Heathkit-AR14-rec-sm 维修电路原理图.pdf

CIRCUIT DESCRIPTIONEach major section of the Receiver will be changed, is used as an AFC (automatic fre-described separately in the following Circuit quency control) voltage to lock-in the localDescription. For ease of explanation, the Source oscillator frequency with the station beingswitch will be described in the FM position. tuned in.Follow the circuit on the Block Diagram (fold-out from Page 66) and on the Schematic (fold-out from Page 79) while reading the CircuitDescription. The letter-number designations(R4, C115, R212) for all resistors, capacitors,and diodes have been placed into the followinggroups to make them easier to locate on thechassis and Schematic.1 - 99100 - 199200 - 299FM tuner section.Amplifier section.Power supply section.The oscillator frequency is locked-in by the AFCvoltage in the following manner: The capacitancebetween the elements of diode Dl changes whenthe AFC voltage that is applied to it changes.This capacitance is connected in series withcapacitor C19. and these two capacitances areconnected in parallel with part of coil L4. Thus,when the capacitance of diode Dl is changed bythe AFC voltage, the total capacitance acrosscoil L4 is changed. This change in the tunedcircuit capacitance changes the frequency of theoscillator in such a way as to maintain propertuning.FM TUNING UNITThe FM signal from the antennaisappliedto theprimary of balanced input transformer Tl in theFM tuning unit. The secondary of transformer Tlforms a tuned circuit with trimmer capacitor Cland capacitors C2 and ClA (antenna section oftuning capacitor). The signal selected by thistuned circuit is coupled through capacitor C3 toRF amplifier transistor Ql.The signal is amplified by transistor Ql. The RFtuned circuit of Ql selects the desired signaland couples it through capacitor C8 to the baseof mixer transistor Q2. The RF tuned circuit con-sists of coil L2, trimmer capacitor C6, and ca-pacitors C7 and C6A (RF portion of tuning capac-itor).The oscillator and the received FM signals aremixed in transistor Q2 to produce a 10.7 mc IF(intermediate frequency) signal, that is coupledthrough transformer T2 and capacitor C21 tothe first IF amplifier transistor, Q4. Theamplified IF signal from the collector of tran-sistor Q4 is coupled through transformer T3to the base of second IF amplifier transistorQ5. This IF signal is again amplified by tran-sistor Q5, coupled through transformer T4,and amplified by transistor Q6. From Q6 thesignal is coupled through transformer T5 andamplified by the fourth IF amplifier transistorQ7.The local oscillator transistor, Q3, operates at afrequency that is 10.7 mc higher than the receivedFM signal. The oscillator frequency is de-termined by a tuned circuit composed of coil L4,trimmer capacitor C14, and capacitors Cl3 andC14A (oscillator portion of tuning capacitor). Theoutput signal from this oscillator is coupledthrough capacitor Cl0 to the base of mixertransistor Q2.A portion of the signal voltage is taken from thecollector of transistor Q6 and rectified by diodeD2 to produce an AGC (automatic gain control)voltage, This AGC voltage, which increases anddecreases with the strength of the received FMsignal,is coupled through resistors R14 andR12 to the base of transistor Q4, where itautomatically controls the gain of the IF signal.A small DC voltage is coupled from the ratio de-tector circuit through resistor R5 to diode Dlin the collector circuit of the oscillator. ThisDC voltage,which changes as the tuning isThe AGC voltage is then coupledfrom the emitterof transistor Q4, through resistor RlO, to the baseof RF amplifier transistor Ql. This automat-ically controls the gain of the FM signal inthe tuner section.RadioFans.CN 收音机爱 好者资料库Figure 11All, or only one of the IF amplifier stages mayoperate as limiters. For a very weak signal,only the fourth IF amplifier Q7 may be limiting,and transistors Q4, Q5, and Q6 would beamplifying the IF signal. For a very strongsignal, all four IF amplifiers may be acting aslimiters. This limiting action removes ampli-tude modulation from the FM signal. Limitingaction is also provided by the self-limitingcharacteristics of the ratio detector circuit.RATIO DETECTOR CIRCUITFrom the collector of transistor Q7, the IF signalis coupled through resistor R26 and ratio de-tector transformer T6 to the ratio detector cir-cuit. This circuit, which separates the audio sig-nal from the 10.7 mc IF signal. is shown re-drawn for greater clarity and simplified in Fig-ure 11. Transformer T6 is represented in thisfigure by primary coil Ll, a center tapped sec-ondary composed of coils L2 and L3. and a thirdor tertiary winding. L4. L4 is just a few turns ofwire tightly wrapped around the bottom of pri-mary Ll. NOTE: In the actual circuit, choke L5and coil L6 are also connected in series withcoil L4, resistor R31, capacitor C39, and re-sistor R39,Consider a separate voltage to be induced by theprimary into each of the windings, L2, L3, andL4. L4, which