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shielded wires - Printable Version

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shielded wires - murf - 11-27-2012

Working on an old grunow 589 teledial, and was wondering why Grunow made so much use of metal shielded wires with the shield soldered to ground.
Why was it necessary to ground all the shields?
THey are a real Pain to repair.
I have a couple broken off on the 589 that I will need to address.
They don't give you any extra length on these either.
Anyone else run into this before?
murf

RE: shielded wires - Radioroslyn - 11-27-2012

Can you say HUMMMMMM? If you need to replace any I'd go with RG-174. It's a small coax. Much easier to manage.
GL
Terry

RE: shielded wires - codefox1 - 11-27-2012

You can get some donor shielded wires from a trashed cassette or 8 track. Tricky to fix shielded cable connections that have been ripped or gnawed off otherwise. If you want to fepair the old wire proceed with caution and an cool iron to make a splint to the sheild without shorting to the center conductor.

Of course you can get a brand new spool of new shielded wire, one or two center conductors if you need it.

RE: shielded wires - TA Forbes - 11-27-2012

I always keep the shielded cables from sets I am parting out. They come in handy.

RE: shielded wires - exray - 11-28-2012

You may be able to feed new wire through the existing shields.

RE: shielded wires - murf - 11-28-2012

Why couldn't a guy just make a replacement using a double wire, one being the ground?
Just a thought.
murf

RE: shielded wires - TA Forbes - 11-28-2012

Would not be as effective as having the entire conductor shielded.

RE: shielded wires - codefox1 - 11-28-2012

Forgot to mention that audio patch cords from the 99 cent stores are an inexpensive source when you only need a few feet of shielded cable. Antenna coax could be used, but it is a real pain to work with.

RE: shielded wires - morzh - 11-28-2012

Murf

Shield works as a total drain of all the charges if grounded: no matter where the electric field comes from, the potential is always zero and so no influence is imparted to the wire within. (this is of course onl'y partly true, as the electrical field if changing begets the magnetic field and the wire shield is not much of a protection there, but it all depends on the frequencies).

If however the shield is not grounded, it is fairly useless as the potential is now floating and capacitive coupling affects the wire within (Hum galore!).

Now, the second grounded wire you suggested: well, depends. One thing is when you have a 2-wire signal, of which one is GND. There the coax and a diff. pair a both effective. Better yet a twisted 2-wire pair.
But if there is just one wire that carries the signal and the shield is just the shield, then the second wire simply won't work as it onlhy protects one side of the signal wire, and you need to protect all the way around.
It actually does work if you deal with a PCB (a printed circuit) as the wires are essentially two-dimensional there, and so-called guard traces are indeed used (not that they are always effective, no, but sometimes, and if routed correctly (stitched to a plane well etc) - they may help).

RE: shielded wires - Arran - 11-29-2012

So are you talking about a braided lead between a tube shield and the chassis or are you talking about grid leads with a metal shield wrapped around them? I don't really understand why a shielded grid lead would be a problem but those tube shields with a soldered on ground lead are a pain to deal with, I wasn't aware that Grunow bothered with those.
Regards
Arran