Re: [TowerTalk] Station Ground

["K8RI on Tower Talk" <k8ri-tower@charter.net>]

> On Sat, 15 Jan 2005 03:16:11 -0500, Pete Smith wrote:
>
> > I had fairly
> > severe AC hum on the audio, apparently the result of a few millivolts
> > potential difference between chassis.  I finally cured it by 
> > experimentally
> > connecting the chassis of the computer and radios directly to one another
> > with heavy copper (not through the bus bar).
>
> You were experiencing a "pin 1 problem," probably in both the computer and 
> the
> radio. Nearly all computer sound cards are built with pin 1 problems, and 
> so
> are nearly all ham rigs. The correct connection of a cable shield is to 
> the
> CHASSIS of the equipment on either end, NOT to the printed circuit board

But what do you do with equipment like the Icom 756 Pro that uses the mike 
shield as a neutral return to the PC board?

I had an intermittent AC pickup.  You know the kind that is always there, 
you make a change and it goes away... only to return the next time you use 
the rig.

Grounding the shield worked well, as did just reaching up and touching any 
bare metal on the rig or amp.  The hum would be gone.  Other times it wasn't 
there at all.

This is an old house and who knows where all the wiring goes, but I'd bet 
dollars to donuts there is a floating neutral *somewhere* on a circuit in 
the house.  I can say with a pretty strong certainty it isn't on either 
circuit in this room.

Unfortunately instead of seperate cirucits for the outlets and lights in 
each room they daisy chained them to save on circuit breakers.  The kind 
where the lights in the living room are on the same circuit as the outlets 
in the garage.  The outlets in the living room are on the same circuit as 
the lights in the back bedrooms which are on the same circuit as the 
bathroom.

I've installed a 200 amp box, consumers has run in a new underground feed, 
I've added a second box with a transfer switch fed by a 9500 watt generator 
(for furnace, pump room lights, Microwave,  refrigerator, freezer, *sump 
pump*, ham station, and yes... the TV in the living  room along with enough 
lights upstairs to find our way around.

Quite possibly as I eliminate circuits and break them into smaller sub units 
I will find the culprit. (I hope)
I did at the time, try turning off breaker by breaker and I don't remember 
any positive results.
I spent hours rerouting cables to find the signal quiet, but the hum was 
back the next time, or the time after that.
The only thing that was sure fire as still continues to work is the 
grounding of the mike shield, but that eliminates about half of the mike 
functions.


(often
> called "audio ground" or "control ground" on the mic connectors and the 
> rear
> panel jacks. One sure sign of a pin 1 problem is an RCA or 1/8" jack that 
> is
> insulated from the chassis. The easy cure is to remove the shield from 
> that

The socket is grounded to the chassis.

Roger Halstead (K8RI, EN73 & ARRL Life Member)
N833R, World's Oldest Debonair (S# CD-2)
www.rogerhalstead.com
<snip> 


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