Studio Technologies 42A 2008 - Manual (Page 18)

Issue 1, February 2008 Model 42A User Guide
Page 18 Studio Technologies, Inc.
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 for more than a few seconds
will typically cause the auto shut-down
mode to become active. A continuous
overload condition will cause the output
voltage to cycle through its 4-seconds
on, 10-seconds off sequence. Restoring
the output load to be within the rated 200
milliamperes will allow the output to again
operate normally. In extreme cases, such
as where the Model 42A is located in an
environment with elevated temperatures,
a few minutes may be required from the
time an overload condition is removed
to when normal operation will again take
place. Please don’t test the Model 42A’s
ability to sustain frequent overload or
short-circuit conditions! The long-term reli-
ability of the unit can be impacted by the
stress caused by these fault conditions.
The four status LEDs make it simple to
know if an excessive load, or a short cir-
cuit, is being placed on one or more of the
IFB circuits. The LEDs provide a direct in-
dication of the IFB circuits’ DC output volt-
age. Each output voltage is directly related
to the amount of current being drawn, as
well as indicating when its output circuit
has entered protection mode and essen-
tially shut down. During normal operation
the DC level on pin 2 of the connectors
associated with an output circuit will range
from approximately 28 to 30 volts. An LED
will begin to flash on and off if the level
falls below approximately 24 volts DC.
This will occur when the current draw is
greater than nominally 200 milliamperes.
As previously discussed, if the fault lasts
for more than 4 seconds the LED will flash
at a faster rate. In addition, the DC output
will shut down for a 10-second period.
There’s really only one piece of advice
when it comes to understanding how
to use the under-voltage status LEDs: if
they’re flashing there’s a problem that
must be corrected! The most likely cause
will be a wiring fault that causes a partial
or full short circuit between pin 1 (com-
mon) and pin 2 (power with audio) on one
or both of the IFB circuit’s XLR-type output
connectors. Another cause can be due to
problems with the IFB user devices. Either
a defective user device can be drawing
too much current, or too many user de-
vices end up being connected to the same
IFB circuit. Troubleshooting an IFB circuit
problem should prove quick and easy.
Begin by disconnecting the IFB user de-
vices. Observe the status LED and see if
the problem has gone away. If not, review
the interconnecting cables and find the
fault condition. Within 10 seconds of the
problem being “cleared” the status LED
will stop flashing.
Determining the actual IFB circuit current
draw won’t often be required but can be
useful in a tough troubleshooting situa-
tion. This measurement can be performed
using any good-quality digital multimeter.
Begin by setting the meter to measure
DC current. Then place the meter leads
in series with pin 2 of the XLR-type female
connector associated with the connec-
tion being made to the IFB circuit to be
tested. The easiest way to measure the
pin 2 current is to create a simple adapter
cable using one female and one male