - found an old device that has me confused

That's a death trap: if you reverse the AC plug the chassis will be live.
Sure in the US & UK the plug it is polarized, but it is very common to have the live and neutral reversed in the outlet.
The only thing I can think is that this is not original:
If it is the company should be brought to justice for it, this is just crazy!
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Old 02-14-2021, 07:19 AM   #3
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Default Re: Question on AC grounding - found an old device that has me confused

nothing unusual back in the old days.
they did the same shit with radio's and tv's in the u.s.
does it have any datecodes on it?

to answer the first question,
it's a modulator, you put it between your antenna and tv set.
then you feed composite video and line level audio into it from a vcr, camera or computer and it mixes it in with the other rf channels.

Last edited by stj; 02-14-2021 at 07:22 AM..
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Old 02-14-2021, 07:27 AM   #4
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Default Re: Question on AC grounding - found an old device that has me confused

Quote:
Originally Posted by stj View Post
to answer the first question,
it's a modulator, you put it between your antenna and tv set.
then you feed composite video and line level audio into it from a vcr, camera or computer and it mixes it in with the other rf channels.
OH I get it ... so then it broadcasts the VCR on channel 3 or 4 (based on the switch) but you can continue to go to other channels and they will just feed off the antenna. Is that right? Like the antenna is just passed through the device with an injected carrier on channel 3 or 4 which is the VCR.??
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Old 02-14-2021, 07:31 AM   #5
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Default Re: Question on AC grounding - found an old device that has me confused

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nothing unusual back in the old days.
they did the same shit with radio's and tv's in the u.s.
does it have any datecodes on it?
No, in the old days they used a wooden box, and plastic knobs.
You could never touch the metal parts which had a 50% chance of being at mains potential.
Today in modern designs this is called double insulated, it is signified with a mark with two squares inside each other.
What these all have in common is that they are non-conductive, i.e. wood or plastic enclosures.
It is completely forbidden to do this with a metal enclosure that the user can come in contact with!
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Old 02-14-2021, 07:35 AM   #6
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Default Re: Question on AC grounding - found an old device that has me confused

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does it have any datecodes on it?
I ripped the whole thing apart including taking out the circuit board and I don't see any numbers printed anywhere. That pic that I posted of the bottom of it is the only information about it given on the device itself.

Here's a picture of the front of it.... but no relevant information there either.

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Old 02-14-2021, 07:37 AM   #7
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Default Re: Question on AC grounding - found an old device that has me confused

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It is completely forbidden to do this with a metal enclosure that the user can come in contact with!
Apparently sometime between the early 80's and probably the late 90's it was not forbidden! Just no idea when they finally deemed the practice a big NO NO.
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Old 02-14-2021, 07:49 AM   #8
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Default Re: Question on AC grounding - found an old device that has me confused

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Apparently sometime between the early 80's and probably the late 90's it was not forbidden! Just no idea when they finally deemed the practice a big NO NO.
I can not imagine this would have ever been up to code in any country in the world at any point in time.
EDIT: If you remove the heatshrink tubing from the wire that goes to the chassis is there at least a resistor in there?
If it is then it will only give you some mild tingles when you touch it if it's connected with "wrong polarity"
If not: then you will die if you touch it with "wrong polarity"

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Old 02-14-2021, 08:41 AM   #9
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Default Re: Question on AC grounding - found an old device that has me confused

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I can not imagine this would have ever been up to code in any country in the world at any point in time.
EDIT: If you remove the heatshrink tubing from the wire that goes to the chassis is there at least a resistor in there?
If it is then it will only give you some mild tingles when you touch it if it's connected with "wrong polarity"
If not: then you will die if you touch it with "wrong polarity"
That's so funny ... I just cut the wire at the solder point and tossed it in the trash then I read your question and thought .... I dunno, that's a good question ... so I went and dug it out of the wastebasket and removed the heat shrink protector and sure enough, it's got a resistor in there. Value is 1.5 Megaohm according to my Fluke.
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Old 02-14-2021, 08:48 AM   #10
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Default Re: Question on AC grounding - found an old device that has me confused

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does it have any datecodes on it?
OK so I was looking for ink type numbering on it somewhere and didn't see any, but then I removed the shielding cover and saw some numbering that would have part of the PCB silkscreen.

And as I was looking at the circuit board, all I saw was just analog components and thought ... how in the heck could they build something like this as small as it is without any ICs in it then when I took the bottom shield cover off of it, I saw the IC which I'm assuming is what sets up the channel injection frequency? There are definitely no crystals anywhere and it's lead-free which means it's probably newer than the '80s.



