Studio Technologies 740 - Manual (Page 22)

Issue 3, May 2004 Model 740 User Guide
Page 22 Studio Technologies, Inc.
Model 740
Reference Tone Frequency
Configuration
Two shorting jumpers are used to select
the frequency range of the reference
tone oscillator. A trim potentiometer is
then used to set the frequency to the
exact desired value. From the factory the
frequency of the reference tone is set to
be 400 Hz. This is accomplished by first
setting the jumpers to the two locations
that are labeled 400HZ. Then the trim pot
is adjusted to give a 400 Hz signal when
measured between the common and tone
test points. Like the jumper locations,
the trim pot and tests points are located
on the Model 740’s printed circuit board
assembly.
To change the frequency to 1 kHz simply
start by moving the jumpers to the posi-
tions labeled 1KHZ. Then, using a fre-
quency counter, adjust the trim pot labeled
TONE FREQ to give 1 kHz on the test
point labeled TONE. Finding the reference
tone frequency configuration location is
not difficult. When viewed with the cover
off, it is located adjacent to the back-panel
headphone output jack.
Be careful not to adjust the trim pot that
is labeled TONE LEVEL. This adjusts the
output level of the oscillator to give pre-
cisely –2 dBu to the main audio bus when
the front-panel tone button is enabled.
Should this pot have to be readjusted the
procedure is quite simple: measure across
the COM and TONE test points and adjust
the trim pot to give –2 dBu. The measure-
ment equipment must have a high input
impedance (>2 k ohms) so as not to load
the oscillator’s output.
Model 740 Limitations
The Model 740 was carefully designed
with simplicity of operation in mind. Only
the required features and functions were
included. The goal was reliable audio
performance with little risk that operator
confusion or errors would occur. Protect-
ing the audio signals associated with live
news “shots” was foremost on the list of
requirements. The Model 740’s design
team spent lots of time reviewing what
could be included, then were told by
field personnel to take everything out
that wasn’t absolutely required! Here’s
a review of some of the features that
weren’t included and the reason why:
• Including microphone phantom power
would have been nice. But it’s almost
never needed for live news broadcasts.
(An external phantom supply can easily
be used for those rare occasions.) And
providing true 48 volt phantom, along
with individual on/off switches, is the
only technically strong way to do it. So
from the start this feature would have
added six more pushbutton switches.
And while generating 48 volts isn’t dif-
ficult, it does add cost and generates
additional heat in the enclosure. Other
mixers don’t worry about supplying
the “real deal,” instead providing 12
volts, or some nonstandard value such
as 15 volts. While often acceptable,
these voltages can’t be depended
upon to power all phantom-powered
microphones.
• Including high-pass filters on the
mic/line inputs was a possibility. But
in the “heat of battle” it’s unlikely that
an operator will have the time to enable
them. Also, this requires another six
front-panel switches for the on/off
functions. If wind noise is an issue,