[Richard Crowley]

"hank alrich" wrote ...
Richard Crowley wrote:

How good are the green-wire grounds?

I don't know. If I measure between contacts at the quad box or power
strips I get:

120.5 V hot to neutral
0.5 V neutral to ground
6.4 V hot to ground

If "6.4 V" is not a typo, it seems like a BIG PROBLEM.

The Neutral (white) and Ground (green) are supposed to
be tied together somewhere back around the breaker box.
Even if this is fed from a 3-phase power drop from the
utility, there should be virtually zero V between neutral
and ground, and the voltage between hot and neutral
should be the same as between hot and ground.

This may be an indication of serious/dangerous problems
with the power wiring. I'd seriously consider telling them
to get a licensed electrician to correct/confirm the wiring
before proceeding with any signal wiring debugging, etc.

If it were a private residence we could discuss debugging
processes for fixing (or at least identifying) this problem.
But since this is a public building, it would be inappropriate
to suggest anything but legal work by a licensed electrician.

Should I run a long lead from the ground rod to the closet and read
Ohms
for that link to see if the building wiring actually connects the
greens
properly to the stake?

I would defer to a licensed electrician. You may have
identified a dangerous mains wiring problem.

Does the system hum with an isolated source (an MP3 player, etc.)?

Lots of hum when Onkyo fed by TiBook running on power supply.

But that happens to most of us. The power supplies on
laptop computers are notoriously reliable sources of noise.

No hum when running on battery with nothing else connected,

THAT is the test of whether the amplifier and speakers by
themselves have a problem. You seem to have confirmed
that the amp/speaker combo are "clean".

but add the CD player and get modest hum,

Can you plug the CD player into the power strip adjacent
to the amplifier? Is the plug on the CD player reversible?

add video receiver, even one leg of one receiver,
and loads of hum.

I suspected those satellite receivers from the start (:-)

The Onkyo doesn't hum when nothing is connected to it, and it doesn't
hum when FM is selected, playing radio, whether or not the CD player
and/or video receivers are connected to the it.

In that case, you might get away with those cheap/simple
stereo line-level isolation transformers they sell at Rat Shack.

What do the isolated outputs of the satellite receivers sound
like by themselves (on headphones, etc.)?

They sound okay feeding phones directly.

So it is only when connected to the amplifier that we start
seeing the ground loops, etc. It may be easier to use the
iso transformer(s) than to try to "fix" the grounding problem
with the satellite receivers. Especially since the grounding
of the RF line is likely required by local code.

Will be checking that next round. I have just discovered a potential
problem, but I haven't gotten a clear answer from the owners. There
are
two separate electrical services (2 separate meters). There is a
possiblity that the downstairs sports bar is fed from one of these
and
the upstairs piano bar from the other. If that's the case it could be
trouble since two of the video receivers in the closet feed screens
upstairs. I assume the video feeds (RF I think, based on Scott's
description of the connector type) are shielded, connecting receiver
chassis to video screen chassis, and that might give us the ground
loop.

Yikes. it would be interesting to know what is the voltage
differential between the green-wire "ground" on each system.
Or perhaps we don't WANT to know. :-/

If the video is being distributed from the receivers to the TV
screens via RF (threaded "F-connectors" vs. RCA or BNC)
then it may be easier to use one of those RF iso transformers
like they sell for home theater people with cable system ground
loop problems.