Instruction manual

TM 11-6625-3017-14
Q2; one via a switch, so that on the 5 to 500kHz
deviation ranges the meter circuit is connected to
terminal 7, and the other from terminal 9 to the DE-
EMPHASIS switch. On the 1.5 kHz deviation range the
meter circuit is connected to the output of the amplifier
by terminal 11.
The remainder of the amplifier is the same as the
first 1. f. amplifier and its output is taken to the front
panel terminals. Headphones may be used here, or a
distortion factor meter, external meter (or oscilloscope).
The output impedance is 600 Q1, and is isolated by C11.
Do not allow d. c. to reach the OUTPUT terminals so as
to avoid damaging C11, which is a low voltage
electrolytic capacitor.
3.12 PEAK READING METER
(Unit A10-Fig. 7-4)
For reading modulation depth and deviation, and for
setting the f. m. calibration, the peak reading meter
circuit is brought into use.
Q1 and Q2 amplify the signal from the 2nd 1. f.
amplifier and RV1 presets the gain of the amplifier. The
remainder of the circuit consists of two similar pairs of
semiconductors arranged in push-pull, driving the meter
via rectifier MR1. The polarity of the meter can be
switched to read trough or peak on a. m., the positive or
negative deviation on f. m. Feedback is used to stabilize
the gain, reduce distortion and broaden the fre-quency
response. The feedback loops include R3, R12, C14,
R16, R19 and R24.
The meter is also used to measure the d. c.
component of the output from the a. m. detector for i. f.
level monitoring and tuning, and in the position SET
FREQ it reads the voltage developed by a counter circuit
in the limiter thus monitoring the frequency from the
mixer for fine tuning purposes.
3.13 A.M. detector
(Unit All-Fig. 7-5)
The diode detector, MR1, is preceded by Q2 and
Q3 which are arranged in a composite emitter follower
configuration to give a large peak-to-peak signal without
distortion, and thus allowing MR1 to operate over the
linear part of its forward characteristic. The input to the
detector is via a common-emitter amplifier, Q1, stabilized
by negative feedback through R6 and R7.
MR1 is fed directly from a tuned circuit, C4 and L1,
which is heavily damped by the low
output impedance of Q3. This gives the circuit a wide
band response which is flat to within the limits of the i. f.
amplifier. MR1 feeds into a resistive load and a fraction
of the voltage developed across the load is tapped off
and fed to a 50 kHz low-pass filter.
When the Function switch, SB, is in positions
TROUGH or PEAK, the detected signal is fed via
terminal 12 to the range switch, the 2nd i. f. amplifier and
the meter circuit.
When making an a. m. measurement, the internal
meter is switched directly to terminal 7 by the Function
switch, SB, and the d. c. component adjusted by RV1,
SET AM control, until the meter reads SET.
The d. c. component is also used for setting the
input level to the i. f. amplifier, thus ensuring correct
loading and good signal-to-noise ratio. On position
TUNE OSCILLATOR AND ADJUST LEVEL of the
Function switch, a fixed resistor, R1, is switched in place
of RV1 and the correct level set up by adjusting the input
attenuator until the meter indicates within the black arc.
3.14 BETWEEN-UNITS CIRCUITRY
(Unit A0-Fig . 7-1)
The A. M./DEV RANGE switch, SC, controls three
Tr pads of 10, 20 and 20 dB respectively. These
attenuating pads are inserted in steps as follows:
Range Attenuation
(1) 1.5kHz f. m. 0
(2) 5 kHz f. m. 0
(3) 15 kHz f. m., 30% a. m. 10 dB
(4) 50 kHz f. m. , 100% a. m. 20 dB
(5) 150 kHz f. m. 10 dB +20 dB
(6) 500 kHz f. m. 20 dB + 20 dB
If should be noted that the meter has two deviation
scales; ranges 2, 4, 6 are associated with the bottom
scale and the other ranges with the upper.
Also on the chassis is a single attenuator pad for
the calibrator. This is switched in place of the range
attenuator on the SET CAL position of the Function
switch. This pad incorporates a variable resistor, RV3,
which is mounted on the top of the chassis immediately
to the rear of the calibrator board. RV3 standardizes the
calibration circuit to the meter.
3-6