Lg 42PJ350 no Picture

[happyade123]

Hoping some of the brains trust in the group can help me figure out which board is the culprit. I am tossing between the Control Board and the Ysus Main Board.Tv appears to turn on with two relay clicks but no picture. I have no input to test sound. All voltages Va and Vs and Vzb are correct but -VY shows as +33v instead of -195 and Vsc only shows 20v instead of 145v.
I have removed the Y sus Main and checked all IGBTs for shorts and there is none. All fuses are ok on the Y main as are most of the resistors and diodes. I am thinking maybe the control board is not sending the logic signal to drive the dc to dc convertor of the Y main, but I am only clutching at straws.
I have also disconnected the Y drivers completely and the voltage still does not increase.

[happyade123]

Re: Lg 42PJ350 no Picture

Does anyone have any ideas?

[tom66]

Re: Lg 42PJ350 no Picture

Is there any LED lit on the control board?

Do Vsc and -Vy return to the correct voltage if the Y buffer is disconnected from the Y-sustain. You may leave it connected to the plasma panel itself. You will need to remove the screws carrying the drive current and the small white ribbon cable, it may be helpful to lift the board out, flip it over and tape it securely to the front of the TV.

[happyade123]

Re: Lg 42PJ350 no Picture

No Tom. The voltages Vsc and -Vy do not return with the Y Buffer disconnected. The light on the control board flashes but very erratically.

[tom66]

Re: Lg 42PJ350 no Picture

If the light on the control board flashes erratically I would try removing all the connections to it besides the main board connection and the power supply connection (if present). Also disconnect power to the Y- and Z- sustains. Then see if the LED behaves differently.

[happyade123]

Re: Lg 42PJ350 no Picture

Hi Tom. Thanks for taking the time to help.The Light does not come on at all if I remove the power to the Y sus and the Z Sustain, and the Logic Ribbon as well as the two ribbons to the X Buffers( I think that's what they are). The only time I get life out of the Led but it flashes erratically is with the Y sus power plugged in.

[tom66]

Re: Lg 42PJ350 no Picture

OK on closer inspection I can see that it is likely that the Y-sustain powers the logic board. There is probably a 15V or 17V supply from the power supply that powers the control board. This may pass through the Y-sustain directly, or it may be converted/regulated on this board.

Can you see a "pin map" next to the ribbon cable that goes between the Y-sustain and logic board? There should be some things like SUS_UP, GND, ER_UP, marked on it.

[happyade123]

Re: Lg 42PJ350 no Picture

HI again Tom. Pls check included pictures for pin out etc on Logic Control Board.

[tom66]

Re: Lg 42PJ350 no Picture

OK, can you please check the +15V and +5V pins? Be careful not to short them together.
Check them with all cables connected up as normal.

[happyade123]

Re: Lg 42PJ350 no Picture

Hi Tom.15 volts all measuring but high at 20 volts , suspect my metre might be lieing. All 5 volts also measuring . Does this help. Led on logic board only has 2.5 volts on one side when triggered.
Tried it with another metre same high reading Tom.

[tom66]

Re: Lg 42PJ350 no Picture

Since the supply voltages are there (they may be high without load - that could be normal), I would suggest replacing the logic board now. The logic boards are known to go bad on these sets. Hopefully it fixes the issue.

[happyade123]

Re: Lg 42PJ350 no Picture

Thanks Tom will order one now. There are a few over your way but none over her in Oz lol. Thanks again Tom. Much appreciated.

[ajshoe]

Re: Lg 42PJ350 no Picture

have exact same set - same symptoms. set comes on, all voltages are present, erratic green led on control bd, no display or sound. logic bd fix this ?

[freakaftr8]

Re: Lg 42PJ350 no Picture

Disconnect lvds from main board and enable test patterns on logic.

[ajshoe]

Re: Lg 42PJ350 no Picture


Vs=205v, Va=60.1v, 15V=20v, Vcsc=17-20v, Vy=20.1v. disconnect lvds conn, jumpered EXT AUTO GEN with small wire. same symptom - no display. logic bd green led erratic

[ajshoe]

Re: Lg 42PJ350 no Picture

set on - all connectors connected green led on logic bd flashes erratic
on Ysus bd - Vs =204.5v, Va =60.1v, Vy=20.4v, Vsc=21.3v on zsus bd Vze=0v.

with buffer bd disconnected & set on - all connectors connected, green led flashes erratic - on Ysus bd - Vs=204.5v, Va=60.1v, Vy=20.4v, Vsc=20.7v on Zsus bd Vze=0v.

I checked buffer bd - good.
checked all IGBTs on Y & Z bds - good.
assume logic bd is bad ??

[ajshoe]

Re: Lg 42PJ350 no Picture

after testing some more - checked on logic bd - 5v = 5, 15v = 4.8v. is the 15v generated by the Ysus board ??

[tom66]

Re: Lg 42PJ350 no Picture

It depends, but I seem to recall that it is on this set. Check the pin map on the Y-sustain, to see what voltages come/go to the Y-sustain, then trace the 15/17V on there and see if it comes from the PSU connector or from somewhere else on the Y-sustain.

[ajshoe]

Re: Lg 42PJ350 no Picture

don't believe 15v comes from PSU. think 15v is generated on Ysus bd.
pic 1 is logic bd pin out- conn comes from Ysus bd. pic 2 is the connector from PSU. pic 3 is conn to Xsus bd

[ajshoe]

Re: Lg 42PJ350 no Picture

on logic bd - 15v to grd shows 1.8K ohms I'm thinking that's too low - time for a new logic bd