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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