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Author Topic: Old 3-prong 300ohm TV jacks - why three prongs?  (Read 1684 times)
NQ4T
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« on: July 28, 2018, 10:04:31 AM »

Hi all,

Now I might be a tad on the young side given the average age of hams, but I do know what twin-lead is and I can recognize the old twin-lead antenna connectors when I see one.

The thing I'm wondering is why the third prong? Is it functioning as a key preventing you from plugging it into something you shouldn't?

73,
Jay
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NE1U
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« Reply #1 on: July 28, 2018, 01:41:24 PM »

A shield.

Although I never saw shielded TV twin lead, but I have heard of it.
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W1ITT
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« Reply #2 on: July 31, 2018, 05:46:27 AM »

I recall shielded TV twinlead.  It was a bit oval, and did indeed have a foil shield with a drain wire so that one had something to connect and solder to.  It made some sense in that it could be taped to a metal mast without disturbing the impedance or, more important, the balance of the line.  I had a length of it spliced in and used it to bring my open wire line through a window to a tuner in the radio room.  My recollection is that it was a Radio Shack product and that the outside plastic jacket was woefully susceptible to UV degradation.
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N3QE
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« Reply #3 on: July 31, 2018, 05:55:05 AM »

I think you are talking about connector D in the picture below? This was common on European FM radio tuners from the 1960's. My recollection from the wiring diagram on the back of the tuners, was that it was "240 ohms" (within margin of error same as 300 ohms) balanced twinlead between the two outer prongs, and you could also use a single-ended 75 ohm coaxial antenna cable with shield to the center pin and center conductor to one of the outer pins. I also recall a drawing just showing a single whip antenna plugged into the outer pin.

I also recall US FM sets using three screw terminals with similar wiring diagrams for twinlead and coax. Although the ground was often not between the two twinlead terminals but instead was off to the side.

« Last Edit: July 31, 2018, 06:04:51 AM by N3QE » Logged
NQ4T
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« Reply #4 on: July 31, 2018, 12:36:18 PM »

I hadn't ever heard of shielded twin lead. I assumed the shielding would screw up the impedance.

It looks similar to that connector...except found pins vs flat. The wall plate side also had TV embossed.
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W9GB
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« Reply #5 on: July 31, 2018, 03:08:26 PM »

When Moseley Electronics was located in St. Louis, MO they had a broader range of products in 1950s and 1960s.  Catalogs from that period will show their product portfolio.
http://www.mosley-electronics.com

They sold a number of Over-the-Air (OTA) television products (Allied Electronics catalogs),
including the Twin-Lead Plugs and Wall Faceplates.  Foam twin-lead 300Ω was popular (lower-loss) for UHF installations, and cheaper.
I installed this system in the addition to my parent’s house (1970).
I replaced it in late 1990s with runs of Category 5 UTP and RG-6/U Quad-Shield.
« Last Edit: July 31, 2018, 03:11:40 PM by W9GB » Logged
N3QE
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« Reply #6 on: July 31, 2018, 06:05:47 PM »

They sold a number of Over-the-Air (OTA) television products (Allied Electronics catalogs),


YESSS!!!! Twinlead faceplates of various kinds!





This is one of the few with a third pin for ground:


My searches turned this one up too: An Octal jack on a faceplate! Obviously a genius at work! I'm jealous!!!


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