Studio Technologies 43 - Manual (Page 10)

Model 43 User Guide Issue 1, July 2004
Studio Technologies, Inc. Page 11
In all foreseeable situations, both nor-
mal and abnormal, the LED should be lit.
However, it’s possible that if the IFB circuit
is presented with an over current or short
circuit condition the circuitry 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 24
volts DC is correctly being applied, try
removing the load from the IFB circuit.
The easiest way to do this is to remove
the 3-pin female XLR-type connector that
is plugged into the Model 43. If after a few
seconds the LED again lights, carefully
check the IFB circuit wiring for a fault
condition.
If the LED still doesn’t light, even after con-
firming that an external source of 24 volts
DC is present and that the IFB circuit
is not shorted, it’s likely that the unit
requires factory service. The Model 43
must be returned to the factory, or an
authorized service location, for review
and repair.
Maintaining Correct Input
Signal Levels
The Model 43’s two audio inputs are de-
signed for nominal signal levels of
+4 dBu. Applying signal levels signifi-
cantly lower than +4 dBu will reduce the
signal-to-noise ratio (raising the perceived
noise floor) and can prevent the connect-
ed user devices from operating optimally.
Applying signal levels significantly higher
than +4 dBu will reduce the headroom
and greatly increase the chance of reach-
ing audio “clipping.” Obviously, these cau-
tions are not unique to the Model 43, but
apply to most audio equipment.
The easiest means of confirming that the
Model 43 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 an IFB or intercom circuit
and provides two useful functions. Two
5-segment LED meters display the au-
dio levels present on pins 2 and 3 of the
circuit. In addition, two “dry” line-level
audio outputs are provided. The Model 72
should prove to be very useful, both dur-
ing initial Model 43 installation and routine
system testing. Complete information on
the Model 72 is available on the Studio
Technologies website.
Maintaining Correct IFB
Circuit Current Draw
The Model 43’s IFB circuit is designed
to provide up to 200 milliamperes of DC
current. By design, the circuit is protected
so that an overload condition, or even a
complete short circuit, should not cause
damage. Exceeding about 220 milliam-
peres will cause the protection circuitry to
come into play. An overload condition will
cause the output voltage to shut off con-
tinually or intermittently. The exact action
will depend on the specific overload con-
dition that is present. In general, the more
extreme the overload condition, the soon-
er normal operation will cease. Restoring
the output load to be within the rated 200
milliamperes will allow the output to again
operate normally. A few seconds may be
required from the time an overload condi-
tion is removed and when normal opera-
tion again takes place. Please don’t test
the Model 43’s ability to sustain frequent
overload or short-circuit conditions! The