Service manual

THE 2ND MIDDLE FREQUENCY & DEMODULATOR CIRCUIT
THE 2ND MIDDLE FREQUENCY & DEMODULATOR CIRCUITTHE 2ND MIDDLE FREQUENCY & DEMODULATOR CIRCUIT
THE 2ND MIDDLE FREQUENCY & DEMODULATOR CIRCUIT
THE 2ND MIXED CIRCUIT TRANSLATES THE 1-IF SIGNAL TO THE 2-IF SIGNAL. DOUBLE
SUPER HETERODYNE METHOD (TRANSLATE RECEIVING SIGNAL TWICE) IMPROVES
IMAGE REJECTION RATIO & GAINS STABLE RECEIVING PROFITS.
FM IF IC (U2) INCLUDES THE 2ND PART OSCILLATOR, THE 2ND MIXER, LIMITER
AMPLIFIER, QUADRATURE DEMODULATOR AND NOISE DETECTOR CIRCUIT, ETC.
THE 1-1F SIGNAL FROM Q6 IS APPLIED TO THE 2ND MIXER OF U2 (PIN 16). AND THE
2ND LO SIGNAL OF 21.25MHz (FOR CREATING THE 2ND IF SIGNAL OF 450 KHz IS
CREATED AND MIXED FROM STANDARD OSCILLATOR
(X2, Q25). THE 2-1F SIGNAL FROM U2 (PIN 3) IS DELIVERED BY CERAMIC FILTER (FL3),
WHICH REMOVES UNNECESSARY SIGNALS. THEN, IT IS APPLIED TO THE 2-1F
(LIMITER) AMPLIFIER AT U2 (PIN 5). THIS SIGNAL IS APPLIED TO FM DEMODULATOR OF
US IN ORDER TO DEMODULATE TO AF SIGNAL. FM DEMODULATOR CIRCUIT USES
QUADRATURE DEMODULATION METHOD (BEELINE PHASE DETECT). IT USES CERAMIC
DISCRIMINATOR(X1) NONE ADJUSTING CIRCUIT IN ORDER TO GAIN PHASE DELAY.
THE SIGNAL, DEMODULATED FROM U2 (PIN 9) IS APPLIED TO AF CIRCUIT.
AF
AF AF
AF AMPLIFYING CIRCUIT
AMPLIFYING CIRCUITAMPLIFYING CIRCUIT
AMPLIFYING CIRCUIT
AF AMPLIFYING CIRCUIT AMPLIFIES DEMODULATED SIGNAL TO OPERATE SPEAKER.
AF CIRCUIT INCLUDES AF MUTE CIRCUIT FOR SQUELCH. AF SIGNAL FROM U2 (PIN 9)
IS APPLIED TO DE-AMPHASIS CIRCUIT (R20, C50).
DE-AMPHASIS CIRCUIT IS A UNITED CIRCUIT, OF WHICH FREQUENCY IS 6dB/OCTAVE.
UNITED SIGNAL IS APPLIED TO ACTIVE FILTER (Q7, Q10). IT REMOVES UNNECESSARY
LOW BAND NOISE SIGNAL BY HIGH PASS
FILTER Q7 FUNCTION, REMOVES HIGH BAND NOISE SIGNAL BY LOW PASS FILTER Q10
FUNCTION. FILTERED SIGNAL IS APPLIED TO THE AF POWER AMPLIFIER (U6, PIN 1) OF
THE NEXT STEP VIA [VOLUME]. SIGNAL, OUTPUT FROM U6(PIN 4) OPERATES
INNER(OUTER) SPEAKER.
SQUELCH CIRCUIT
SQUELCH CIRCUITSQUELCH CIRCUIT
SQUELCH CIRCUIT
SQUELCH CIRCUIT CUTS AF SIGNAL, WHEN NO RF SIGNAL IS RECEIVED. SQUELCH
CIRCUIT CHANGES AF MUTE SWITCH, BY DETECTING NOISE COMPONENT FROM AF
SIGNAL. A PART OF AF SIGNAL FROM FM IF IC (U2, PIN 9) IS APPLIED TO ACTIVE
FILTER (U2, PIN 8), AFTER PASSING SQUELCH ADJUSTER (VR1). ACTIVE FILTER