Pioneer ARP-230-0 Service Manual - Page 6

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5 . C I R C U I T D E S C R I P T I O N S

FM Front End

A unitized variable capacitor type front end

unit is used consisting of an FET RF single stage

amp, Iocal oscillator/mixer IC and €rn IF trans-

former.

FM lF Amp, Detector and MPX Circuit

A 2-transistor IF amp with ceramic filter is used

and the next stage has an IC (M51533L) contain-

ing the FM IF detector and MPX circuit. The block

diagram of IC M51533L is shown in Fig. 5-1. This

IC uses a peak detection method which can be

adjusted with a single coil. The PLL method is

employed to reconstitute the 38kHz sub-carrier

for the MPX circuit.

The IF signal is input at pin 1 and the detection

output is obtained from pin 5. The detection out-

put passes through the muting Q4, goes to pin 9,

passes through the MPX circuit and the stereo

signals are obtained from pins 11 and L2. In

addition, this unit detects the presence or absence

L O U T

R O U T

of a pilot signal to automatically switch between

stereo and mono reception by R14 which is con-

nected to pin 14. When a stereo signal is received,

pin 13 drops to the low level to light the stereo

indicator. When pin 2 is grounded, the PLL VCO

and FM IF operation stop (for AM).

FM Muting and LED Driver

With this unit, muting is automatically activated

when the antenna input drops below 10pV. As the

antenna input decreases, the voltage at pin 3

drops, Q5 is turned off and Q6 is turned on. As a

result, the gate voltage of Q4 drops, Q4 is turned

off and the detector output circuit is blocked

(Fig. 5-1). When an FM or AM station is tuned

in, the base potential of Qb is raised, Q5 is turned

on and the tuning indicator lights.

M 5 r 5 3 3 L

R E G U L A T O R

L E V E L

M E T E R

C I R C U I T

I N P U T

B U F F E R

S T E R E O

S W I T C H

A M

s t - 3

F i g . 5 - 1

F M l F , D e t e c t o r a n d M P X c i r c u i t