Kenwood TS-990S Service Manual
Also see for SP-990: Instruction manual
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TS-990S17CIRCUIT DESCRIPTION3-9. Noise Blanker (NB) Circuit■ Noise DetectorThe Noise detector removes pulse noise of short cycle.The 8.248MHz IF signal passes through the buffer (Q16)and NB post-amplifi er in the RX unit (X55-313 A/2), and issent to the same NB circuit block on the RX unit through thecoaxial cable.The IF signal passes through the buffer (Q36) in the NBcircuit and is amplified by the 3-stage amplifier (Q21/Q24,Q28/Q30 and Q33). The IF signal then passes through thebuffer (Q37) and is noise detected by D21.A voltage from the noise detector (D21) switches Q35,turns Q38 ON, attenuates the IF signal in the blanking cir-cuit (D26 to D29) depending on the noise, and removes thepulse noise.The AGC works when a long cycle signal is applied, andthe gains of NB amplifier 2 (Q21 and Q24), NB amplifier 3(Q28 and Q30) and NB amplifier 4 (Q33) fall, which resultsin a low detecting voltage, and Q38 is turned OFF. Thus, theIF signal is not attenuated. Since the time constant of theAGC is long, the AGC does not work for short pulses, whichresults in a high detecting voltage, and Q38 is turned ON.Therefore, the blanking circuit operates.When the NB function is turned ON, a dc voltage MNBL(threshold control voltage) is applied to the emitter of Q35from the D/A converter (IC1), so that the Noise Blanking ef-fect can be adjusted by varying the emitter voltage.■ Noise Blanking CircuitThe noise blanking circuit attenuates the IF signal by thesignal detected in the noise detector and removes pulsenoises.When pulse noise is detected in the NB circuit, theMBLNK goes to “H”, Q54 is turned ON and Q53 is turnedOFF. Diodes D26 to D29 are then turned OFF and the IFsignal is attenuated.Fig. 14 Noise detection circuitIF input(8.248M)Q16BufferQ18NB POST AMPCN80 CN81Coaxial cableQ36BufferQ21,24NB AMP2Q28,30NB AMP3 Q33NB AMP4Q37BufferD21DetectionQ32NB AGC AMPAGCMNBL(Noise blankerthreshold level)Q35SwitchingQ38SwitchingMIBKMBLNK signal(NB blank control)Fig. 15 Blanking circuitBlanking OFF : 1.65VL77Blanking ON : 1.7VIN+8VL79Blanking ON : 8VBlanking OFF : 0VMBLNKH : Blanking ONL : Blanking OFFOUTD26 D28D27 D29Q53, Q54 |
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