Radio with AC mains connected to antenna, safety hazard - Page 1
Forum section: electronics / general

author: electr_peter
author_rank: Unknown
timestamp: September 09, 2016, 08:47:55 pm
content: Browsing local electronics forum I noticed an interesting post about dangerous radio receiver.Link to Lithuanian forum post.Short summary- someone bought a small AC powered FM radio in local market place and noticed"tingle"in metal antenna. Electronics repairman deduced that this is becauseAC mains is connected to antenna via diode bridge, meaning no isolation. It is not a capacitive coupling, test with light bulb proved it to be direct. See added pictures with traced circuitry.Obviously, such design is verydangerous(and 99.99% illegal).Other posts suggested that some USSR period (and older western/chinese) radio designs also have antenna connected to mains via diode bridge or autotransformer. As a remedy installation of proper isolation transformer or high voltage caps in series with antenna were suggested.Question- do you know of any common radio that has antenna connected to mains? Is this an old design practice (from times when earth was not used and live chassis were common)? Or is it rather stupidity of a designer?

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 10, 2016, 02:27:38 am
content: Many old CRT TVs and tube radios used nonisolated power supplies. That's perfectly fine as long as there are no exposed connections accessible from the outside. Most likely the antenna should have a Y rated capacitor in series but it either removed by a tech not understanding why it's there or is not an actual Y rated capacitor.

author: AG6QR
author_rank: Frequent Contributor
timestamp: September 10, 2016, 03:12:28 am
content: Not quite exactly the same thing, but here's a description of a similar problem in the power supply of the Hallicrafters S-38, a very common shortwave radio produced from the postwar 1940s through the early 1960s or so. I believe a very similar design was used for many tube-based radios of that era. Depending on which way you plugged in the (unpolarized) power cord, the chassis could have 120V present on it.http://www.the-s38-guy.com/Page_2.htmlI had an early S-38 in my bedroom when I was a child (I'm not that old -- it was an antique when I got it). Yes, I got shocked by it a few times.It would not meet any modern safety standard.

author: Len
author_rank: Frequent Contributor
timestamp: September 10, 2016, 03:59:52 pm
content: As far as I can tell, old live-chassis radios generally had the antenna connected to ground through a capacitor, not directly. So the antenna wouldn’t give you a big shock but sometimes there were exposed mounting screws that could be dangerous.Modern devices are supposed to have a grounded chassis or be double-insulated, but of course there’s plenty of cheap crap that doesn’t follow the rules. Here’s a teardown of a “slightly dangerous” product by Big Clive:

author: Cyberdragon
author_rank: Super Contributor
timestamp: September 11, 2016, 04:39:44 pm
content: Quote from: AG6QR on September 10, 2016, 03:12:28 amI have one too (S-38A). I've gotten shocked so I now wear gloves when using it. Usually AC-DC (live chassis) radios are fine as long as you don't stick your hands inside, but metal case ones can be nasty since the case is connected to the chassis through a capacitor.