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[RFI] Fixing Pin 1 Problems

To: rfi@contesting.com
Subject: [RFI] Fixing Pin 1 Problems
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Thu, 13 Jan 2011 12:08:02 -0800
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
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.


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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