I thought it would be different to post a complete case i just put on my bench a 8p4834 big screen tv. The complaint by the customer is dead, to me this does not resolve to one simple solution, that most people posting would want, this set will require some troubleshooting. This set will be very easy for me to diagnose, i know this tv very well, so the troubleshooting phase should go quickly to determine the cause i will post a step by step diagnosis of this tv, because i know this chassis well, i will make many assumptions based on my experience so it may look like i am missing a step but i will explain if i catch myself.
After taking the back off, i am going to plug in the tv, and check the stand-by power supply to see if it is functioning.
j403 pin 7 has the standby 5 volts so i know the inital voltage is present, i will now try to turn on the tv and monitor the power on command and see if it trys to turn on the tv
j403 pin 3 with tv off has 5 volts when you press the power button it drops to zero volts, this is correct, so the micoprocessor is sending the start command, now i need to see if the power supply is accepting the on command.
i checked j468 pin 5 for 32 volts comming from the main power supply. It was not present, so i know the main power supply is not turning on even after a successfull on command was sent.
now this is where my experience with this tv kicks in, i know if i see this problem and the test points i just tested, i know the "on" command is not being transmitted through the opto-coupler ic to the hot side controler is the prime target so i will change this without any more troublshooting to possiablly save some time.
Ok, that did NOT fix this tv, i am now going to trouble-shoot the power-on circuit to see exactly why this set did not turn on, since the quick fix did not work more time will now need to be dedicated to this repair. I will consult the schematic to work out my game plan
the power on command is processed by ic402 i checked pin 16 to see if the power on command actualy got to this ic, and yes it did, power on command will show about .6 volts
The power supply for this ic comes in on pin 15 at 13 volts and another power supply comes in on pin 10 (vcc) at about 9 volts, all of these voltages are from the stand-by power supply, and they check good
now i will see if the optocoupler ic405 led is emitting, by checking for approx 1.2 volt drop across teh anode and cathode of the diode which is pin 3 and 2, -- after checking the voltage drop it was only .4 volts, this indicates that the led is not lit trying to send info over to the photo transistor. IC402 the cold side controler also controls a shutdown opto coupler, ic406. i wanted to see if this led was lit, so i check the voltage drop across pin 3 and 2 of this ic, looking for a voltage above 1.2 volts --- yes i found a voltage it was 1.4 volts, so i know this led is lit, signaling a shut-down situation
ran out of time for today, i will continue on this monday.