Heathkit AR 14 Schematic
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Page 1
Figure 11 All, 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 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
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RIC>HT 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 center frequency 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 AUDIO MULTIFLEX SIGNAL sca signals Figure 14 lls TTED IONS 1
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5544332211DDCCBBAASTEREO INDICATOR PHASE 38KC OSCILLATOR ON PHASE ADJUST CONTROL AUDIO AMPLIFIER AMPLIFIER AND EMITTER FOLLOWER19 KC6 3 4Q8STEREO LEFTSTEREO RIGHTB +17VC +8VC45 390pfR40 220C58 .002MFC48 .2MFC53 470pfQ8 2N2712C49 2700pfR33 33Q14 2N2712R50 10KR51 220KR43 10KC50 .01MFR37 2.2KR41 47KR46 6.8KR44 560R49 47KR.E.C.231Q9 2N2712+C56 10MF +C59 10MFC55 .001MFC51 .01MFADJ NORMALR38 47KC41 680pfR47 2.2KC42 .01MFC44 .01MFR35 120KQ11 2N2712L8R.E.C.231R42 560R48 15KC46 .01MFR39 10KL75142R34 1KQ10 2N2712T71625347 +C52 10MFQ12 2N2712R45 39KC57 .002MF +C43 10MFC47 .01MFQ13 2N2712R36 68KREAR OF WAFER #3R52 220KC38 200pfLAMPC54 .001MF