Pioneer KEX-70 Service Manual page 4

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2.1 TAPE SECTION
e Dolby NR Circuit
The Dolby NR system functions to reduce the amount of tape
hiss and expand the dynamic range of the high frequencies. It
is able to improve the S/N ratio by 10 dB.
When the Dolby NR switch is at the OFF position, the Dolby
NR circuit functions as a flat amplifier, thereby allowing tapes
which have not been recorded with the Dolby system to be
played back.
The Dolby NR switch is set to ON when playing tapes which
have been recorded with the Dolby NR system.
REVERSE
© Equalizer Amplifier
The MB3106M dual-type IC is employed in the equalizer am-
plifier.
When the 70us switch is OFF, Q1 (Lch) and Q2 (Rch) go OFF
and the amplifier functions as a normal equalizer (3180ys
+120us).
2.2 MUTING CIRCUIT
When the switch is at OFF, + B is applied to the base of tran-
sistor O5 which then turns OFF since it is a PNP type. Since
the signal components no longer pass through the highpass
filter, the circuit functions as an ordinary flat amplifier.
When the Dolby NR switch is at ON, + B is no longer applied
to the base of transistor O5 which then turns ON. +B is ap-
plied to the Dolby NR indicator which lights. Since 05 is ON,
the highpass filter is connected to the output and the Dolby
noise reduction effect is produced.
OFF
O
| op oovey wR
SWITCH
ON
1
DOLBY NR
G tNO.
Fig. 3
When the same switch is ON, Q1 and Q2 turn ON, the 70us
equalization network is added and the amplifier functions as a
CrO2 (Metal) equalizer (3180us + 7Ous), enabling the play-
back of CrO2 tapes.
CONNECTOR
Fig. 4