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SIGNAL lndicatorCircuit

Fig .4 - 7 SIGNALindicatordrivecircuit

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The SX-3800SIGNAL indicator consistsof an FL tube 5-point indicator display.The signalmeter drive signal obtained from the FM IF system IC (PA300?-A)and AIVI tuner IC (HA119?) is first amplified and then applied to the indicator drive IC (HA12010). This IC contains12 pairsof voltage comparators similar to those employed in the power indicator circuit, b of thesepairsbeingused to drive the SIGNAL indicator.

TUNING lndicatorCircuit

The TUNING indicator consists of a center tuning indicator (which lights up when a broad- casting station frequency is properly tuned) and

2detuning direction indicatorswhich indicatethe direction in which the station hasbeentuned awav

from. The correspondingdrive circuits are outlinei in Fig. 4-Bbelow.

The TUNING indicator is activated once the station has been tuned to within t100kHz of the

center frequency.This is because is turned

e16 on and Q21 turned off (resultingin the detector dif_ feren^tialamplifier le19 & e20J being tumed off and Q24 beingturned on) by the FM muting signal appearing at pin no.lB of the IF system IC {PA3007-A) and passedvia the Schmitt circuit (Q17 & Q1B) rvhen the station is tuned away by morethant100kHz .

The DC voltage on pin no.4 of pAB007_Ade- scribesan S curvewhen tuning to and away from a particular broadcastingfrequency, the voltage on pin no.2 serving as the referencelevel. This DC

SCHMITT

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Fig.4-8 TUNINGindicatordrive.circuit

voltage is amplified by the differential amplifier (Q19 & Q20) and then appliedto a polarity detec_ tor switch circuit (eZZ & e2g).

When tuning to a frequency from the high fre_ quency side (or tuning away from the frequency to a higherfrequency),the voltageon pin no.4 will be higher than that on pin no.2. The e20 collector voltage will thus be lowered and the e1.g collector voltage raised, resulting in e2B being tumed on, and the higher frequency (right hand side) detun- ing direction indicator also being turned on. When.

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