Thanks Jim.
I will have a look at your tutorial.
The old TV sets of a few years ago are/were most troublesome. We have new sets
here and they are miles apart as far as RFI goes from the old sets. I checked
my cabling to the external boxes and it appears pin one of the XLR does go to
chassis of the two external boxes. I have them in a rack which the rails are
taken to station ground. So far I have no RFI issues now on 10, 15 or 20, 20
being my worst case band for RFI here. I also have many #31 and 43 mix
snap-on's all over everything. BTW, I burnished the paint off the surface
where the rack ears attach to the Behringer box. I felt that paint might be
preventing a good RF ground to the rack ears. I also installed a couple of
split cores on internal cabling from the front panel to the rear PCB which
holds all of the XLR/TRS receptacles.
73
Dale, k9vuj
On 13, Jan 2011, at 14:08, Jim Brown wrote:
> On 1/12/2011 2:49 PM, dalej wrote:
>> How would I cure a pin one problem in a particular piece of equipment.
>> Would I look at the input XLR or TRS and put that pin one directly to ground
>> or the chassis somewhere close to the receptacle?
>
> Let's stop confusing things by using the word "ground" when we talk
> about a connection to the chassis, or to circuit common. The key info
> here is that common mode current, usually, but not always shield
> current, needs a complete circuit (or an antenna). Our objective is to
> provide a path that does NOT include circuit common, thus keeping it
> outside of equipment.
>
> Low frequency current usually finds a path to the earth, or to the power
> system equipment ground (Green Wire), and NEC REQUIRES that the the
> Green wire go to the chassis of equipment, and to the earth at panel
> where power enters a building. Thus, a connection to the CHASSIS (or
> more correctly, the SHIELDING ENCLOSURE) for all cable shields, return
> of power cables, solves that problem. That is, all the common mode
> current stays outside the box.
>
>> What about the PCB? Some of those doggone connectors are mounted to PCB
>> and not that easy to find the pin one connection. I like the theory, but
>> implementation is another matter, in some cases.
>
> YES, that's exactly the root CAUSE of most pin 1 problems, and because
> of the way stuff is built, they are usually VERY difficult to fix
> without rebuilding and rewiring equipment (and perhaps causing more
> problems than you solve). In those situations, the best solution is
> usually to simply kill the current on that cable, either with a common
> mode choke, or by diverting the current to a lower impedance path.
> Common mode chokes are most effective at RF, while diversion is usually
> most effective at audio and power frequencies. Both are typically needed
> on telephone equipment (including voice phones and DSL modems).
>
> RF current is on those cables because they are acting as receiving
> antennas. At high frequencies, no earth connection is needed to provide
> a return path for that antenna current -- the local wiring within a ham
> shack, or an A/V rig, or even the chassis of equipment may be large
> enough as a fraction of a wavelength to be the other half of the antenna.
>
> There is a lot of tutorial material on my website about this. The Power
> Point on Ham Interfacing, and the Solving Problems In the Shack chapter
> of the RFI tutorial, address the audio frequency solutions. The RFI
> tutorial also includes lots of detail about how ferrite chokes work, and
> how to use them to kill RFI coupled by pin 1 problems.
>
> http://audiosystemsgroup.com/publish.htm
>
> Two sets of people need to understand all of this, and the two must
> react to it differently. Those of us who USE equipment must usually
> work around those problems with band-aids like diversion and ferrite
> chokes, and by making sure that our stations and systems are properly
> bonded together, to the earth, and to the power system. Those who
> DESIGN and BUILD equipment must BUILD IT DIFFERENTLY! In 1994, when the
> pro audio world was first alerted to the Pin 1 Problem by Neil Muncy
> (ex-W3WJE), a very good pro audio mfr called Rane acknowledged the pin 1
> problems in their gear, and over a period of about five years made
> running production changes to all of their more than 50 products to
> eliminate them. When that transition was complete, they found that their
> customer support costs, most of which addressed problems with hum,
> buzz, and RFI, had dropped by 90%!
>
> 73, Jim Brown K9YC
>
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