[TenTec] TT Orion 2 and Yaesu CM-500 - RF problem sorted! - classic Pin 1 issue

[Steve Ireland]

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).  

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.

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.

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

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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