Philips MC-21, Service Manual

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9-2 9-2
Brief introduction
General
1. Playback Mode
Signal from the playback head (Deck B) is selected and fed through by the Mode Selector IC7710 (HEF4952BT). The signal
is amplified by amplifier IC7720 (AN7318S) before feeding to the IC7740 (HEF4952BT) and out to the AF Board via connector
1701.
2. Recording Mode
Recording Signal is selected and fed through by the Mode Selector IC7710 (HEF4952BT) which is then amplified by the
amplifier IC7720 (AN7318S). The amplified output signal will pass through IC7730 (HEF4952BT) for record equalization and
back to IC7710 (HEF4952BT) before registered into the Rec/PB Head (Deck B).
3. Mode Selector
The Mode Selector IC7710 (HEF4952BT) handles 2 inputs signal, Playback Signal from Deck B and Recording Signal.
4. Amplifier PB/REC
Amplifier IC7720 (AN7318S) is for the purpose of amplifying the Playback and Recording signal from the Mode Selector.
5. Automatic Level Control (ALC)
ALC circuit consists of resistors (3760, 3765, 3766, 3767), capacitors (2762 , 2763) and control by transistor 7787 (BC847B).
ALC limits the amplifier output to a constant value when input signal becomes too large, thus limiting recording current to
below saturation level, to prevent recording distortion.
6. Muting Circuit
Switch S4 of the IC7740 (HEF4952BT) is for the purpose of muting the output during Recording mode. During Recording
mode, S4 is closed and shorted to the ground.
7. IC7740 (HEF4952BT)
The function of the IC7740 (HEF4952BT) is to change time constant between 120us Ferro (IEC I) and 70us Chrome (IEC
II) during playback mode. It will automatically determined whether the tape type is 120us Ferro (IEC I) or 70us Chrome (IEC
II). This IC will switch to Flat Gain during the Recording mode.
8. IC7730 (HEF4952BT)
The function of the IC7730 (HEF4952BT) is to change gain and time constant according to tape type and recording speed
to boost recording current at higher frequency during recording to compensate for head loss. It will automatically determined
whether the tape type is 120us Ferro (IEC I ) or 70us Chrome (IEC II).
9. Bias Level
Bias Level making use of the Variable resistor (3773) for adjusting the optimal level of the bias current for Ferro or Chrome.
10. PB Switch
FET 7786 (J111) acting like an electronic playback switch provides a virtual ground for the Rec/PB Head (Deck B). During
the Playback mode, the FET is turn on shorting pin 2 of connector 1710 to the ground. During Recording mode, the FET is
turn off to allow the oscillator signal to be superposition onto the Recording signal for recording.
11. IC7610 (HEF4094BT)
IC7610 (HEF4094BT) is a Shift Register use for issues the logic for cmos switch ICs (HEF4952BT) via 1A0, 2A1 and 2A2.
It also issues logic to On/Off SOL_B and MOT. Recording speed is controlled via NS/HS.
CONNECTORS ASSIGNMENTS:
CONNECTOR 1701 INTERCONNECTION TO AF BOARD
1 REC-L Record input left
2 REC-R Record input right
3 GND A AF Ground
4 TAPE-L Playback output left
5 +12V D.C. supply (+12V) for AF electronics
6 TAPE-R Playback output right
7 -CMOS Negative d.c. supply (-9V) for CMOS ICs
CONNECTOR 1703 INTERCONNECTION TO AF BOARD
1 GND M Motor Ground
2 +MOTOR D.C. supply (+12V) for tape deck motor & solenoid
CONNECTOR 1706 INTERCONNECTION TO FRONT BOARD
1 AD2 Deck sensing switches output voltage / Deck A EOT
2 AD1 Deck sensing switches output voltage / Deck B EOT
3 +5V DC 5V for ADC network
4 GND P Control & Oscillator Ground
5 CLK HEF4094BT shift register Clock line
6 DATA HEF4094BT shift register Data line
7 STROBE HEF4094BT shift register Strobe line
CONNECTOR 1710 DECK B HEADS CONNECTON
1 B R/P HD L+ R/P Head left channel positive
2 GND A R/P Head return ground
3 B R/P HD R+ R/P Head right channel positive
4 ERASE HEAD Erase Head
5 GND A Erase Head ground
CONNECTOR 1730 DECK A HEAD CONNECTIONS (Not For Single deck)
1 A PB HD L+ Pb Head left channel positive
2 GND A Pb Head return ground shield
3 A PB HD R+ Pb Head right channel positive
CONNECTOR 1740 DECK A & B CONTROL INTERFACE
1 REC REW Record tab protection status switch (reverse) [open=on: close=off]
2 REC FWD Record tab protection status switch (forward) [open=on: close=off]
3 CrO2 B Chrome tape detection switch deck B [open=Cr: close=Fe]
4 PHOTO B Photo sensor output (tape movement indication)
5 SOL B Solenoid supply for deck B
6 Vcc Deck / Motor supply
7 MODE B Mode switch (head engagement) [open=off: close=engaged]
8 GND M Deck / Motor ground
ABBREVIATIONS USED IN THIS DOCUMENT
CR Chrome (IEC type II)
DB Dolby NR type B
DD Double Deck
DM Double Motor
FE Ferro (IEC type I)
FF Non-Autoreverse
FR Autoreverse Deck B
Gnd x Ground x
HSD
High speed dubbing
ND Non Dolby
NR Noise Reduction
NSD Normal speed dubbing
PB Playback
REC Record
S/A Sub-assy
SD Single Deck
SM Single Motor