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Thought I would never find a copy of the Technics SX-EN2 Service Manual until I found Owner-Manuals.com. Price was very fair and I received the download promptly. While a photocopy, it is quite readable and includes all the pertinent information and diagrams. Thank you Owner-Manuals!
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11. CIRCUIT DESCRIPTION
11.1 POWER SUPPLY:
· Socket PL800 is the 220VAC input. · 3.5A fuse F800 is used to protect the device against short circuit and unexpected overloads. · Line filter and capacitors L800, C801 and C803 are used to block the parasitic coming from the mains. They also prevent the noise, produced in the circuit, from being injected to the line. · Voltage is rectified by using diodes D805 diode bridge. Using capacitor C815 (100uf) a DC voltage is produced. (310- 320VDC). · The current in the primary side of the transformer TR800 comes to the SMPS IC (IC800 MC44608). The SMPS IC has a eight-pin DIP-8 package and an external MOSFET with a cooler is mounted on it. It has a built-in oscillator, overcurrent and overvoltage protection circuitry and runs at 100kHz. It starts with the current from the primary side of the transformer and follows the current from the feedback winding. · Feedback current is deteceted by optocoupler IC803. Depending on the control current coming fro m the secondary side, SMPS IC keeps the output voltage constant by controlling the duty cycle of the ~30kHz signal (PWM) at the primary side of the transformer.
· Voltages on the secondary side are as follows: +30 Volts at D811, +8 Volts at D808, +15V at D810, -22 Volts at D812, +12Vst at Q804. · Using the output of the D808, a photo diode inside of the IC803 generates feedback signal bu using optocoupler's photo transistor. This photo transistor adjusts the control voltage at the IC800 pin3. The voltage at this pin effects the pwm output frequency on the IC800 pin5. And finally output voltages reach their correct values by this way. · Voltage regulator IC805 (LM7805) supplies +5 Volts, IC807 supplies +5V (off on standby mode), IC809 supplies +3.3V (by using output of the IC807, off on standby mode), Q804 supplies +12Vst, IC806 supplies +12V (off on standby mode), Q808 supplies -5V, D812 supplies -22V. Standby mode controlled by standby control transistors Q805, Q806, Q807. · �22 Volts is used to feed the VFD (Vacuum Fluorescent Display) driver IC on the front panel. Using diode R844, �22V is decreased and connected to the filament winding to produce the DC offset for the filaments.
11.2 FRONT PANEL:
· All the functions on the front panel are controlled by IC300 (MT1379) on the mainboard · IC300 sends the commands to IC101 uPD16311 via socket PL101 (pins 2,3 and 4). · There are 16 keys scanning function, 2 LED outputs, 1 Stand-by output and VFD drivers on IC101. · Pin 52 is the oscillator pin and is connected via R107 56K. · LED D1 is blue in stand-by mode and off when the device is on.
· Vacuum fluorescent display MD1 is specially designed for AV2300. · The scanned keys are transmitted via IC101 to IC300 on the mainboard. · IR re mote control receiver module IC102 (TSOP1836) sends the commands from the remote control directly to IC300. · Socket PL102 carries the VFD filament voltage and �22 Volts.
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