Heathkit-AR14-rec-sm维修电路原理图.pdf

CIRCUIT DESCRIPTION Each 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 local Description. For ease of explanation, the Source oscillator frequency with the station being switch 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 Circuit Description. The letter-number designations (R4, C115, R212) for all resistors, capacitors, and diodes have been placed into the following groups to make them easier to locate on the chassis and Schematic. 1 - 99 100 - 199 200 - 299 FM tuner section. Amplifier section. Power supply section. The oscillator frequency is locked-in by the AFC voltage in the following manner: The capacitance between the elements of diode Dl changes when the AFC voltage that is applied to it changes. This capacitance is connected in series with capacitor C19. and these two capacitances are connected in parallel with part of coil L4. Thus, when the capacitance of diode Dl is changed by the AFC voltage, the total capacitance across coil L4 is changed. This change in the tuned circuit capacitance changes the frequency of the oscillator in such a way as to maintain proper tuning. FM TUNING UNIT The FM signal from the antennaisappliedto the primary of balanced input transformer Tl in the FM tuning unit. The secondary of transformer Tl forms a tuned circuit with trimmer capacitor Cl and capacitors C2 and ClA (antenna section of tuning capacitor). The signal selected by this tuned circuit is coupled through capacitor C3 to RF amplifier transistor Ql. The signal is amplified by transistor Ql. The RF tuned circuit of Ql selects the desired signal and couples it through capacitor C8 to the base of 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 are mixed in transistor Q2 to produce a 10.7 mc IF (intermediate frequency) signal, that is coupled through transformer T2 and capacitor C21 to the first IF amplifier transistor, Q4. The amplified IF signal from the collector of tran- sistor Q4 is coupled through transformer T3 to the base of second IF amplifier transistor Q5. This IF signal is again amplified by tran- sistor Q5, coupled through transformer T4, and amplified by transistor Q6. From Q6 the signal is coupled through transformer T5 and amplified by the fourth IF amplifier transistor Q7. The local oscillator transistor, Q3, operates at a frequency that is 10.7 mc higher than the received FM signal. The oscillator frequency is de- termined by a tuned circuit composed of coil L4, trimmer capacitor C14, and capacitors Cl3 and C14A (oscillator portion of tuning capacitor). The output signal from this oscillator is coupled through capacitor Cl0 to the base of mixer transistor Q2. A portion of the signal voltage is taken from the collector of transistor Q6 and rectified by diode D2 to produce an AGC (automatic gain control) voltage, This AGC voltage, which increases and decreases with the strength of the received FM signal,is coupled through resistors R14 and R12 to the base of transistor Q4, where it automatically controls the gain of the IF signal. A small DC voltage is coupled from the ratio de- tector circuit through resistor R5 to diode Dl in the collector circuit of the oscillator. This DC voltage,which changes as the tuning is The AGC voltage is then coupledfrom the emitter of transistor Q4, through resistor RlO, to the base of RF amplifier transistor Ql. This automat- ically controls the gain of the FM signal in the tuner section. RadioFans.CN Figure 11 All, or only one of the IF amplifier stages may operate as limiters. For a very weak signal, only the fourth IF amplifier Q7 may be limiting, and transistors Q4, Q5, and Q6 would be amplifying the IF signal. For a very strong signal, all four IF amplifiers may be acting as limiters. This limiting action removes ampli- tude modulation from the FM signal. Limiting action is also provided by the self-limiting characteristics of the ratio detector circuit. RATIO DETECTOR CIRCUIT From the collector of transistor Q7, the IF signal is 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 this figure by primary coil Ll, a center tapped sec- ondary composed of coils L2 and L3. and a third or tertiary winding. L4. L4 is just a few turns of wire tightly wrapped around the bottom of pri- mary Ll. NOTE: In the actual circuit, choke L5 and coil L6 are also connected in series with coil L4, resistor R31, capacitor C39, and re- sistor R39, Consider a separate voltage to be induced by the primary into each of the windings, L2, L3, and L4. L4, which is closely coupled to the primary introduces a voltage that is in series with both L2 and L3. This voltage across L4 is