On 1/13/2011 3:08 PM, 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.
I choose to use the word ground because that is what I used. I had an
intermittent problem with something that appeared to be pin one and then
it didn't. The hand mike on the 756 Pro worked just fine and did for
several years. I added an Alpha 76A and things still worked fine 160
Then one day it started picking up a signal. I turned the amp off and
it was still there. Key the mike and it was full output and it didn't
matter what level I had the drive set. Be it 5 watts I had 5 out, 100
and I had 100 out. It sounded like 60 or 120 cycle AC and was not RF
feedback. Then I discovered if I reached up and put a finger on any
"grounded" bare metal in the station the hum would go away. So, I
physically grounded the shield in the mike cable to the "grounded"
chassis. No more hum. Of course Icom uses that shield as a return for
some control functions which I no longer had after grounding it. Then
again I never used the mike controls except for the PTT.
More recently, switching to a Heil pro headset and no hum with it
either. No RF feedback with either the Alpha or the Emtron at the legal
limit although I did have to install a current balun in the feedline to
the 75 meter antenna at the tower near the remote antenna switch to get
rid of the RF coming back into the shack and lighting all the LEDs. <:-))
The only noise I get from the Heil headset is due to background noise in
the shop. Humidifier, furnace, and who shop filter. Too bad the mike
isn't directional AND noise canceling like the ones in aviation headsets.
My voice has a very high peak to average power and with the sensitive,
non directional mike I can use very little compression or it does a good
job of putting out all the noise in the shop. I usually turn off the
humidifier, but the furnace and filter have to keep going. With just a
little compression the darn thing even picks up the fan on the amp.
> 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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