Yaesu FT-901DM Maintenance Service Manual page 53

TECHNICAL NOTES
When the unlock signal is received from the PLL
UNIT, Q3o:¢6 (MC14011B) generates a blanking
signal which is fed to the LED drivers Qy917.
Qro22. and Qro23-
To calibrate the display, the heterodyne crystal
frequency can be shifted by VC299; from the
front panel.
Voltage regulator Qj923 (uPC14305) stabilizes
the supply voltage.
MEMORY UNIT (PB-1787)
The VFO signal delivered to pin 4 of PB-1787 is
amplified by Qogo9 (3SK40M) and Q3549 (2SC
785), and fed to counter gate Q3s,3 (SN74
LSOON). A clock signal generator Q35,;, (MSM
$562) produces 50 Hz clock pulses from a crystal
frequency of 3276.8 kHz. The pulses which passed
through the gate are fed to a six digit decade
counter Q)s,; (TC5032P). which counts the
VFO frequency. The BCD output from Qx1¢ is
fed through Qyg22 - Qrg26 (all MC14042B) to
up/down counter Q3g27 - Qog3; (SN74LS192N).
which presets the VFO frequency,
BUFF AMP
02807 2SC372Y
ouTO—
55—S.OMHz
The 5.0 - 5.5 MHz signal generated by Qoso1
(JF1033B) is fed through buffer/amplifier Q2so2
(JF1033) and Q2s93 (2SC785) to pulse shaper
Qrs03 (2SC785).
The switching pulse generated by Qogo4 is also
fed to Qog27. where the signal is divided to
1/5000 to 1/55000 in accordance with the pro-
gram preset by Q2527 - Q2g31- The output from
Q2g3; (approximately 100 Hz) is fed through a
one-shot multivibrator Q2332 (SN74LS123N) to
the phase comparator Q)g2; (MC4044P) where
the phase of the 100 Hz signal is compared with
the phase of the 100 Hz signal which is delivered
from Qogi1-
The DC output from Q3g2; passes through a low
pass filter, operational amplifier Q)g29 (LM308),
to remove the 100 Hz ripple. This DC voltage is
applied to varactor diodes Dzg96 - D2go7 (MV-
104), thus locking the VCO frequency.
When the M (memory) switch is pushed, the
counter output from Q>x;5 is latched and the
VCO frequency is locked.
When the MR (memory recall) switch is pushed,
the output from VCO buffer Q2go2 (JF-1033B)
SSTAGE UP/DOWN COUNTER
ozez7-cz83!1 SN74LSI92N
TIMING SIGNAL GENERATOR
ge8i3,44 SN74LSOON
TIMING SIGNAL
6 DIGIT DECADE COUNTER
c28is TCSO32P
WRITE Sw
O.1Sec z
200Hz
Dc CONV
joz808 MPS-AI3.
MEMORY UNIT
CLOCK PULSE GENERATOR
c28!1 MSMS562,0ze:2MM74C90 | -b 3276.8 KHz
aca
5.S—5.OMHz
BUFF AMP & PULSE SHAPER
2809 3SK40M, C2810 2SC785-0
© VFO IN
5.5—-S.OMHz
2-26