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

[Jim Brown]

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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