Tom,
Thanks for replying. I have found this to be true with my computer
I/F. In fact a common ground caused erratic PTT in the radios or
serious hum in audio. In the current control box, I interfaced RTS
and DTR thru opto-isolators to isolate the computer ground and the
audio is coupled thru 1:1 600-ohm transformers with the computer side
floating from the radio ground. The audio lines plug into the
computers normally with unbalanced shielded audio cables, thus are
grounded there. This is mainly for interface with my FT-847 as the
K3 provides isolated audio interfacing directly. The RS-232 is
routed thru the control box to either radio (switched) with normal
DB9 (sub-D9) connections. They are connected all the time.
As I related to Tom, off the list, I do not experience lightning at
this QTH. The coastal maritime influence apparently does not produce
it. We only hear thunder maybe once in a few years. So my questions
address only the ground-loop and safety issues. We only hear
lightning crashes on 160 or 80m when propagation reflect activity at
a distance (on 600m my noise floor runs -115 dBm on the SDR-IQ or S3
on the K3). Interior Alaska is subject to lots of lightning caused
wildfires, but not here.
73, Ed - KL7UW
------------------------------
Message: 13
Date: Tue, 20 Jul 2010 18:38:16 -0400
From: "Tom W8JI" <w...@w8ji.com>
Subject: Re: [Elecraft] K3: Added protection for RS-232 port (summary)
To: <Elecraft@mailman.qth.net>
Message-ID: <8c568fc5ee5144a78e090bd4c6af9...@tom0c1d32a93f0>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=original
I rarely disagree with Jim on audio issues, but I am going to disagree with
this:
> On Tue, 20 Jul 2010 09:38:23 -0800, Edward R Cole wrote:
>>The only dc-isolated wiring in my shack is the soundcard audio lines
>>from radios to computers.
Jim replied:
> Why are these isolated? They should NOT be. See my tutorials on The Pin 1
> Problem and on simple bonding of every equipment ...
...snip...
>... do that,
> there is no need for isolation, AND if properly bonded to all your other
> grounds, you have a safer installation.
The problem is Ham radios do not have isolated negative power sources, and
many or most supplies do not have floating negative terminals. Peak current
on SSB approaches 25 amperes. Voltage from the negative rail current, even
with .02 ohms resistance, would be .5 volts peak. A good portion of this
could easily appear across the length of the shield.
In broadcast studios or wiring, we would never allow an unbalanced audio
line to be grounded to the chassis at both ends. The same is true for my
headphone lines, or the lines from my radio to the computer. My microphone
lines are that way also, grounded ONLY at one point (generally the input
port).
Even when we ran balanced lines, shields were grounded only at the input
ends.
Isolating the grounds on audio lines is the thing to do. It never hurts, it
often helps, and it reduces the chances of fault currents damaging
equipment.
> Both your ham gear and your computer have Pin 1 Problems, and Pin 1
> problems are excited by shield current. That shield current results in
> hum, buzz, and RFI (RF in the shack). When you bond the chassis of
> equipment together, you virtually eliminate shield current.
Virtually is often not good enough. If the output end has an isolated
ground, ground loops are eliminated. Rather than bolt things on my desk
together with 00 ground cables, I isolate audio ground loops.
73 Tom
73, Ed - KL7UW, WD2XSH/45
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