i. AF Amplifier
The voltage that can pass through the analogue switch of IC5 is very low. The
voltage is amplified approximately 20dB in IC12: B to get higher AF input voltage
to following IC13 (voltage controlled electronic volume). Also a part of this output
is picked up and output to Pin6 of microphone as non-squelched audio output.
This output is used as the terminal of packet, RTTY, SSTV, etc.
j. Electronic Volume, AF Amplifier
IC13 is the dual electronic volume controlled by the voltage.
The volume is controlled by the AF GAIN VR on the front panel. Pin5 is the control
terminal. The value of the attenuation is the minimum when the control voltage is
about 3.4V, and the value is 90dB or more at maximum when the control voltage is
about 3.1V.
One of the circuits is for volume control of the demodulation sound, and the other
for the volume control of the beep and sidetone. The beep and sidetone can be
heard even if the volume is set to the minimum point and sound tone is related with
AF GAIN VR.
The squelch circuit (IC14:A, Q14) controls Pin5. The output of IC14: A activates to
close the squelch when transmitting, so Q14 is turned OFF in D85 to control the
volume of the sidetone.
The receiving sound is fed to Pin6 and applied from Pin7. As for the beep and
side tones, Pin2 is for input and Pin 1 for output. These two outputs are combined
with the inpul of LPF amplifier IC12:A. The high tone noise that is generated in IF
amplifier is decreased by LPF amplifier.
The output of IC12: A is attenuated in R309 and R310 to get the same level with
IC20, and also to decrease the noise. IC20 is the AF power ampliiier which can
get the output of 2W or more (THD 10%) at 8£2 load. The ripple filter consists of
Q51 and C260.
The AGC voltage is supplied one stage to first IF amplifier and three stages to
second IF amplifier. These IF amplifiers consist of 3SK131. AGC voltage is
applied to the Gate2. The IF amplifiers are designed that the gain is changed
linearly corresponding to the AGC voltage.
D53 and D54 are the rectifier, and Q26 is DC amplifier. D50, the anode is set to
about 2V in R176, D110, D111 and R177. Usually AGC voltage is applied 2.4V.
The strong signal rectifies D53 and D54 resulting in DC voltage. Q26 decreases
the AGC voltage.
When AGC-FAST is selected in SSB/CW mode, C205 and C206 are connected
between 5V and AGC line in parallel. The attack time of AGC is determined in
R 1 67 and C206, then the release time is determined in R168 and C206. The
characteristics are "fast attack" and "slow release". In case of AGC-SLOW, the
analogue switch IC6 is turned ON, then R175 and C287 are connected in parallel.
The release time is lengthened because of C287. In case of AM, C206 is con
nected in parallel, then the attack time is delayed, which is the average type. D110
and D111 is the thermal compensation of D50.
in receiving AM, AGC is the average type not to follow the modulation.
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