Heathkit IM-4180 - Page 21

Page 58
ST
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AT
HEIT
circuit U101. U101 is a wide-band amplifier with a
gain
of
80.
The
IF
signal,
as
it
is
coupled
from
output
pin
6
of
U101
to
the
input
of
U102,
is
routed
across
a
limiter
network
consisting
mainly
of
diodes
DiGi
and
D102.
This
limiter
circuit
removes
the
peaks
from
the
IF
signal
and
effectively
creates
square
waves
from
it
for most input signal levels.
The square-wave signal is routed into operational
amplifier IC U102 pin 3 which functions as a
waveshaper and squeich circuit. Positive feedback
fro rad
from C115 causes the output square wave from pin 6
to switch up and down very rapidly. An offset bias on
the input from the limiter diodes provides a squelch
action for low-level signals.
Transistor Q102 functions as a trigger circuit. Positive
current spikes are coupled into the base of Q102
through capacitor Cii6. The collector output is
positive-going trigger pulses; which are coupled to
integrated circuit U103 through capacitor C118 and
resistors R131 and R132. Operational amplifier IC
U163 is a monostable multi
normally low. When a tri
present on input pin 2, its
is provided by capacitor C121
resistor R136. The output pulse
ps by
set to approximately 1.6
trol R138.
vibrator whose output is
ger pulse from (102 is
tput rises. Regeneration
to input pin 3 through
oxwsadth a aati. Of
ewiatn at U 163 pin ois
alibration (CAL) con-
Tig
ou
mm al am
Two operationa! amplifier aww, WV 104 CELA WY Aa, CLL
low-pass filters. The inputs to the filters are pulses of
fixed width and amplitude with spacing that varies
according to the deviation frequency.
AUDIO CIRCUITS
25C5
The pulses are inte d
low-pass filter network and, at the out
age
Lo
B.S =
audio signals. A resistive volta ivider consisting
ist 58, fo orm a range di-
6 is is a : full- scale | 7.5 kHz signal. At the junction of
resistors R156 and R157, the signal is 20 kHz full-
scale, and at the junction of R157 and R158, the signal
is 75 kHz full-scale.
The 2 kHz range uses the same signal as the 7.5 kHz
e, but the gain of the peak detector is changed by
switching resistor R161 into the circuit, along with
low-pass filter resistor R155 and capacitor C138.
a
whose input is a signal selected by the range switch
attenuator circuit. The output of U106 is rectified by
diode D107 and the signal is stored in capacitor C143.
The direct current developed at the output of U106
flows through the meter and develops a negative
feedback voltage across resistor R167. Resistor R161
shunis resistor R167 to reduce negative feedback on
the 2? kHz range,
Resistors R152 and R153 with capacitor C1 form a
de-emphasis circuit. Audio signals coming from ic
U105 arerouted into both resistors and through one of
them, as determined by the setting of De-emphasis
switch SW1, and on into capacitor C1 and Audio
ral prawnten 1
GL contr UL R4.
Audio signals from the center tap of control R4 are
routed to U107 pin 7. The amplified audio output
signal is coupled from U107 pin 12 through capacitor
C149 to the primary winding of audio transformer
T101. Audio output signals are coupled from the sec-
ondary winding of T101 to the output Speaker jack J4
on the Deviation Meter front panel. Switch SW2 is
ganged onto the shaft of Audio Level control R4.
When control R4 is turned fully counterclockwise,
SW2 turns off the power to the audio amplifier circuit
to conserve battery energy.
Eel its Vase
POWER SUPPLY
Power for the FM Deviation Meter is taken from ten
Type AA (NEDA Type 15) batteries and (or) an op-
tional Battery Charger/Eliminator Accessory. Zinc-
carbon or alkaline non-rechargeable cells with a use-
ful life of approximately 80 hours may be used with-
out the Charger/Eliminator nack
RAE NSA U4 Bae aeeee at peu,
nickel-cadmium batteries may be used for a charged
useful period of approximately 35 hours. These bat-
teries are opiional as the Deviation Meier can operate
directly from the accessory power pack with no bat-
teries at all. The 15-volt power source is taken directly
from the battery or power pack through On-Off switch
CarTana mee bee At
SW101A contacts. Power for the 8-volt supply also
comes from this source.
or rechargeable
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