Yaesu FT-223 Instruction Manual

THEOR Y OF OPERA TION
:;.ENERAL
i'unctional operation of the model FT -223 VHF IFM transceiver
s ìllustrated in the Block Diagram, Figure 6
lefer to the schematic diagram for the circuit description.
['be transceiver consista of a crystal controlled transmitter and
:eceiver operating on any of the 23 channels within the frequency
:ange of 144 to 148 MHz. In addition to 22 channels which are
;elected by the CHANNEL selector switch, one "most-often-used"
:hannel may be preset for ease of operation. Solid state circuitry
s employed throughout and the transceiver is designed to operate
rom a 13.5 Volt +lO o/o DC negative ground power source.
CRANSMITTER SECTION
Che transmitter section produces an FM (phase modulation method)
Jutput signal.
Che audio signal from the microphone is amplified by an integrated
: ircuit Q201 (TA706IAP) which works as arnplifier and an IDC
Instantaneous Deviation Contro!) circuìt. The IDC circuit, clips
lOth positive and negative peaks when they exceed a predetermined
evel to li.Init the maximum deviation of the transmitter. The IDC
:ontrol, VR202, permits the deviation to be adjusted, and is nominally
actory set fora deviation of +5kHz. When wide band transmission
s desired, VR202 must be readjusted to provide a maximum deviation
•f +10 kHz.
FT- 223
BLOCK OIAGRAM

The tone signa! passes through a low pass filter by uni t 11c
11
c
0501 and is fed to 0502, NE567. When the tone signal has t}
same frequency as preset for transmitting, the voltage of pi
of 0502 becomes low causing 0504, 2SC372Y to
1
'0FF". In t
proper bias voltage is applied to 0119 for normal operation.
Without proper tone signal, Q504 conducts, removing the prc
bias from 0119 to disable the audio circuit.
As the conventional carrier squelch circuit is operative wher
the tane squelch is switched in, the busy lamp lights up when
any carrier is received.
l SIV A )
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( TOJYE. SQ S W l ICH)
TONE SQUELCH <Pe -r555A) OPTION
Figure 9
Figure 10
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1e frequency modulated 146 to 148 MHz signal is amplified by Q210,
:c741, Q211, 2SC730 and Q212, MRF212 and applied through a two
age pi-network to the antenn a. A diode D204, 1Sl88FM rectifies
s mall portion of the RF output and applies the resultant DC voltage
the meter where is provid es an indication of relat i ve power output
om the tr ansm itter. The meter sensitivity is adjusted by VR204
Ld it is set for indication of 8 on the meter scale a t l O watts on a
l ohm r esisti ve load.
the transmitter is keyed without an antenna conn ec t ed, or ifa high
\T'R exists in the antenna syst em , the reflected power is detected
rough T210 and a diode D207, 1Sl88FM produces DC voltage.
~15, 2SC372Y conducts with the DC voltage applied through VR209
.uaing Q216, 2SC3 72Y to decrease its collector current. Thus ,
e
emitter voltage of Q217 , 2SA496(0) is lowered causing Ql,
:D313 to decrease current and the supply voltage to the PA amplifier
nz is low ered to prevent damage of the transistor. The protection
vel is set by VR209. This circuit is also used to switch the output
· wer down to l watt where the ID/LOW switch is s et to LOW position.
1e amou nt of power reduction may be adjusted by VR208.
1e antenn a change -over circuì t consista of the switching diodes
~05 and D206, Ml 301.
~CEIVE R SECTION
te 146 to 148 MHz input signal from the antenna is am plified by
~T QlOl, 3SK 40Mon RF board, and applied through five hi-Q
)t-coupl ed resonators to the first mixer, Q102, 3SK40M .
...
di
Q,
Q
Q ,
a..
X,
a ..
Q,
Q,
Q
Figure 8
Top View
14 -
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C'he limited audio signa! is applied through a low-pass filter to
2202, 2SC372Y where it is amplified and applied to the phase modulator
-aractor diade D20l, lS 1658. The low- ; pass filte:t limits the transm.itt€
nodulation spectr\.un by attenuating frequencies above the speech range.
~he oscillator, Q203, 2SC372Y, operates on the crystal frequency
o generate the initial RF signal. The crystal frequency is in the 12 MH
·ange, and is determined as follows:
Crysta1 Frequency (MHz)
= Output Frequency (MHz)
12
~rhnmer capacitors TC40l through TC423 permit each of the 23
:rysta1s to be individually setto frequency. Output from Q203 is
.mplified by Q204, 2SC372Y, and applied across transformer L203.
~he audio signal from Q202 varies tl1.e bias applied to DZO l and
>202, in turn causing the capa city of the diodes to vary in accordance
rith the audio voltage. As D201 and T201 and D202 and T202 are in
eries, the capacity change in the diade effectively changes the
esonant frequency to produce a phase shift at an audio rate in
:1e input of Q205, 2SC372Y in the exciter stage.
'he angu1ar phase shift fronì the modulator is relatively small,
1erefore the crystal frequency is multiplied twelve times to obtain
1e desired deviation at the output frequency of 144 to 148 MHz.
'he nìodulated 12 MHz signal is applied through the buffer amplifier
~205, 2SC372Y to the multiplier chain consisting of a doubler Q206,
SC710D, a tripler Q207, 2SC710D, and a doubler Q208, 2SC710D
·here the necessary frequency m~ltiplicatio-n is provided.
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PB-1539
(RX CRYSTAU
r---- - -- --
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33 P1
1 Tc:n:J Te JOI--.J ZJ ' oP :
L- - -- - -- - -- .J
N()J?.
Q IOI 3SK'l0
PB- !5 3 8.C · (RX)
~I
0115
2S C372Y
0102 3 SK40
Oil6 2S CI04 7 0117 2 SC 1 04 7'
- -- ------ -,
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02 05
2 SC372Y
01 0 :3
0202
2SC372Y
L---- -- - --- - ---- -- -- ----- - --
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J,.JDNJ70R
1Jf1-oJI
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NOTES.
l
ALL RES1STORS IN n 1/ 4\11 ±.10%
UNLESS OTH ERW I.SE NOTE O.
21lLL CA PACITORS IN"F
UNLESS OTHERWISE HOTEO .
+*=- 16\IIV
-· -..,
FT - 223
CIR CUIT OIAGRAM
--== - ........,
AN7 .
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