Pioneer SX-3800 manual t t t

4 . 7 P O W E R] N D I C A T O RC I R C U I T

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O C A M P

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O F F S E T C O M .

P E N S A I O R

F / L A M E N T

Fig .4 - 12 Power indicatorcircuit

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The SX-3800 output power indicators feature fluorescentindicator tube (FL tube). In this tube, thermionic emissions from the cathode are ac- celerated into the fluorescent substance of the segmentalanodes,resulting in the emissionof light. This tube is used to indicate numerals,letters,and other symbols.

An outline of the FL tube drive circuit is shown in Fig. 4-12. The output circuit sj"tnalis appliedto pin no.6 ( ) of the IC (TA?318f-A).The IC con- tains a detector circuit, conpressor (40d8), and peak hold circuit for both left and right channels.

The dynamic range of the signalis thus contracted by 40dB to obtain a "peak held" DC voltage.

The output pou'erindicator segmentsof tire FL tube are driven by the HA12010 ICs (one for each channel) equipped rvith 12 pairs of differential amplifiers. Theseamplifiers are biasedat increasing levels,so each amplifier lvill commenceto operate separatelyas the input level increasesAnd. since these amplifiers apply the voltages to the output power indicator segmetrts,each successivesegment will light up in turn as the input level rises.

4 . 8 P R O T E C T I O NC I R C U I T

The purpose of this circuit is to protect the speakersand the power amplifiers.The relay in the output circuit is automatically opened in any of

the following cases:

L. During the "transient operations" lvhen the power supply is turned on and off.

2.Upon detection of an overload, causedby a short circuit in the ioad.

3.Upon detection of a DC voltagein the output causedby componentfailure or accident.

MutingOperationwhen PowerSupply is Turned On and Off

With reference to Fig. 4-13 when the porver supply is turned on, Q3 turns off due to *81 (l'he time constant of the *81 circuit is very sma11.). If thereis no input (DC) on Qb and Q6, they will be off, and the timing capacitor C1 chargesup through R8 and R6, and thus Q4 turns on. \Vhen Q4 condr,rcts,tire relay operates,and the output muting on the power amplifier will be removed.

When the polver supply is turned off, -81rvill abruptly decay,and Q3 will conduct owing to the resiclualcomponent of +81. As a result, C1 rvill rapidly discharge,Q4 will ceaseto conduct, rvhere- upon the relay will become de-energizedand restoremnting.

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