lg 42pt85 no power standby lights up

[wreksta]

Re: lg 42pt85 no power standby lights up

tom how can I fault find this board heres some pictures of it

this is one showing burn mark on pads



this one was burned until I cleaned it



this big coil acts as a resistor doesn't it what could have caused it to go bad
or could cold solder joints do it I might try fix this

[wreksta]

Re: lg 42pt85 no power standby lights up

any ideas ?

don't think this is broke tbh could be age that's blackened it as I know its a big resistor sort of thing so all 8 pins are connected or should they not be
I checked the ipm on the x & y board and nothing shorted there nor is there any blown fuses anywheres

[tom66]

Re: lg 42pt85 no power standby lights up

That is an inductor. It should test as shorted.
The marks are flux.
If you have a short between Vs and GND, the only real cause can be the IPM.

[wreksta]

Re: lg 42pt85 no power standby lights up

that's just it though I have no short mate only thing is there is a resistor on y main has no continuity don't know if its supposed to the rest of the resistors all test ok except this one from the underside of the board it has the the symbol like a zig zag between each leg as do the working ones so may be that or a mosfet nr there

the blackened part of that coils was burned though proper ashy sooty until I cleaned it up also the heat pad it sits on had the same burn out mark should this be like that

[tom66]

Re: lg 42pt85 no power standby lights up

Resistors shouldn't necessarily have continuity. Their value should test according to the colour code printed on them. Resistors -rarely- fail without obvious burn marks.

[wreksta]

Re: lg 42pt85 no power standby lights up

Tom just throwing this out here if the control board was bad the main tcon/ldvs coud that give these symtoms as everything checks out between the x and y ast thng is the buffers but they have heatsings and sicone covering them i thnk i may take them out and test for shorts

i think i asked this before but i can discconnect buffer from y and try powering up never tried that

all i knmow is the x is linked through y and if you disconnect y it fires up and stays on

[tom66]

Re: lg 42pt85 no power standby lights up

You can power it up without a buffer, but you will not get any image on the panel.
I don't think these older sets control boards detect errors in sustains, they will continue sending signals even if the Y-sus/Z-sus are bad.
The Z-sus shares an energy recovery circuit with Y-sus that's what the large inductor is for. (Inductor-Capacitor Resonant Energy Recovery.)

[wreksta]

Re: lg 42pt85 no power standby lights up

But it will stay on ?

What about my theory on control board or even main av board

[tom66]

Re: lg 42pt85 no power standby lights up

See edit.

[wreksta]

Re: lg 42pt85 no power standby lights up

So the fault falls on either the y x or buffers then ?

[tom66]

Re: lg 42pt85 no power standby lights up

Well as PSU does not start up with Y-sustain, then it's with the Y-sustain (I presume you have isolated Z-sustain from Y-sustain board first and the same symptoms were observed.)

A shorted buffer would NOT cause the Y-sustain to pull down Vs. In rare cases (never heard of it on an LG) a shorted buffer can damage the Y-sustain causing a similar fault. I would not be expecting this on this model.

Due to how the energy recovery circuit chains between boards it may be a good idea to replace both Y- and Z-sustain at the same time with known good boards.

[wreksta]

Re: lg 42pt85 no power standby lights up

Okies mate well il do some tests etc see where it goes frm there thanks tom