Switch S4B switches the keyline to transmitter module pin XA3, A3J1-11. When an attached 50-ohm mike is
keyed (by a ground, switched at the mike), keying switch A3A2CR8 is forward biased, permitting the transmitter
module 50-ohm mike control circuit to pass signals originated by the microphone.
Switch S4G grounds transmitter module pin XA3, A3J1-8 through contacts 5 and 6. When pin XA3, A3J1-8 is
grounded, A3A2C1 parallels A3A2C2, narrowing the pass band of sine wave pass filter A3A2C1, C2, L1 for nsk
tones. Ground is also applied through contacts 12 and 11 to mike return pin E of MICROPHONE jack J3. Pin D
of RCVR TRANS AUDIO jack J2 is also connected to ground through contacts 12 and 11. This action grounds
the secondary of transmit output transformer A6T1, resulting in an unbalanced output for the 85-Hz + voice
Section IX. METERING STAGE ANALYSIS AND SWITCHING
A detailed discussion of the operation of the metering circuitry Is given In paragraph 1-71. The following figure
shows that the metering circuitry consists of A5A2R19, R11; meter multiplier resistors A5A2R10, R20 and A7R6; meter
M1; meter bias board A7; sampler resistors; and METER FUNCTION switch S3.
1-73. METERING CIRCUITRY
METER FUNCTION Switch Set at DC LOOP NO. 1 . When a loop current flows in dc loop No. 1, a voltage drop
proportional to the current appears across sampler resistor A5A2R19. When METER FUNCTION switch S3 is set at DC
LOOP NO. 1, the negative terminal of meter M1 is connected through S3 contacts 7 and 8 to one side of sampler
resistor A5A2R19. The positive terminal of M1 is connected through S3 contacts 1 and 2 and meter multiplier resistor
A5A2R20 to the other side of sampler resistor A5A2R19. The zero on M1 is at center scale, so the direction of the
current through A5A2R19 can be observed by deflection of the meter indicator. Current flow is shown for an internal
supply and results in a deflection of M1 to the right. Current flows in the opposite direction in this portion of dc loop No.
1 when supplied from a source external to the modem, resulting in a deflection to the left.
METER FUNCTION Switch Set at DC LOOP NO. 2 . In this position of the METER FUNCTION switch, the
metering circuitry operates as described above, except that dc loop No. 2 is sampled.
METER FUNCTION Switch Set at DISCRIMINATOR . Received tty tones, converted to tty pulses, are sampled at
the output of receiver module A4 through meter multiplier resistor A7R6. The positive terminal of M1 is connected to
meter multiplier resistor A7R6 through switch contacts 1 and 4; the negative terminal is connected to pin XA4, A4J1-24
through contacts 7 and 10.
METER FUNCTION Switch Set at REGULATED DC . Voltage divider A7R2, R1 samples the output from the +20
volt regulator. When S3 is set at REGULATED DC, M1 parallels A7R1 through switch contacts 1, 5 and 7, 11.
METER FUNCTION Switch Set at RCV LEVEL . In this setting only, zero on M1 is shifted from center scale to left
scale. A positive bias, derived from voltage divider A7R4, R3 is applied to the negative terminal of meter M1 through
switch contacts 12 and 7. Tty tones and/or voice from the modem input circuitry is applied to detector A7CR2, CR1, C1,
C2. Resistor A7R5 reduces the input to the desired level. The rectified input reduces the level of the bias applied to the
negative terminal of M1, causing the meter to indicate upscale for an audio input to the modem. The meter is returned to
ground through switch contacts 1 and 6.