Re: Sports Bar Hum Job Questions

Richard Crowley wrote:

"hank alrich" wrote ...
I'm helping folks who have a sports bar get some of their audio action
working better. Following info from RAP in other ground problem threads
I've determined that the ground is good and that the AC to the closet
with the CD changer, Onkyo receiver and a stack of DishNet video
receivers is good, too.

There are two hum-related problems here:

1. A certain amount of hum seems related directly to the Onkyo receiver
in that about 3.5 volts is showing between the chassis of the CD player
and the receiver (probing at the ground collars of their respective RCA
connectors) when the system is first started cold after being turned-off
overnight. As it warms up that rises, to as much as 12 volts. When the
CD player AC power was sourced from an outlet on the back of the
receiver I measured 10.5 volts at cold start, and lord only knows what
it must have been after warming up. When I hooked the CD changer's AC
cable to the same power strip feeding the rest of the gear in the closet
the voltage dropped to 3.5 cold.

Do any of the units have 3-prong power plugs/cables?

The seven video receivers are 3-prongers.

Or 2-prong?

The Onkyo receiver and the Sony CD changer are 2-prong.

Exactly where/how are these plugged into the source of power?

There's a quad box outlet on the wall behind the units.

Are they all running from a power strip? Or more than one?

There are two power strips, connected to adjacent outlets in the quad
box.

Is everything on the same branch circuit (breaker)?

Everything in the closet is, but I wonder about the TV's to which the
receivers feed signal.

How new/reliable is the building wiring?

Good question; most of it's definitely not new, has obviously been
expanded in various stages.

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

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?

What are the startup and warm-up chassis voltages relative to
green-wire ground? Else you are only measuring the differential
between reciever and CD player, etc. which is the equivalent of
one equation and two unknowns. We don't know which unit is
behaving badly (or perhaps both).

Right now I'm in the closet. I've had the gear powerd-up for about three
hours, so it's well warmed-up. I measure 0.35 V Onkyo chassis to green
wire ground (via one of the power strips), with nothing connected to the
Onkyo inputs, and the same for the Sony Cd player to ground. I measure
12.8 V chassis-to-chassis (Onkyo-to-Sony).

But some of these measurements seem not always repeatable. Sometimes I
get that 12.8 V, next time I get 10 V, then 12.8 again, and so forth,
and that seems to apply to all I read at the Onkyo and Sony, but not for
the measurements made at the quadbox or power strips, which remain far
more stable.

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

Lots of hum when Onkyo fed by TiBook running on power supply.
No hum when running on battery with nothing else connected, but add the
CD player and get modest hum, add video receiver, even one leg of one
receiver, and loads of hum.

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.

What is the chassis voltage of the receiver with nothing attached?
(relative to green-wire ground)

See above.

Does it change with the speaker lines connected?

No.

Is this a sign that the power supply filter caps in the receiver are
leaking, and leaking more as they heat?

Filter caps in the power supply should have NO effect on the
chassis voltage. Regardless of how bad they are or what temp.
If they do, something is seriously wacko (perhaps dangerous?)
Certainly bad filter caps will contribute to hum in the system,
but you should NOT see any of it at the chassis ground point.

2. There is a ridiculous amount of hum coming via the audio feeds from
the DishNet video recievers, fed to the Onkyo receivers audio inputs.
Yet there is no comparably appreciable voltage showing between those
chassis; measuring again RCA's-to-RCA's I see single-digit millivolts.

Normally one would see zero volts between the chassis of the
various components. Audible hum could have other sources than
differential ground potentials.

Does the receiver hum with no inputs connected?
What is the receiver chassis voltage relative to green-wire gnd?

Classic case of disconnect everything and then restore it one
connection at a time and see the effect of each individual
connection.

Yep, been doing that today.

While everything is disconnected, measure each chassis
voltage against a common reference (mains green-wire gnd)

I plan to go back with the MIO and feed audio from the video receivers
to SpectraFoo to see if noise associated with those audio feeds is
actually being delivered along with the intended audio, i.e., if that
hum is not simply inter-component ground related, but part of the
"signal".

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

They sound okay feeding phones directly.

What is the chassis voltage of the sattelite recievers (each
of them isolated) relative to green-wire ground?

0.9 to 1 V.


There is a standardized way of testing the safety of the exposed
metal parts of equipment. I will look up the details when I get
home. IIRC, it is a resistor and a capacitor and a multimeter.
I didn't find anything online. Likely using the wrong search term.

Is the RF feed from the satellite dish(es) grounded at any
other point?

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.

I can't proceed further today, as the joint is open for St. Paddy's Day
and I can't go 'round flipping breakers to see what's what. I've been
poking around in this long enough for an average Saturday anyway. "Get
me out of this closet!" <g> There's going to be a jam here tonight and
it's time to deFluke, go get food, and then break out the mandolin,
banjo and guitar.

Frequently there is a "grounding block" before
the coax enters the building. This is supposed to be connected
with a stout wire to a physical ground (water pipe, etc.) as a
primary protection against lightning/static entering via the
sattelite dish cable. If the grounding blocks are connected
to some kind of ground that is different than the ground used
by the electrical power system in the building, that could be
a source of significant ground potentials right there. This is
frequently a problem with cable systems, also where the
cable system's notion of ground isn't necessarily the same
as the power mains.

Okay, thanks for that, as well as for the rest of this, Richard. I'll
chase that stuff next round if the possibility of two juice feeds for
the down and up stairs rooms turns out to be a false lead.

--
ha
Iraq is Arabic for Vietnam
.

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