[Elecraft] K3: Added protection for RS-232 port (summary)

[Tom W8JI]

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 chassis to every other
> chassis, including the computer with big short copper. Once you 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