Chassis ground lift – Allen&Heath Scepter SERVICE MANUAL User Manual
Page 9
Audio Owner Manual
Section 5
CHASSIS GROUND LIFT
Normally the chassis of the Scepter is connected to the audio ground with a
link on the DC Input connector.
The chassis can be lifted from the audio ground by removing this link.
This should not be confused with disconnecting the ground on an AC plug.
The AC ground is a safety ground; it is there as protection from electrocution if
some component should fail and cause an exposed surface to become live. The
safety ground is designed to shunt this voltage to ground, causing the equipment or
outlet fuse to blow. The Power supply chassis is attached to this AC ground since
there are potentially hazardous voltages inside the case. The Scepter chassis and
audio ground are isolated from this AC ground since the Power Supply feeds only
low-voltage DC to the mixer. The Scepter chassis and audio ground are floating in
respect to the AC ground; they will obtain their ground from the external equipment
they are attached to.
In most rack situations, there are a number of grounds tied together via the
metal rack rails. This can lead to ground loops which may cause hum in a system.
These ground loops can be eliminated in many
isolating the equipment
audio ground from this metal rail. If you need to isolate the Scepter chassis then
follow the procedure outlined below:
The Scepter is also provided with Pin-l lifts on all its Balanced outputs. In
some cases lifting Pin 1 is all that is required to fix a ground loop problem.
Chassis-Audio Ground isolation procedure:
1
Release the DC Power connector from the rear panel by removing the 2 fixing
screws
2
Carefully ease the connector out and locate the chassis lug (see diagram)
3
Locate the bare wire link that is attached to this
diagram)
4
Cut this wire and dress it to prevent shorting.
5
Re-install the connector and screws.
REMOVE THESE 2
SCREWS TO FREE
DC POWER JACK
FROM REAR
PANEL
REAR PANEL
DC POWER WIRES
UT THIS BARE
FROM THE SOLDER
DC POWER INPUT
5-PIN XLR
LUG TO ISOLATE
CHASSIS FROM THE
AUDIO GROUND
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