Yaesu FT-901DM Maintenance Service Manual page 48

The VOX GAIN control VR, on the front panel
provides adjustment for relay sensitivity, and
VReo1 controls antitrip sensitivity. Relay hold
time is determined by delay control VRgo2-
The tone oscillator Q.¢,5 (2SC373) operates when
the MODE switch is in the CW position. It is a
phase shift oscillator operating at approximately
800 Hz. The sidetone level is adjusted by VRyo3-
The sidetone output is also coupled to the VOX
circuit for semi-break-in operation.
Located on the VOX UNIT, the crystal marker
generator Q,,, (2SC1815Y) generates a basic 3200
kHz crystal controlled signal which is divided into
100 kHz multiples by frequency divider Qg;2
(4024PC). The voltage regulator Qo; (2SC
1815Y) regulates the supply voltage to the VOX
circuit.
FM UNIT (PB-1707)
The speech signal from pin 20 of PB-1707 is ted
to amplifier limiter Qgo9 (TA7061 AP), which clips
both positive and negative peaks when they ex-
ceed the level set for maximum deviation of the
transmitter frequency. Qog,9 (2SCI8I1SY) acts as
TECHNICAL NOTES
an active low pass filter to eliminate harmonics
caused by the clipping. VRoo2 determines the
maximum deviation.
The output from Qoo,; is amplified by Qo,
(2SK34E) and then fed to the modulator Doo9
(FC63). Qgi2 (2SC1815Y) oscillates at a crystal
frequency of 8987.5 kHz and is frequency mod-
ulated by varactor diode Dgg9. The output from
Qoi2 is amplified by Qg,3 (2SC1815Y) and then
fed to the NB UNIT.
TUNE UNIT (PB-1720)
When TUNE switch S3,9, is pushed, the voltage
across Cz;9, is applied to the base of Q310)
(2SCI815Y), causing Q2;9, to turn ON: this
activates RL3,92 which grounds the cathodes of
Dzros and Dai96 (both 181555). As Day0s is
connected in parallel with the PTT switch, the
transceiver is placed in the transmit mode.
Diode D2,9¢ grounds the common terminal of
the MODE switch S,;g and S;h, and the bias
voltage of the PA stage is lowered from the cut-
off condition to the normal operating level.
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KEYER UNIT
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TUNE,CW KEYING CIRCUIT