On 1/13/2011 4:31 PM, Jim Brown wrote:
> On 1/13/2011 1:15 PM, Roger (K8RI) wrote:
>> So, I
>> physically grounded the shield in the mike cable to the "grounded"
>> chassis. No more hum.
> Yes, that fixed the problem. But that had NOTHING TO DO WITH GROUND (the
> EARTH). It was simply that the cable SHIELD needed to be connected to
> the SHIELDING ENCLOSURE to make it function as a shield. Now, a
> connection to earth is important for other reasons, including lightning
> protection and preventing hum and buzz from power line leakage currents
> when equipment is interconnected, but it has no effect on SHIELDING.
True but in this case I did connect it physically to ground/earth. The
question remains, why did it work for several years and then suddenly
start picking up AC? I'd like to know where it was coming from or what
changed to let it cause a problem.
That room has a lot of computers, network equipment, and house
electrical wiring. Using a scope and just taking hold of the probe tip
will show substantial voltage being picked up. AFAIK I had changed
nothing in the station within weeks of the problem showing up. Same
computers, same network, same router, same network switch, same cables.
OTOH it is "one mass of wires" behind the desks, on the floor, and in
> 73, Jim Brown K9YC
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