Studio Technologies 42 - Manual (Page 11)

Issue 1, July 2004 Model 42 User Guide
Page 12 Studio Technologies, Inc.
power present LED should be lit. This LED
is powered by the internal 36 volt DC
power supply that is used by the circuitry.
If the LED is not lit confirm that AC mains
power is active (“hot”) and that the cord
is securely mated with the inlet connector
on the Model 42’s back panel.
In all foreseeable situations, both normal
and abnormal, the front-panel LED should
be lit. However, it’s possible that if all four
IFB circuits are being presented with a
short circuit-condition, the internal 36 volt
power supply may enter its protection
mode and shut down. In this case the LED
will not light, or will light intermittently. If
the LED presents this scenario, even after
confirming that mains power is correctly
being applied, try removing the loads
from the IFB circuits. The easiest way to
do this is to remove the 3-pin female XLR-
type connectors that are plugged into the
Model 42’s back panel. If after a few sec-
onds the LED again lights, carefully check
the IFB circuit wiring for fault conditions.
If the LED still doesn’t light, even after
confirming that mains power is present
and that the IFB circuits are not shorted,
it’s likely that the unit requires factory
service. For safety in the event of a major
internal failure, the internal 36 volt power
supply contains a fuse in series with the
incoming mains power. This fuse will
open (“blow”) only if a serious failure
occurs inside the unit. The fuse is not
field-replaceable. The Model 42 must be
returned to the factory, or an authorized
service location, for review and repair.
Maintaining Correct Input
Signal Levels
The Model 42’s four 2-channel audio
inputs are designed for nominal signal
levels of +4dBu. Applying signal levels
significantly lower than +4dBu will reduce
the signal-to-noise ratio (raising the per-
ceived noise floor) and can prevent the
connected user devices from operating
optimally. Applying signal levels signifi-
cantly higher than +4dBu will reduce
the headroom and greatly increase the
chance of reaching audio “clipping.”
Obviously, these cautions are not unique
to the Model 42, but apply to most audio
equipment.
The easiest means of confirming that the
Model 42 is being presented with the
correct audio levels is to use a Model 72
Level Meter/Interface, also available from
Studio Technologies. The Model 72 is
a compact, portable device that plugs
directly into IFB or intercom circuits and
provides two useful functions. Two
5-segment LED meters display the audio
levels present on pins 2 and 3 of the
connected circuit. In addition, two “dry”
line-level audio outputs are provided.
The Model 72 should prove to be very
useful, both during initial Model 42
installation and routine system testing.
Complete information on the Model 72
is available on the Studio Technologies
website.
Maintaining Correct IFB
Circuit Current Draw
Each of the four IFB circuits is designed
to provide up to 200 milliamperes of DC
current. By design, the IFB circuits are
protected so that an overload condition,
or even a complete short circuit, should
not cause damage. Exceeding 200 milli-
amperes will cause the protection circuitry
to come into play. An overload condition
will cause the output voltage to shut off
continually or intermittently. The exact