LG 42LD420 no power

[rtstorm]

Hello, a friend sends me this LG TV that has been thunderstrike. When plugged in TV show red light and on the power press button TV show a blue LED. I have 3.3V and 20V lines coming to the main board but no picture on the TV.

I found that one Zenner diode was shorted (i tested all other diodes) but when pulled out from the board it turn out just fine (no short). Next to the diode is a resistor that shows 0 Ohm but i do not know what value should be that resistor.

Can you please help me to determine the value of the resistor?

Also, I notice that a few Schottky barrier rectifiers (W2206) have shorted pins 1 and 3. (TO220 package). When taken out the board they are ok, no short.

Do you have any clue where should I start? besides that resistor :-)

[nomoresonys]

Re: LG 42LD420 no power

.

[nomoresonys]

Re: LG 42LD420 no power

.

[rtstorm]

Re: LG 42LD420 no power

Hmm now when i look it again it seams to be yellow, orange, silver, gold
And that turn out to be 0.43 Ohm
But id that is true why is zenner diode shorted

[nomoresonys]

Re: LG 42LD420 no power

ok, that makes sense, its ok, leave it in.

[nomoresonys]

Re: LG 42LD420 no power

zener reads short because of other components in the circuit, you know its good because you measured it out of circuit and it was ok right? like about .5xx to .7xx.

[rtstorm]

Re: LG 42LD420 no power

Yea, outside the board it is 0.5xxx

[nomoresonys]

Re: LG 42LD420 no power

yes those 2 are ok.

[nomoresonys]

Re: LG 42LD420 no power

is the light of the optical audio out on mainboard lit red?

[rtstorm]

Re: LG 42LD420 no power

No, but TV respond to remote (on/off)

[nomoresonys]

Re: LG 42LD420 no power

ok, can you VERY CAREFULLY measure the voltage between the legs of the big filter capacitors, only touch them with your meter probes NOT HANDS, might be near 400 volts which can be deadly. mention it for those who may be looking on and might not know.

[rtstorm]

Re: LG 42LD420 no power

Yes, 392V

[nomoresonys]

Re: LG 42LD420 no power

can you get a good clear picture of the map/legend of expected voltages of the connector on powerboard?

[nomoresonys]

Re: LG 42LD420 no power

Yes, 392V

ok, that looks good.

[nomoresonys]

Re: LG 42LD420 no power

I wonder if the strike just messed up the firmware, need to look at the firmware update procedure for this model, some will go straight to it, some need remote or maybe button pushes.

[rtstorm]

Re: LG 42LD420 no power

Here is the pictue, all 3.3V,12V and 20V are present. On the other labelas are like this
Error = 1V
P-DIMM = 1V
A-DIMM = 3.3V
INV ON/OFF = 0V

[nomoresonys]

Re: LG 42LD420 no power

ok, probe inverter on-off, leave probe in place, take note of voltage with tv in standby, then turn on power see if there is any change or if it just stays 0 volts.

[rtstorm]

Re: LG 42LD420 no power

It is the same 0V in stand-by and when turn on. Always 0V

[nomoresonys]

Re: LG 42LD420 no power

ok, that's a problem it should probably be 3 volts or so, so looks like you are not getting the inverter on signal coming from the mainboard.

[nomoresonys]

Re: LG 42LD420 no power

ok, we can try this, unhook cable from powerboard to mainboard, then test the pins on powerboard connector, which pins have voltage and how many volts?