TM 11-6625-596-45
CHAPTER 2
FUNCTIONING OF EQUIPMENT
impedance of the earphone being tested.
2-1.
General
c. Microphone. Test microphone 1A2A4A4
designed to perform eight different types of tests on all
measures the acoustical output from a receiver to which
types of transducers and telephone communications
a known electrical input has been applied from the
equipment. To perform such a wide variety of functions,
power amplifier. The output of the test microphone is
the test set consists of a number of separate circuits and
coupled to LEVEL control A4AT1 by input transformer
assemblies which are selected and interconnected by
1A4T2 when TEST CONDI-TION switch 1A4S9 is at
pushbutton and rotary switches.
COUPLER. With 1A4S9 at ART REC or TEL LINE,
b. The TB-716/U tests equipment in one or
1A4T2 is connected to the MICROPHONES panel
more of the following ways:
terminals. MICROPHONE LOAD switch 1A4S2 selects
(1) By applying a known input signal,
the correct tap on the primary winding of 1A4T2 to
either electrical or acoustical, to the equipment under
match the impedance of the microphone being tested.
test, and measuring either the electrical or acoustical
output of the equipment under test.
d. LEVEL Controls A and B. LEVEL controls
(2) By applying an input to the equipment
A (1A4AT1) and B (1A4AT2) are decaded ladder type
under test which simulates its normal operating
variable attenuators providing a total attenuation of 0 to
condition.
110 decibels (dB) in 1-dB steps. The LEVEL attenuators
(3) By measuring the output of the
control the input to preamplifier 1A3A1 so that a
equipment under test.
midscale reading of meter 1A4M1 can always be
(4) By comparing the equipment under
obtained, regard-less of the sensitivity of the
test to a standard circuit in the test set.
microphone or receiver being tested.
2-2.
Block Diagram
e. Preamplifier.
Microphone
preamplifier
1A3A1 provides a fixed gain of 60 dB to raise the level
of the weak signals from the microphone ahead of the
716/U. The location of the various controls, circuits,
switching and filter circuits.
connectors, and assemblies is shown in figures 3-1
f.
Filters. Filters 1A3FL1 through 1A3FL4
a. Noise Generator.
Noise generator
divide the output of the preamplifier into four frequency
assembly 1A3A3 produces a random noise signal, with
bands. By measuring the electrical energy in each
band, it is possible to determine the frequency response
300 to 5, 100 Hertz (Hz).
characteristic of a micro-phone or receiver.
b. Power Amplifier. Power amplifier assembly
1A3A4 raises the output of the noise generator to a
g. VALUE Controls C and D. Since the output
nominal level of 1 watt. The power amplifier output is
of a microphone is usually not uniform but increases as
applied to a resistive load and to either test loudspeaker
frequency increases, VALUE controls 1A4AT3 and
1A2A2 or to NOISE GEN terminals 1A4E12 and
1A4AT4 are placed between the CENTER (1A3FL2) and
ARTIFICIAL MICROPHONE terminal 1A4E as selected
HIGH (1A3FL3) filters and meter amplifier 1A3A2 to
by NOISE GENERATOR switch 1A4S3. The power
provide additional attenuation in these frequency bands.
amplifier may also be applied to EARPHONES panel
terminal 1A4E8.
RECEIVER LOAD switch 1A4S2
selects the proper resistor network to match the
2-1
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