SONY TRINITRTON WEGA - [KV-TG21L70 _ KV-TG21M80 _ KV-TG21P55] -fFAILS TO SWITCH ON EVEN WITH REPEATED ATTEMPTS.
Set fails to switch ON even after repeated switch OFF and switch ON; by its front panel power ON/OFF button, or by power ON button on remote control.
There are two possiblilites for this fault
The first is with failure of 9VDC regulator [IC604 - PQ 09RD11]; or loose solder terminals to this IC.
Check wether the standby light comes On, at least for a moment, when you switch ON the TV by its front panel power switch. If yes, make sure that 5VDC and 9VDC supply rails are OK. If not, check the voltage at the cathode of [D609] at SMPS power supply section. If there is voltage at this point, check the voltage at pin number [1] of regulator IC stated above. If there is no voltage found, check and repalce [R614 & R625]. If there is voltage at pin number [1] of the regulator IC, and there is no voltage at pin [3] of that IC, check the voltage at pin [4] (control). This pin is connected by a 1K resistor to pin number [3] of main system IC (Standby). If there is no voltage at the output to this 9V regulator IC, means this is a bad one. It must get a control voltage from main system control IC pin [3]; approximately 4.5VDC, to switch it On and to supply the 9VDC output. So before repalce it, make sure that this control voltage is there or not.
If not; is the second case.
Check the voltage at pin number 64 of main system IC. It should be 5VDC. If this voltage is absent, Check the voltages at the reset cum regulator IC [MM 1319AFBE]. The pin voltages to this IC are;
Pin number 1 = 0V [Ground]
Pin number 2 = 4.6VDC [Reset voltage]
Pin number 3 = 4.6VDC [Reference voltage]
Pin number 4 = 11VDC [Supply in]
Pin number 5 = 5VDC [Output voltage from IC to supply main system control IC]
Pin number 6 = 0V [Ground]
Pin number 7 = free
Pin number 8 = Free.
Check the voltages to these pins, especially to pins 4&5. If there is no voltage at pin-4 check the supply rail from SMPS for any break. If there is voltage at this pin-4 and not voltage at pin 5 (5VDC out); make sure that this IC is faulty. Replace it with same number.
Check the voltage at pin number 2 of this IC. It is connected with pin number 36 of main system control IC via a 1k resistor. The pin number 36 of main system control IC is the reset pin [Strobe]. If this pin do not get a voltage, the microcomputer IC [system control IC] will not start up, and this is the fault to the set
Repalcing the the RESET IC stated above will solve the problem.
In my service experience, I've replaced several of this reset cum regualtor ICs, for this fault.
Be very carefull to take voltages to both main system control and rest ICs. Never make a short circuit with meter probes with adjecent close soder terminals. It may migh make main system control too to get damaged.