relatively constant in amplitude as long as the voltage across Ll does not change. (Remember, the voltage across Ll will stay relatively constant due to the limiting action of transistor Q7.) Notice that each diode has its own separate loop through which its current flows (indicated by the arrows). Current flowing in diode D3 is con- trolled by the voltage induced in L2 and L4 which charges capacitor C33. The current flowing in diode D4 is controlled by the voltage induced in 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 C33 and C34 in the same direction. Electrolytic capacitor C35 is connected across both of these capacitors through resistors R27 and R30. This large capacitor keeps the total voltage across these two capacitors from changing, thus, any amplitude changes on the IF signal are damped out by this capacitor. The audio output signal from the ratio detector circuit is applied to the base of Q8. Note that the two loop currents are flowing in opposite directions through coil L4, resistor R31, capac- itor C39, and the input resistance of Q8. At the FM IF center frequency of 10.7 mc, the diode currents are equal, thus they cancel each other out and no voltage appears across the input re- sistance of Q8. When the IF frequency deviates from 10.7 mc due to FM modulation (audio signal), the current in one diode loop increases while the current in the other loop decreases. These changes are caused by a change in phase relationship in the signal current across coils L2 and L4. and L3 and L4. Now current flows through the input resistance of Q8 in the direction of the larger signal,and an output voltage is developed RadioFans.CN RICHT signal R Figure 12 across the input resistance of Q8, The ampli- tude of this output voltage is determined by how far the IF frequency deviates from the centerfrequency of 10.7 mc. The frequency of this audio output voltage is determined by how often the frequency deviates from 10.7 mc. .- The slug in the secondary of coil T6 is used to balance the ratio detector circuit. Capacitor C36 and L5 removes any remaining 10.7 mc IF signal from the audio signal. Resistors R28 and R29 are load resistors for diodes D3 and D4. FM STEREO MULTIPLEX CIRCUIT Figures 12A and 12B show two sample signals that might appear from the left (L) and right (R) channel microphones of a radio station that is broadcasting a stereo FM signal, The trans- mitting circuits then combine these signals to produce the L+R signal shown in Figure 13Aand the L-R subcarrier signal shown in Figure 13B. The L-R subcarrier signal is a suppressed carrier amplitude modulated signal on a 38 kc subcarrier, and is called the subcarrier channel. Figure 13 These two signals, L+R and L-R, arc then com- bined with the 19 kc pilot signal shown in Figure 13C. This whole complex signal modulates the FM carrier and is then radiated from the broadcasting antenna. Figure 14 shows the locations of the different components that modulate an FM stereo signal. The“main channel” signal is from 50 cps to 15 kc. Monaural FM tuners use only this part of the signal, and the remaining parts are atten- uated by the tuners de-emphasis network, L + R I L - R A U D I O M U L T I F L E X S I G N A L sca signals Figure 14 lls T T E D IONS 1 RadioFans.CN A 19 kc pilot signal is transmitted to give the proper phasing for the demodulated subcarrier channel, The 38 kc subcarrier channel is AM modulated from 23 kc to 53 kc. A second subcarrier signal is transmitted by some stations at 67 kc. This is usually a com- mercial music signal. This signal is called the SCA (Subsidiary Communications Authorization) channel. The signal thatis used for stereo multiplex operation is coupled from the ratio detector. through capacitor C39 and the SCA filter, to audio amplifier transistor Q8. The SCA filter, whichconsists of coil L6 and capacitors C4O and C41, removes the 67 kc SCA signal. These signals are not used for stereo reception. The complete stereo multiplex signal consists of the (L+R) main channel, the (L-R) subchannel, and the 19 kc pilot signal, The complete stereo signal is amplified by transistor Q8 andcoupled through capacitor C43 to 19 kc amplifier tran- sistor Q9. The collector circuit of transistor Q9 is 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 phase of the 19 kc signal can be adjusted, The 19 kc signal is then coupled to the base of the 38 kc oscillator transistor, Q10. where it locks the 38 kc oscillator in phase and frequency with the transmitted 38 kc subcarrier signal, The 38 kc oscillator signal from transistor QIO is applied through transformer T7 to the base circuits of switching