LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

[jason123]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

What voltage do you have across C5? is it also fluctuating in standby mode?
What is the main filter (C2/C3) voltage? does the pfc turn off in standby and back on when powered on?

Yes C5 fluctuates between 14 and 20V
C2/C3 have some ripple 392 to 406v, but PFC is on. I dont think it turns off on the model

[nomoresonys]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

ok, forgot you had one working, that makes it easier, check each pin on those ic's in question with the good board, take note, then check them with original board, also check the transistors on the heatsinks and the diodes and resistors, check your voltages on the transformers too.

[nomoresonys]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

if all that checks out, what's left? down to caps or optos?

[nomoresonys]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

maybe check Q8, Q10, Q407, compare to good board.

[nomoresonys]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

and those 2 transistors by c57 and d24, d25.

[nomoresonys]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

look out for bad solder joints.

[nomoresonys]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

Q5. Q404. Q405. F3.

[jason123]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

Q5. Q404. Q405. F3.

These check out.

The only big difference I see between the boards is the orange Capacitor section (LED Drive Circuit) - C403, C404, C405

Good board reads 0.6 MOhm across these orange caps
Problematic board, reads 4.1 MOhm across these caps

Opto (and zener dioded - circled) Hot Side:
Good board 23.9 k
Bad board 2.4k

[nomoresonys]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

"Opto (and zener dioded - circled) Hot Side:
Good board 23.9 k
Bad board 2.4k" That looks like a pretty big difference.

[nomoresonys]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

I might be tempted to change all 5 if I had them on hand.

[jason123]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

"Opto (and zener dioded - circled) Hot Side:
Good board 23.9 k
Bad board 2.4k" That looks like a pretty big difference.

Yes.. factor of 10
Though I find it odd that there seems to be 2 failures.. one on the hot side (somewhat isolated?) and one on the cold side

[nomoresonys]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

maybe switch out c404.

[nomoresonys]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

do you have about 1 ohm across the opto?

[jason123]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

do you have about 1 ohm across the opto?

cold side about 1k on both boards
hot side 23k on good board, and 2.3k on the bad board

[nomoresonys]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

what are you getting on that zener in diode mode?

[jason123]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

what are you getting on that zener in diode mode?

0.7 VF... tests as good. Out of circuit (when diode removed) hot side of opto now measures 23k (same as good board)

[nomoresonys]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

hmm, seems ok, wonder if it's those caps to blame?

[jason123]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

So transfered the good zener from good board to bad. Now bad board reads 23k across hot opto side. TV powered up for 3 seconds then off. Something else is a problem.

Disconnected LED Connector (to exclude these caps) and no change. I don't think these caps are to blame. Took them off voard, and did ESR and C measurements. No significant difference

[nomoresonys]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

ok, yeah the ceramics are usually pretty reliable, wonder if it's the opto?

[nomoresonys]

Re: LG 50UH5530 Turns Off And Wont Turn On: diagnoses included

so you tested the pins of those 2 ic's to make sure both boards give the same readings? and tested all the transistors including the little ones standing by themselves?