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Re: [TenTec] TT Orion 2 and Yaesu CM-500 - RF problem sorted! - classic

To: tentec@contesting.com
Subject: Re: [TenTec] TT Orion 2 and Yaesu CM-500 - RF problem sorted! - classic Pin 1 issue
From: Jim Brown <k9yc@audiosystemsgroup.com>
Reply-to: k9yc@arrl.net, Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Fri, 10 Apr 2015 21:43:00 -0700
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
Steve,

No matter what the manual says, much of what Ten Tec and other ham mfrs do is WRONG, including much of what you are describing. CAPS added for emphasis.

The ONLY proper connection for a cable shield is the CHASSIS at the point where it enters the box. PERIOD. Not "rig ground." What IS OK is for an overall cable shield to go directly to the chassis and a signal pair within that shield to go to Mic and Mic return, but I seriously doubt that's what they're doing.

On Fri,4/10/2015 6:30 PM, Steve Ireland wrote:
As Jim K9YC says, a Misconception Alert’ here is very important – and part of 
the problem is about what we name things.

In the TT Orion 2 manual, as per what seems the general convention in electronics today,  the 
–ve connection from a mic element is the connection called ‘MIC GND (mic signal ground 
– pin 7).

It's a common mistake, called the Pin One Problem.

As checked by my ohmmeter, there is no direct connection between the MIC GND at 
the rig and the actual rig ground (labelled GND (pin 5).

The good news is the Orion 2 manual says: ‘Keeping the chassis ground and mic signal 
ground separated are done to reduce the possibility of inducing stray hum or RFI into the 
transmitted signal” and gives a nice diagram to help ensure you do this.

Again, this is a CAUSE of hum, buzz, and RFI, not a solution to it.

What I said in the previous email was what I physically did, but the idea and 
the end result was to avoid a path for RF current flowing on the shield of the 
microphone cable (which runs from the TT Orion 2 to the interface box) into the 
electret microphone element.

As K9YC points out, this was achieved by connecting the shield/chassis ground 
to the interface box, which kept the RF outside the box.  The so called MIC GND 
(microphone -ve) travels through the interface box without any connection to 
chassis ground.

See above. The only way that this would be good circuit is if the two mic wires (mic and mic gnd) are a twisted pair inside a braid shield that is bonded to the chassis at the point of entry, and the circuit inside the box is some form of balanced input.

The CM-500 enclosure is made of plastic and so (of course) offers no shielding 
to the electret element.

Like most hams, you are fixating on shielding, when the problem is almost never a lack of shielding, but rather improper connection of cable shields.

Here's a tutorial I've done on proper power, bonding, and audio for ham radio.

http://k9yc.com/GroundingAndAudio.pdf

Soon after I moved to CA, I was invited to guest-op at N6RO, a superstation about 70 miles east of San Francisco. The power there had been very well installed, but lots of dumb stuff had been done with power strips, MOV boxes, and interconnects for computers and other gear, and the buzz from power system leakage currents were absolutely awful on the air. In an afternoon, I applied the principles outlined in that tutorial to Ken's six legal limit stations, each with its own computer, interface boxes, etc . By the end of the afternoon, buzz had gone from awful to inaudible. And all without a single audio transformer anywhere in the station. Just proper bonding of both power and the gear.

73

As one of the antennas I now use – a 40m Moxon Rectangle – has feeders which 
come into the shack, so I can vary the amplitude and phase of the current in the reflector 
for maximum gain/F to B and its tuning unit currently has no cover, there seems to be a lot 
more RF in the shack than there used to be...

Despite this situation, I was grateful (and a little surprised) that doing the 
above to the interface worked OK without the need for any further RF 
suppression.

 Part of it is that you got lucky. :)

73, Jim K9YC

Sorry for any confusion caused – and thanks to all who helped.

Vy 73

Steve, VK6VZ


On Fri,4/10/2015 12:44 AM, Steve Ireland wrote:
Isolating mic ground from the rig ground, along with making sure the interface 
aluminium box was connected to the rig ground, stopped the RF from getting into 
the CM-500 electret element.
Misconception Alert!
We are not talking about a connection to Mother Earth. That is NOT part
of a solution to RFI. What we are talking about is avoiding a path for
RF current flowing on the shield that goes THROUGH the box. When we
connect the shield to the chassis, we keep the RF outside the box.



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