detector transistors Q11 and Q12. At the same time, the main channel (L+R) and subchannel (L-R) signals are coupled from the emitter of transistor Q9 to the emitters of transistors Qll and QI2, When the main channel and subchannel signals are combined with the 38 kc oscillator signal in the switching detector circuit, the 38 kc carrier that was removed at the transmitter (suppressed carrier transmission) is reinserted into the stereo signal (waveform 3 on Block Diagram), Figure 15 shows the various waveforms that are present in the switching detector circuit, Waveform 1 is the suppressed carrier steres and main channel signal that comes from tran- sistor Q9. Waveform 2 is the 38 kc oscillator signal that is reinserted in the stereo signal at the same phase and frequency as the original 38 kc carrier. Remember, this 38 kc oscillation was locked at the correct frequency and phase by the 19 kc pilot signal from Q9. The actual detection process takes place in the following manner:When waveform 3 is applied to the switching detector transistors, Q12 only conducts on that part of the waveform that car- ries the L waveform, Thus, only the L wave- form 4 appears at its output. Transistor Qll only conducts on the R portion of the 38 kc waveform, thus only the R waveform 5 appears at 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 kc peak 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 L signal from transistor $12, charges capacitor C54: the R signal from transistor Qll, charges capacitor C55. L W A V E F O R M Q12 C O N D U C T S Q11 c u t - o f f ,o 3 ? W A V E F O R M ( Q 1 2 2 C U T - O F F Cl11 C O N D U C T 5F igure16 LEFT WAVEfGRh -_ .- ,r I RlCdT WAVEcCRhr i I- RIGHT 1 IEF WAVEFGRN WAVEFORk/ SWITChcD - DFTFICTIChl - il? CGNCMTS O N L Y Stv THESi 3e KC FEAKS -.- . -_ LEFT WAVEFORM I LFFT WAVEFORN 38 ICC r- - -. l/jIli/lIljIIilI 0 5 RIGH r WAVEFORM 31 i CONDUCTS CNLY Oh THESE 38 KC PEAKS I RIGHT WAVEFORM I Figure 75 between the reinserted 38 kc carrier and the 38 kc subcarrier signal, This insures maxi- mum separation from the receiver, (The Phase control is adjusted by listening for maximum sound in the subcarrier signal: the presence of main channel sound would make this adjust- ment impossible.) The left and right channel audio signals then are applied to individual 38 kc PEC filters that remove any remaining 38 kc signal. Proper de-emphasis of each signal is provided by the combinations of the PEC components and ca- pacitors C57 and C58, The stereo signals are then connected to the Left and Right channel outputs. When the Phase switch is in the “out” position, a 38 kc bandpass filter is connected into the circuit (coil L8 and capacitor C52. This circuit allows only the38 kc subcarrier channel to pass through to transistors Qll and Q12. The phase control is used to adjust for proper phasing A small amount of the 19 kc signal is coupled from coil L7, through capacitor C38, to the base of stereo indicator amplifier transistor Q13. This 19 kc signal is only present when a stereo signal is being broadcast. MONOPHONIC FM OPERATION The monophonic signal is coupled from the ratio detector circuit to the base of transistor Q8. After amplification,this signal is coupled to transistor Q9. Q9 acts only as an emitter follower since no 19 kc signal is present. The monophonic signal is then coupled through capacitor C52 to the emitters of transistor Q11 and Q12. No oscillator signal is coupled to transistors Qll and Q12, since the 38 kc oscil- lator circuit is disabled by the Source switch. Transistors Qll and Q12 will conduct when no 38 kc signal is applied to their bases. Then the same monophonic signal is present at the col- lectors of transistors Qll and Q12. These signals are then coupled through the de-emphasis cir- cuits to the Left and Right channel outputs, LEFT CHANNEL AMPLIFIER The complete amplifier section of the Receiver consists of a left channel amplifier and a right channel amplifier, The left and right amplifiers are identical; therefore, in order to simplifv this description, only the left channel amplifier will be discussed, The FM signal from the FM tuner section is applied through resistorR105 and Source switch 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 phono cartridge.Transistor Ql operates as a high- gain low-noise amplifier to increase the level of the incoming signal, The amplified signal from the collector of Ql i s applieddirectly to the base of transistor Q3. Transistor Q3 amplifies the signal again. DC feedback is applied from the emitter of Q3 to the base of Ql through resistor R117. From the collector of Q3, for P