Yaesu FT-2400H Technical Supplement page 9

signals. This pulse train is converted to DC by
charge pump Q1026/Q1027
pass filtered, and then fed back to varactors D301
x
and D302 (1 T363 2) on the VCO Unit.
Changes in the level of the DC voltage applied
to D301/D302 affect the reactance in the tank cir­
cuit of VCO Q301, changing the oscillating fre­
quency according to th� phase difference between
the signals derived from the VCO and the crystal
reference oscillator. The VCO is thus phase-locked
to the crystal reference oscillator.
The output of receiver VCO Q301, after buffer­
ing by Q305, is delivered to the Main Unit for
amplification by Q1005 before application to the
)
1st mixer, as described previously.
Transmitter VCO Q303
between 140 and 17 4 MHz according to the pro­
grammed transmit frequency. The remainder of the
PLL circuitry is shared with the receiver. However,
the dividing data from the microprocessor is such
that the VCO frequency is at the actual transmit
frequency (rather than offset for IFs, as in the re­
ceiving case). Also, the transmitter VCO is modu­
lated by the filtered sp � ech audio applied to
modulating varactor D305, as described previous­
If
ly. the FRC-6 Digital Coded Squelch option is
installed, DCS modulatio.n is applied both to the
VCO and to the PLL frequency reference, via varac­
tor D1021
(HVU202).
Transmit Inhibit
When the transmit PLL is unlocked pin 7 of PLL
chip Q1024 goes to a logic low, turning on Q1020
and turning off half of Q1018. This
( DTA143EK)
unlock signal produces a low impedance at the
base of the other half of Q1018, which then turns off
Automatic Power Controller Q1019 to disable the
supply voltage to transmitter RF amplifiers Q1010
and Q1011, disabling the transmitter.
CIRCUIT DESCRIPTION
low­
(IMX3/IMZ2),
(MM8R9S1 L) oscillates
Miscellaneous Circuits
Power-On Sequencing
POWER
Pressing the
turns on Q2007 (28A1162GR)
applying 5 volts from regulator Q2004
via Q2003
4SAA) (NJU7201 USO)
supply line to the base of Q2012
pulls PO KEY pin 38 of microprocessor Q2001 low,
causing PO ON pin 19 to go high. The PO ON line
then pulls the base of Q2018
Q2015 lighting the pilot lamps via Q2014
(FMG8),
(28D1033K) and Q2010
brightness is controlled by CdS photosensor
CS2001, opamp Q2009
sistor Q2011 (28C2712GR),
Q2013
(NJU7201 USO).
When Q2015 is turned on by the PO ON line, it
turns on Q3001 (DTA114EK)
which in turn allows Q3002
13.8 volts to regulator Q3003
9-volt regulator Q1013, audio amplifier Q1023 and
APC switch Q1019 on the Main Unit.
Push-To-Talk Transmit Activation
The PTT switch on the microphone is connected
to pin 17 of microprocessor Q2001, so that when the
switch is closed pin 12 (tx/rx) goes low. This signal
is delivered to power bus controller Q1021
on the Main Unit, which then disables the receiver
b y disa bling the 9 - V s upply bus at Q 1022
,
(OT A 143 EK) to the front end, IF, discriminator and
receiver VCO circuitry. At the same time, Q1015
)
(2889868) activates the transmit 9-V supply line to
enable the transmitter.
&
Panel Lamps
switch on the front panel
on the Control Unit,
(RHSVA-
and the 13.8-V
This
(DTC114EK).
(2881301-ZQ) low via
(2881301-ZQ). Lamp
(NJM2904M-%), and tran­
powered by regulator
on the Interface Unit,
(28811408) to pass
(HA 17808P), and to
(IMHS)