Power Supply; Opto-Coupler Feedback (PSVR82/PSVR62/VR684HF/VR674HF/VR673HF/VG4239/VG4238)

Model Number:  PSVR82/PSVR62/VR684HF/VR674HF/VR673HF/VG4239/VG4238

The switching mode power supply utilized in these units (see model numbers above) incorporate an IC regulator (IC821) as well as an IC power output device (IC803).  The power supply also utilizes optical feedback (see Fig. 22)  in the primary regulation circuitry.  An optical device is also used to couple the 60 Hz AC line voltage across the Hot-Cold ground barrier.  This signal is then used as the AC Clock signal that is input to the system control microcomputer.   A DC-DC converter module is used to generate the higher DC voltage required by the tuner and the fluorescent display.

The 120 volts AC is input to the full wave bridge rectifier D803 via fuse F801 (1.6A).  The main filter capacitor for raw B+ is C805.  The raw B+ is applied to pin 6 of  the power transformer T802.  A Snubber circuit consisting of R805 & R806, C809, C806 and diode D806 is utilized to protect the main power switching transistor IC803 from damage due to the peak reverse voltage (collector-emitter) developed by the switching transistor.  The snubber circuit connected between pins 6 and 3 of  T802 suppresses the high voltage spikes during start-up as well as during the on-off switching of the power transistor.

Transistor Q1 of IC803 is turned on the first time by coupling B+ across resistors R813 and R807 to pin 2 of IC803.  Q1 then turns on the first time and pulls current through the primary winding of T802 causing a magnetic field to build up across the winding between pin 6 and 3.  Q1 stops conducting when saturation is reached through the winding.  This allows the field to collapse thus inducing a voltage into the secondary windings.  When the power transfer is completed to secondary winding between pin 9 and 14, diode D821 turns off and a reverse EMF voltage is induced back to the winding between pins 1 and 2.   This causes a positive voltage to develop at pin 2 of T802.  This positive voltage is coupled across diode D805 to the base of Q1 inside IC803.  This causes the power transistor to turn on again.  The free-run frequency of the power supply is determined by the R/C time constant of the internal resistor and capacitor across the base-emitter junction of transistor Q2 in IC803.

The secondary voltages are developed at pins 9, 10 and 11 of T802 by diodes D821, D822 and D823.  The 14VE (volts ever) is monitored and used for primary regulation of the power supply.  The 14VE is input the regulator IC821 at pin 13 and is applied to an error amp.  The output of the error amp (pin 12) is then applied the anode of the LED in opto-isolator Q802.  Operation of Q802 and how it regulates will be discussed in detail a little later.  The 14VE that is input to the error amp at pin 13 is also applied the On/Off 9V error amplifier within IC821.

CAUTION:  DO NOT under any circumstances attempt to defeat or bypass the optical regulation feedback path (Q802) during servicing.  Extensive damage to power supply and/or VCR will result. 

The output of this error amp exits at pin 9 and is applied to the base of regulator transistor Q822.  The On/Off 9V source is developed at emitter of regulator transistor Q822.  The 6.6VE that is developed at pin 10 of T802 is applied to pin 5 of IC821.  This voltage is used to generate the 5VE and the On/Off 5V by their respective regulators within IC821.  The input at pin 2 of IC821 is from the system control microcomputer and turns on and off the On/Off 5V regulator as well as the On/Off 9V error amplifier within IC821.

Primary regulation of the power supply is accomplished by monitoring the 14VE output at pin 9 of T802.  The output of the error amp at pin 12 of IC821 is used to control the current through the LED section of Q802.  Pin 1 of Q802 is tied to the 14VE source and acts as the reference voltage.  As the error voltage at pin 2 of Q802 goes down, the current through the LED goes up.  This causes more light to be generated which in turn causes the transistor portion of Q802 to conduct harder (current goes up).  This action pulls the base current away from Q2 and prevents the capacitor on the base from charging up enough to turn on Q2.  Q2 is turned on in order to turn off the power transistor Q1.  When Q2 (of IC803) is held off longer, Q1 is allowed to conduct longer therefore transferring more energy into the secondary of the power supply.  This causes the output voltage to rise.  The opposite occurs if the output voltage goes too high (error voltage goes up).  As the error voltage moves up toward the reference, the current through the LED (Q802) goes down.  This means less light to the base of the opto-transistor.  This causes less emitter current thus pulling less base current away from Q2.  This allows its base capacitor to charge up and turn Q2 on and Q1 off.  With Q1 turned off longer the output voltage of the power supply drops.  If a failure occurs in the regulation feedback path (Q802, etc.) the output voltage may rise as much as two to three times normal.  This could possibly cause catastrophic failures in the rest of the power supply including IC803, IC821, etc.  

The opto-isolater Q801 is used couple the AC line signal across the Hot-Cold barrier and is the used to generate the AC clock signal needed by the system control microcomputer.  One side of the incoming AC line is tapped off and rectified by diode D802.  The cathode of D802 is connected to pin 1 of Q801 (anode of the LED).  The pulsing current flow through the LED causes a clock signal to be generated at pin 3 of Q801.  The clock signal is then routed to the system control.  Its important that this signal be present.  The system control uses this signal as an indicator that AC power is applied to the system and for different internal timing operation.  If this signal is lost, the microcomputer assumes that AC power has been lost and it initiates a shutdown routine causing it to go into a backup or sleep mode.

CAUTION:  DO NOT under any circumstances attempt to defeat or bypass the optical regulation feedback path (Q802) during servicing.  Extensive damage to power supply and/or VCR will result. 