Yaesu FT-2500M Technical Supplement page 11

phase detector section of Q1024, which produces
a dual 5-V pulsed output with pulse duration
depending on the phase difference between the
input signals. This pulse train is converted to DC
by charge pump Q1026/Q1027 (IMX5/IMZ2),
lowpass filtered, and then fed back to varactors
D301 and D302 (1 T363
Changes in the level of the DC voltage applied
to D301 /D302 affect the reactance in the tank
circuit of VCO Q301, changing the oscillating fre­
quency according to the phase difference be­
tween 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 (28C3356) oscillates
between 140 and 174 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 micro­
processor is such that the VCO frequency is at the
actual transmit frequency (rather than offset for
IFs, as in the receiving case). Also, the transmitter
VCO is modulated by the filtered speech audio
applied to modulating varactor D305, as de­
scribed previously.
Transmit Inhibit
When the transmit PLL is unlocked pin 7 of
PLL chip Q1024 goes to a logic low, turning on
Q1020 (OTA143EK) and turning off half of Q1018.
This unlock signal produces a low impedance at
the base of the other half of Ql018, which then
turns off Automatic Power Controller Q1019 to
disable the supply voltage to transmitter RF am-
2) on the VCO Unit.
x
Circuit Description
plifiers Q1009 and Q1010, disabling the transmit­
ter.
Miscellaneous Circuits
Power-On Sequencing
Pressing the POWER switch on the DISP Unit
turns on Q2017 (OTC114EK) on the CNTL Unit,
applying 5 volts from regulator Q2012 (L7805)
via 9-volt regulator Q1013 (AN6541) and the 13.8-
V supply line to the base of Q2016 (28811408).
This pulls RESET pin 75 of microprocessor Q2006
low, causing PO pin 66 to go high. The PO line
then pulls the base of Q2017 (OTC114EK) low,
lighting the pilot lamps via Q2005 (28C2712GR)
and Q2004 (28811438). Lamp brightness is con­
trolled by CdS photosensor CS3001, which pro­
vides an input to microprocessor Q2006, pin 49.
An
pulsed DC ouput is then applied to the inte­
grator circuit formed by C2015 and R2021, which
produces a DC bias proportional to the pulse
train input, which is then applied to the base of
Q2005 (28C2712GR), and Q2004 (28811438),
controlling voltage to the panel and display
lamps.
Push-To-Talk Transmit Activation
The PTT switch on the microphone is connec­
44
ted to pin of microprocessor Q2006, so that
when the switch is closed pin 69 (tx/rx) goes low.
This signal is delivered to power bus controller
Q1021 (IMH5) on the MAIN Unit, which then
disables the receiver by disabling the 9-V supply
bus at Q1022 (OTA 143EK) to the front end, IF,
discriminator and receiver VCO circuitry. At the
same time, Q1015 (28811438) activates the
transmit 9-V supply line to enable the transmit­
ter.
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Panel Lamps
1-9