TK-90
25
Connected to the first AF ALC amplifier output, switching
to the second AF ALC amplifier (IC602), are the recording
output for VGS-1, the play input, and the I/O circuit for the
scrambler circuit.
The second AF ALC amplifier does not operate for ALC
when it is SSB. It instead operates to prevent AM over-
modulation. The AF signal which went through the second
AF ALC amplifier should reach appropriate modulation sensi-
tivity by DA Converter (IC554).
It then goes through buffer amplifier (Q255) and into the
balanced modulator (IC251).
CIRCUIT DESCRIPTION /
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BW 2.4kHz
XF252
BW 6kHz
XF251
Q253
Modulation
output
SSB/AM Mod.
input
CAR level
adjust by DC.
IC251
CAR
10.695MHz
■
Modulation Circuit
The AF signal input into the balanced modulator (IC251) in
USB mode is modulated with a 10.6965MHz carrier and be-
comes a DSB signal. (LSB: 10.6935MHz, AM: 10.695MHz,
FSK changes depending on the shift width and tone setting.)
The signal modulated to DSB is amplified by IF amplifier
(Q253), goes through SSB IF filter (XF252), attenuates unnec-
essary side band component, and becomes the first IF fre-
quency SSB signal.
In AM mode, by adding direct voltage to the modulation
signal, it breaks down the balanced modulator, and by emit-
ting the carrier, generates an AM signal. The AM-modulated
signal is amplified by the IF amplifier (Q253), goes through
AM IF filter (XF251), and attenuates the out-of-band modula-
tion component.
In CW and FSK mode, the carrier signal is generated in the
same way as in AM mode, by adding direct voltage to the
modulation signal terminal.
IC605
MIC AMP
IC602
1st AF
ALC AMP
IC602
2nd AF
ALC AMP
MIC
IC554
D/A
converter
VOX circuit
IC551~553
Analog SW
VGS-1, Scrambler
Q255
Buffer
to Modulator
■
From Modulation Circuit to Drive Output
The transmission signal which passed through the first IF
filter (10.695MHz) is amplified at the second gate of the IF
amplifier (Q186). While being sent, ALC voltage is added to
control the gain.
The transmission signal from the IF amplifier (Q186) is
passed through the buffer (Q187) and the first transmission
mixer (D183), and is converted to the second IF frequency,
73.095MHz.
The transmission signal is then amplified by the IF ampli-
fier (Q182) where the total gain (resulting from the Final unit
frequency characteristics) is corrected for each transmission
frequency by the TGC (Transmission Gain Control).
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