SANSUI SVX – 2106R COLOR TV - USING STR S6708 - Switching Regulator

STR S6708 SMPS Power supply faults

       Power supply of this set is composed of STR S6708, and its regulation is controlled by the optocoupler TLP621GR F2.  The most common fault to the power supply section to this set is, blown out STR Switching regulator.  Before replacing the STR S6708, check first the two ceramic capacitors soldered to its pin number [1], C860-470PF & C862-IK8PF IKV.  Check the soldering to this C862 and check this capacitor for open. If this capacitor has some defect with it, the STR replaced will blow out as soon as the set is power ON.  Check L861 too for open circuit.

     We can check the STR S6708 for defect, with a multimeter.  The IC itself has internal control circuit, and one power transistor integrated.  Normally, the collector to emitter junction of this internal power transistor will get shorted, and a defective STR.  Pin number [3] of the STR is the base to internal switching transistor. Pin number [1] is its collector out, and pin number [2] is its emitter.

      Check these pins like we check an NPN transistor with a multimeter. Check whether there is shorted collector to emitter junction.  Just check the resistance between pint number [2] & [1].  In no case, this junction should show any conductance. If it show conductance, the STR is faulty and should be replaced.

Along with the STR, it is advised to replace the opto-coupler [IC 862 – TLP 621GR F2] too. Check the transistor [Q872 C3852] and two Zener diodes connected in series to its base connection [D872 and D871][both of them are 6.8V Zener diodes] These diodes along with the transistor [Q872] determines the input supply voltage to optocoupler, and should be very constant, otherwise conduction to the internal photo transistor of the optocoupler will differ, which in turn result a non-constant secondary power output [115VDC] from the secondary of the SMPS transformer, which in turn feeds the horizontal output section of the TV.

     The control voltage to the internal LED of the optocoupler has been derived from the collector of [Q884 – 2SA1015], and the base of this transistor is controlled by the voltage from the error amplifier [Q883 –SE117].  This error regulator senses the output voltage of the SMPS, and if any difference comes with 115VDC output, will be controlled.

     So when repair the power supply of this set for blown out STR S6707, all the above stated components must be checked.  If any one among them is defective, replaced STR will blow out.