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Old 02-14-2021, 09:03 AM   #11
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Default Re: Question on AC grounding - found an old device that has me confused

2010??
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Old 02-14-2021, 09:41 AM   #12
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Default Re: Question on AC grounding - found an old device that has me confused

Only the ground terminal of an outlet can be connected to the chassis of a device, if it is not ClassII - Double insulated. This is wrong. Perhaps it relies on a plug with a 3rd dummy pin, so you cant reverse the plug. But it's still wrong. Replacing the plug or improperly wired outlet could lead to you being electrocuted. If these were our unpolarized sockets, there would be dead people.
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Old 02-14-2021, 12:58 PM   #13
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Default Re: Question on AC grounding - found an old device that has me confused

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Perhaps it relies on a plug with a 3rd dummy pin, so you cant reverse the plug. But it's still wrong. Replacing the plug or improperly wired outlet could lead to you being electrocuted. If these were our unpolarized sockets, there would be dead people.
Here's the end of the cord that was on it

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Old 02-14-2021, 01:06 PM   #14
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Default Re: Question on AC grounding - found an old device that has me confused

polarised - sort-of
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Old 02-14-2021, 01:19 PM   #15
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Default Re: Question on AC grounding - found an old device that has me confused

I bet there is a resistor (1~5meg?) in that heat shrink between ground and the line neutral wire, and it is NOT directly connected. There is usually a capacitor (0.01f) across the resistor as well but not always.
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Old 02-14-2021, 02:13 PM   #16
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Default Re: Question on AC grounding - found an old device that has me confused

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I bet there is a resistor (1~5meg?) in that heat shrink between ground and the line neutral wire, and it is NOT directly connected. There is usually a capacitor (0.01f) across the resistor as well but not always.
No cap, but yes, there was a 1.5 Megaohm resistor in there.
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Old 02-14-2021, 02:17 PM   #17
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Default Re: Question on AC grounding - found an old device that has me confused

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Originally Posted by televizora View Post
Only the ground terminal of an outlet can be connected to the chassis of a device, if it is not ClassII - Double insulated. This is wrong. Perhaps it relies on a plug with a 3rd dummy pin, so you cant reverse the plug. But it's still wrong. Replacing the plug or improperly wired outlet could lead to you being electrocuted. If these were our unpolarized sockets, there would be dead people.
I think it's been determined that the inline 1.5 MΩ resistor would be sufficient at keeping people from dropping dead... if I had not ripped the thing completely apart, I might have run some tests on it... but under the assumption that it was just a straight connection, I wanted nothing to do with it ... other than harvest the cord for a different device and the transformer for an unknown future project.

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Old 02-15-2021, 12:38 PM   #18
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Default Re: Question on AC grounding - found an old device that has me confused

Yep, the high value resistor (see posts #8 and on) makes it "sorta ok" in the old days.
It is still retarded though: because if there is a failure in the 20 cent Chinesium transformer that puts the primary across on the secondary and it finds a way over to the ground:
Well then there is no low impedance return path to the breaker, so the fuse wont trip, and you will be dead if you touch the chassis.
There can be other failures too that cause this, like a wire coming loose from that 0.01 cent crimp connector for example.

All this stupid shit is to reduce interference in sensitive equipment that require a ground to work properly to begin with.
Well here is an idea: use a fucking cord with aground then you numbwit manufacturers!
-But ohh, what if the customer don't have a grounded outlet?
Well that's his fucking problem then, isn't it?
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Old 02-15-2021, 12:58 PM   #19
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Default Re: Question on AC grounding - found an old device that has me confused

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you will be dead if you touch the chassis.
unlikely in a country that doesnt have *real* mans voltage.
(yes, i spelt "mains" wrong! )
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Old 02-15-2021, 01:01 PM   #20
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Default Re: Question on AC grounding - found an old device that has me confused

They still make 2-prong extension cords and three conductor cords are less physically flexible and heavier than two conductor cords.

The device also marks UL - so it theoretically is UL listed and thus tested for electrocution hazards. I didn't think RatShak was so bad to sell (under their name) things that could kill people, so I would have suspected the resistor or capacitor. As a device that actually creates radio frequencies they had to make it so that it doesn't generate too much RF, and the hope that the FCC symbol printed indicates it also passed part 15 rules...
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