Yaesu FT-51R Technical Supplement page 23

The modulating audio is delivered first to the Con­
trol Unit, where it may pick up a generated CTCSS
tone, then is next delivered to both Mother Units,
where it is either disabled (on the receiving band) by
Q2003 (VHF) or Q23004 (UHF, both OTC142EE), or
delivered to modulating varactors D5003 (in the UHF
VCO) or D4003 (in the VHF VCO), f r equency modu­
lating the PLL carrier up to ± 5 kHz from the unmodu­
lated carrier at the transmitting frequency.
L ate VHF Transmit Stages
On the 144 VCO Unit, the modulated signal from
VHF VCO Q4001 (28C4226) is buffered by Q4002
and Q4003 (both 28C4226) and returned to the 144
Mother Unit for amplification by Q2002 (28C4537),
and then final amplification by VHF PA module
Q2001 (8-AV28) on the 144 Mother Unit. The transmit
signal then passes through antenna t/r switch D2003
before returning to the duplexer network on the 144
Mother Unit and then a low-pass filter consisting of
L2002, L3003, C2017, C2018, C2012 and the duplexer
consisting of C2019 (mounted on the 144 Mother
Unit) and L3010, C3034, C3165 and the LPF (L3004,
L3005, C3027, C3028, C3014) mounted on the 430
Mother Unit, before delivery to the antenna.
VHF Automatic Transmit Power Control ( APC)
RF power output from the VHF final amplifier is
sampled and rectified by D2001 (188321 ). The result­
ing DC is fed back through Q2004 (28A 1586) and
Q2008 (28C4617)to driver Q2002 (28C4537) on the
144 Mother Unit, controlling the level of drive to the
PA module, and thus the power output. The micro­
processor selects either high or one of four low power
levels via Q2011 (XP1401 ).
When the VHF transmitter PLL is unlocked, or
while receiving, the unlock signal from Q2007 causes
the ULT3V line to be dropped via Q2006 (OTA124EE),
and Q2013 (OTC124EE), which disables transmission
by biasing final amplifier Q2001 off and opening VHF
t/r switch D2003.
VHF Transmit/Receive Switching
Closing the PTT switch on the Key Unit pulls one
side of the microphone low, which turns on Q1010
(OTA 144EE), causing pin 95 of microprocessor Q1002
on the Control Unit to go high. The microprocessor
then delivers appropriate serial data to sub-CPU
QlOOl to raise the TXV line to the 144 Mother Unit
(via the Contact Unit) where Q2020 (UMBlO) turns off
the receiver, and Q2023 (OTC144EE) and Q2015
(DTA123EE) turn on the transmitter. This tx signal is
delivered to the 144 Mother Unit where it causes the
FT-51R Technical SumJ/ement
ULT3V line to go high, which in turn biases the VHF
PA module on and closes t/r switch D2003 on the 144
Mother Unit. At the same time, Q2005 on the 144
Mother Unit turns on Q2002 on the 144 Mother Unit
to drive the PA module.
L ate UHF Transmit Stages
On the 430 VCO Unit, the modulated signal from
UHF VCO Q5001 (28C5006) is buffered by Q5003
and Q5002 (28C5006) and returned to the 430 Mother
Unit for amplification by Q3003 (28C4245), and then
final amplification by UHF PA module Q3002
(M67799MA) on the 430 Mother Unit. The transmit
signal then passes through antenna t/r switch D3003
before returning to the duplexer network and the
high-pass filter consisting of L3003-3005, C3021,
C3022, C3014, C3027-3028, C3165 and the high-pass
filter consisting of L3009, L3012, C3010, C3011, C3020
and C3029, mounted on the 430 Mother Unit before
delivery to the antenna.
UHF Automatic Transmit Power Control ( APC)
RF power output from the UHF final amplifier is
sampled and rectified by D3001 (188321 ). The result­
ing DC is fed back through Q3005 (A 1586Y) and
Q3008 (28C4617) to driver Q3001 (28C4537) and
Q3003 (28C4245) on the 430 Mother Unit, controlling
the level of drive to the PA module, and thus the
power output. The microprocessor selects either high
or one of four low power levels via Q3011 (XP1401 ).
When the UHF PLL is unlocked, or while receiv­
ing, the unlock signal causes the ULT3U line to be
dropped via Q3007 (OTA144EE), and Q3012
(OTC124EE), which disables the transmitter by bias­
ing final amplifier Q3002 off and opening UHF t/r
switch D3003.
UHF Transmit/Receive Switching
As for UHF, closing the PTT line raises the TXU
line via Q1010 and Q1002 on the Control Unit. This
line connects to the 430 Mother Unit (via the Contact
Unit) where Q3026 (UMB1 ON) turns off the receiver,
and Q3017 (XP4213) and Q3022 (UMB1 ON) turn on
the transmitter. This tx signal is delivered to the 430
Mother Unit where it causes the ULT3U line to go
high, which in turn biases the UHF PA module on and
closes t/r switch D3003 on the 430 Mother Unit. At the
same time, Q3004 on the 430 Mother Unit turns on
Q3003, Q3001 on the 430 Mother Unit to drive the PA
module.
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