LG 42LG50FD-AD Intermittent NoGo

[budm]

Re: LG 42LG50FD-AD Intermittent NoGo

Pin 9 PWMCNTL is an open Drain, that MOSFET is normally low (on) but goes high if the Output is higher than the set limit and that is sensed by pin 8 HVSEN which it works in conjunction with VSENSE..
I would check Voltage at pin 7 VINAC since those high Ohm resistors in that sensing network tend to drift in values.
Look at page 10 for 'TYPICAL CHARACTERISTICS'
I did not read the whole spec sheet yet.

[Amraks]

Re: LG 42LG50FD-AD Intermittent NoGo

Pin 7(VINAC) = 4.131v
this is 1 volt over characteristics

[Amraks]

Re: LG 42LG50FD-AD Intermittent NoGo

Just noticed pin 7 is connected to pin 8. so basically I have just checked voltages on both lines.

VINAC and HVSEN

[Amraks]

Re: LG 42LG50FD-AD Intermittent NoGo

Found some resistors that look low. 7503(7.5m) they test 1000k lower about 6.5m these lead to pin 1 ucc2806.
Also on that pin 7 circuit of the resistor 5101(5.5k) is showing really low about 3.692k.

[Amraks]

Re: LG 42LG50FD-AD Intermittent NoGo

Probed the VCDA and VCDB 500 - 560 mv my understanding this is meant to be zero only 2 mv

[Amraks]

Re: LG 42LG50FD-AD Intermittent NoGo

VSENSE Voltage = 6.59v
Found some more resistors in that network that are 7503's they are all over the place in measurements

[Amraks]

Re: LG 42LG50FD-AD Intermittent NoGo

My bad the 7503s are 750kohms the calculator i was using wasn't very good. rechecked them with a phone app.
They are all over the shop will I have to pull these out of circuit to test as I don't see them in parallel with anything.

http://www.ebay.com.au/itm/200pcs-RK...YAAOSwLnBX5MJI

[Amraks]

Re: LG 42LG50FD-AD Intermittent NoGo

The 8x 7503 resistors have been replaced now the TV works thanks budm for the help you was right about them resistors.

end up replacing the 7503 resistors with Bourns CHV0805-FX-7503ELF

[budm]

Re: LG 42LG50FD-AD Intermittent NoGo

Glad to hear you got it fix, have to watch out for those resistor in the high Voltage section, they tend to drift off the values and cause problem in sensing circuits.