is closely coupled to the primaryintroduces a voltage that is in series with bothL2 and L3. This voltage across L4 is relativelyconstant in amplitude as long as the voltageacross Ll does not change. (Remember, thevoltage across Ll will stay relatively constantdue to the limiting action of transistor Q7.)Notice that each diode has its own separate loopthrough which its current flows (indicated by thearrows). Current flowing in diode D3 is con-trolled by the voltage induced in L2 and L4 whichcharges capacitor C33. The current flowing indiode D4 is controlled by the voltage inducedin coils L3 and L4 which charges capacitor C34.Current flows through L4 in both directions,since this coil is common to both current loops,The two currents flow through capacitors C33and C34 in the same direction. Electrolyticcapacitor C35 is connected across both of thesecapacitors through resistors R27 and R30.This large capacitor keeps the total voltageacross these two capacitors from changing, thus,any amplitude changes on the IF signal aredamped out by this capacitor.The audio output signal from the ratio detectorcircuit is applied to the base of Q8. Note thatthe two loop currents are flowing in oppositedirections through coil L4, resistor R31, capac-itor C39, and the input resistance of Q8. At theFM IF center frequency of 10.7 mc, the diodecurrents are equal, thus they cancel each otherout and no voltage appears across the input re-sistance of Q8.When the IF frequency deviates from 10.7 mcdue to FM modulation (audio signal), the currentin one diode loop increases while the current inthe other loop decreases. These changes arecaused by a change in phase relationship in thesignal current across coils L2 and L4. and L3and L4. Now current flows through the inputresistance of Q8 in the direction of the largersignal,and an output voltage is developedRadioFans.CN 收音机爱 好者资料库RICHT signal RFigure 12across the input resistance of Q8, The ampli-tude of this output voltage is determined byhow far the IF frequency deviates from thecenterfrequency of 10.7 mc. The frequencyof this audio output voltage is determined byhow often the frequency deviates from 10.7 mc.-The slug in the secondary of coil T6 is used tobalance the ratio detector circuit. CapacitorC36 and L5 removes any remaining 10.7 mc IFsignal from the audio signal. Resistors R28 andR29 are load resistors for diodes D3 and D4.FM STEREO MULTIPLEX CIRCUITFigures 12A and 12B show two sample signalsthat might appear from the left (L) and right(R) channel microphones of a radio station thatis broadcasting a stereo FM signal, The trans-mitting circuits then combine these signals toproduce the L+R signal shown in Figure 13Aandthe L-R subcarrier signal shown in Figure 13B.The L-R subcarrier signal is a suppressedcarrier amplitude modulated signal on a 38 kcsubcarrier, and is called the subcarrier channel.Figure 13These two signals, L+R and L-R, arc then com-bined with the 19 kc pilot signal shown in Figure13C. This whole complex signal modulates theFM carrier and is then radiated from thebroadcasting antenna.Figure 14 shows the locations of the differentcomponents that modulate an FM stereo signal.The“main channel” signal is from 50 cps to15 kc. Monaural FM tuners use only this part ofthe signal, and the remaining parts are atten-uated by the tuners de-emphasis network,L + RIL - RA U D I OM U L T I F L E X S I G N A L sca signalsFigure 14llsT T E DIONS 1A 19 kc pilot signal is transmitted to give theproper phasing for the demodulated subcarrierchannel, The 38 kc subcarrier channel is AMmodulated from 23 kc to 53 kc.A second subcarrier signal is transmitted bysome stations at 67 kc. This is usually a com-mercial music signal. This signal is called theSCA (Subsidiary Communications Authorization)channel.The signal thatis used for stereo multiplexoperation is coupled from the ratio detector.through capacitor C39 and the SCA filter, toaudio amplifier transistor Q8. The SCA filter,whichconsists of coil L6 and capacitors C4Oand C41, removes the 67 kc SCA signal. Thesesignals are not used for stereo reception.The complete stereo multiplex signal consists ofthe (L+R) main channel, the (L-R) subchannel,and the 19 kc pilot signal, The complete stereosignal is amplified by transistor Q8 andcoupledthrough capacitor C43 to 19 kc amplifier tran-sistor Q9. The collector circuit of transistor Q9is tuned to 19 kc by coil L7 and capacitor C44.Phase control R43 and capacitor C45 are con-nected across a portion of coil L7 so the phaseof the 19 kc signal can be adjusted, The 19 kcsignal is then coupled to the base of the 38 kcoscillator transistor, Q10. where it locks the38 kc oscillator in phase and frequency with thetransmitted 38 kc subcarrier signal,The 38 kc oscillator signal from transistor QIOis applied through transformer T7 to the basecircuits of switching detector transistors Q11 and Q12. At the same time, the main channel(L+R) and subchannel (L-R) signals are coupledfrom the emitter of transistor Q9 to the emittersof