Yaesu FT-901DM Maintenance Service Manual page 47

TECHNICAL NOTES
tor). The microphone signal is fed to pin 17 of
the CARRIER UNIT, amplified by Qyo, (TA
7063P), and fed to Q797 (2SC373). When a suffi-
cient level of speech input is applied to the micro-
phone, Q797 conducts, causing the output of
NAND gate Qyos (MCI4011B) to be "low."
This condition cuts off AMGC controller Q7o9
(2SC1815Y). The collector circuit of Qyo9_ is
connected with the output circuit of the micro-
phone amplifier Q7o5-
When the microphone input signal becomes signi-
ficantly lower than the normal speech level (back-
ground noise only), Qz97 turns OFF, causing the
output of Qyog to be "high." This condition
causes Qy99 to conduct, grounding the output
circuit of Qro5.
The gate holding time can be adjusted by means
of VRyo2, to allow for differences in operator
preferences.
VOX UNIT PB-1846 (PB-1685-3330)
A portion of the microphone input is delivered
to pin 10 of the VOX UNIT. The signal is ampli-
fied by Qeos and Qeo9 (2SC373) and fed to the
base of Qe19 (2SC373). When the signal is applied,
Q610 conducts, causing C,2, to discharge. The
DC voltage of C,2, is then fed to the gate circuit
of Q¢o4 (MC14011B). When pins 1 and 2 of Qeo4
are high, the output of the VOX gate of Qeo4
becomes low, causing Qgog (2SC181S5Y) to cut
off, thus turning relay driver Qe, (2SC735) ON
to actuate the VOX relay RL,.
The antitrip circuit provides a bucking voltage
to prevent the speaker output from tripping the
transceiver into the transmit condition. The
receiver audio output is fed through ANTITRIP
control VRgo; to amplifier Qo, (2SC1815Y)
and is then rectified by Deo, (1N60). The rectified
DC voltage tums Qgo2 (2SC373) ON, placing its
collector at a low level, thus preventing the gate
circuit from activating the relay driver Qgo7. The
collector of Qgo2 is connected through Reog to
the base of Qso3 (2SA564A) which shorts the
supply voltage for Cg2,; thus, Cg, is fully charged
when the speaker output disappears, turning the
gate into the transmit mode immediately with the
appearance of microphone input.
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