transistors Qll and QI2,When the main channel and subchannel signalsare combined with the 38 kc oscillator signal inthe switching detector circuit, the 38 kc carrierthat was removed at the transmitter (suppressedcarrier transmission) is reinserted into thestereo signal (waveform 3 on Block Diagram),Figure 15 shows the various waveforms thatare present in the switching detector circuit,Waveform 1 is the suppressed carrier steresand main channel signal that comes from tran-sistor Q9. Waveform 2 is the 38 kc oscillatorsignal that is reinserted in the stereo signal atthe same phase and frequency as the original 38kc carrier. Remember, this 38 kc oscillationwas locked at the correct frequency and phase bythe 19 kc pilot signal from Q9.The actual detection process takes place in thefollowing manner:When waveform 3 is appliedto the switching detector transistors, Q12 onlyconducts on that part of the waveform that car-ries the L waveform, Thus, only the L wave-form 4 appears at its output. Transistor Qllonly conducts on the R portion of the 38 kcwaveform, thus only the R waveform 5 appearsat its output. These are the left and right sig-nals originating at the broadcasting station.in Figure 16, the 38 kc signal is shown super-imposed on the stereo signal. At each 38 kcpeak on the L waveform, Q12 conducts and Qll.is cut off. At each 38 kc peak on the R wave-form, $12 is cut off and Qll conducts. The Lsignal from transistor $12, charges capacitorC54: the R signal from transistor Qll, chargescapacitor C55.L W A V E F O R MQ12 C O N D U C T SQ11 c u t - o f f,o3? W A V E F O R M( Q 1 2 2 C U T - O F FCl11 C O N D U C T 5F igure16LEFT WAVEfGRh-_ .-,rIRlCdT WAVEcCRhriI-RIGHT1 IEF WAVEFGRNWAVEFORk/SWITChcD -DFTFICTIChl -il? CGNCMTSO N L Y Stv THESi3e KC FEAKS-.- . -_LEFT WAVEFORMILFFT WAVEFORN38 ICCr- - -.l/jIli/lIljIIilI05RIGH r WAVEFORM31 i CONDUCTSCNLY Oh THESE38 KC PEAKSIRIGHT WAVEFORMIFigure 75between the reinserted 38 kc carrier and the38 kc subcarrier signal, This insures maxi-mum separation from the receiver, (The Phasecontrol is adjusted by listening for maximumsound in the subcarrier signal: the presenceof main channel sound would make this adjust-ment impossible.)The left and right channel audio signals then areapplied to individual 38 kc PEC filters thatremove any remaining 38 kc signal. Properde-emphasis of each signal is provided by thecombinations of the PEC components and ca-pacitors C57 and C58, The stereo signals arethen connected to the Left and Right channeloutputs.When the Phase switch is in the “out” position,a 38 kc bandpass filter is connected into thecircuit (coil L8 and capacitor C52. This circuitallows only the38 kc subcarrier channel to passthrough to transistors Qll and Q12. The phasecontrol is used to adjust for proper phasingA small amount of the 19 kc signal is coupledfrom coil L7, through capacitor C38, to the baseof stereo indicator amplifier transistor Q13.This 19 kc signal is only present when a stereosignal is being broadcast.MONOPHONIC FM OPERATIONThe monophonic signal is coupled from the ratiodetector circuit to the base of transistor Q8.After amplification,this signal is coupled totransistor Q9. Q9 acts only as an emitterfollower since no 19 kc signal is present.The monophonic signal is then coupled throughcapacitor C52 to the emitters of transistor Q11and Q12. No oscillator signal is coupled totransistors Qll and Q12, since the 38 kc oscil-lator circuit is disabled by the Source switch.Transistors Qll and Q12 will conduct when no38 kc signal is applied to their bases. Then thesame monophonic signal is present at the col-lectors of transistors Qll and Q12. These signalsare then coupled through the de-emphasis cir-cuits to the Left and Right channel outputs,LEFT CHANNEL AMPLIFIERThecomplete amplifier section of the Receiverconsists of a left channel amplifier and a rightchannel amplifier, The left and right amplifiersare identical; therefore, in order to simplifvthis description, only the left channel amplifierwill be discussed,The FM signal from the FM tuner section isapplied through resistorR105 and Sourceswitch contacts 6 and 3 t o the base of tran-sistor Q1.R e s i s t o r R11 1 1s u s e d to p r o -vide the proper load impedance for the phonocartridge.Transistor Ql operates as a high-gain low-noise amplifier to increase the levelof the incoming signal,The amplified signal from the collector of Qli s applieddirectly to the base of transistorQ3. Transistor Q3 amplifies the signal again.DC feedback is applied from the emitter of Q3 tothe base of Ql through resistor R117. From thecollector of Q3, for Phone operation, a portion ofthe signal is applied through a frequency selec-tive network. which corisists of capacitors Cl05and ClO9, resistor Rl25, and the Source switch.This network provides RIAA equalization,The equalized signal from the collector of Q3is applied through capacitor Cl07 to Volumecontrol R129. The amount of signal required toproduce the desired listening level is tappedoff by