LG 32LF20FR no 12v or 24v on psu!

[MARHD]

Hi guys im trying to repair my Tv.

Initially there was no standby power at all, but i probed around and replaced the recommended components from this thread https://www.badcaps.net/forum/showthread.php?p=837468. Now I have 5v(the red power led shows up on the face of the tv) but 12 and 24 are still missing in action.

(I have attached the schematic) some more probing showed that zd503 has no continuity either way, and R258, R251, R252 are showing unsteady values or are simply way off. I have yet to replace them. I have checked the rest of the through hole diodes, resistors and inductors and all look good. I basically haven't touched anything with more than two pins tho. And i dont have a meter capable of measuring capacitance for the caps, but none of them are leaky or bulged. Fuses look good as well.

Im not sure what to do now or where to look so any help is greatly appreciated

[attainteddragon]

Re: LG 32LF20FR no 12v or 24v on psu!

on p201 (connector to main), when you press the power button, what voltage do you get on the bottom right pin (power on)?

[R_J]

Re: LG 32LF20FR no 12v or 24v on psu!

If you want to check the power supply, disconnect the cable going to the main board and place a jumper wire between Standby +5 volts and power on (pin2 of P201)
That should give you +12 & +24 volts, if not the power supply still has a problem, If you do get +12/24 then the fault usually is with the main board.
I,m not sure about zd503, but you could measure the voltage across it, This is the HOT side so don't use the matal chassis as ground for your meter. Use the negative of C607/608. If you get 16 volts the zener is ok, if not check the voltage on Q501 collector. (using hot ground)

[budm]

Re: LG 32LF20FR no 12v or 24v on psu!

Do you have about 400VDC measured right between the two legs of the main filter cap in the hot side when PS-ON is present?

[MARHD]

Re: LG 32LF20FR no 12v or 24v on psu!

Ok so I've done as you guys instructed, and jumpered the 5v to power on, now im getting 10v on the 12v pin which im guessing is still an ok voltage, but nothing on 24v pin. I measured the power on voltage and its 0. I think the zener zd503 is dead im only getting like 9v grounding to the top half of the board(there is a black seperator line indicating the halves). And im getting like 340v across the main filter caps c607 and c608

[MARHD]

Re: LG 32LF20FR no 12v or 24v on psu!

Update: I have tested bd101, D251, D252 and D255 out of the circuit and they seem good. In circuit the three diodes(excluding bd101) have continuity in both directions

[dick_barton]

Re: LG 32LF20FR no 12v or 24v on psu!

BD501 is probably a ferrite bead. Can you post a picture of it.

I think the PFC voltage is low and should be closer to 380 -390V

Again with the mainboard disconnected from the power supply and the 1K resistor still connected can you measure the voltage at the three points C,B & E of Q501.
Black meter probe lead connected to the "-" of C607.

[budm]

Re: LG 32LF20FR no 12v or 24v on psu!

1) Are you 100%sure that you are not getting any Voltage at the 24V section? That 24V is generated by the same transformer that generates the 12V.
2) 340V reading with forced on means the PFC Voltage booster is not working correctly, the Voltage on the main caps should be closed to 400VDC.

[dick_barton]

Re: LG 32LF20FR no 12v or 24v on psu!

Can you post a picture of BD101. It may be a ferrite bead.

The PFC voltage may be low and I would have thought it to be closer to 380 - 390V

With the power supply disconnected from the main board and the 1K resistor still connected can you measure the voltage at "C" of Q501. The black meter lead connected to the "-" of C607. It should be approximately 16V DC.

[R_J]

Re: LG 32LF20FR no 12v or 24v on psu!

If you are getting 10-12 volts on the 12 volt line and 0 on the 24 volt line,
Check the connections on the transformer where the red lines are marked, then check the resistance of the transformer winding across the 2 red lines, it should be around 1 Ω or less.
Check the connections going to the diodes
And last check the resistance across the 24 volt line to ground, it should be very high resistance and not a short

[vinceroger69]

Re: LG 32LF20FR no 12v or 24v on psu!

have you got a different multi meter you can double check your voltages with

[MARHD]

Re: LG 32LF20FR no 12v or 24v on psu!

Ok guys I was being really stupid and when I was connecting the 5v to power on, i was basically just shorting them together for 1 second expecting it to act pike a power switch. Now after soldering the 1k from 5v to power on im getting the full 12v and 24v, the pins of Q501 were all roughly 13+ volts which is only because(im assuming) zd503 is still bad and i havent had a chance to go and buy a replacement. C608 and C607 both now have a voltage of 387 across them. The resistance of the transformer windings were between 0.3 ohm and 0.5 ohm depending on where O connected my leads. And I have attached a picture of bd101 for you. Is it fine if those readings were taken with the board completely unscrewed and removed from the chasis grounding?

[dick_barton]

Re: LG 32LF20FR no 12v or 24v on psu!

It looks as though there's nothing wrong with your power supply since you now have your secondary supplies and the PFC voltage across C607/8 is fine.
If you now connect the power supply to the mainboard and monitor the Power_On pin to check if it changes when you attempt to come out of standby.

[MARHD]

Re: LG 32LF20FR no 12v or 24v on psu!

Ok I will try once I have replaced zd503 just incase it causes any problems

[dick_barton]

Re: LG 32LF20FR no 12v or 24v on psu!

I doubt there's anything wrong with it since you said you have 13V across it.

[budm]

Re: LG 32LF20FR no 12v or 24v on psu!

Power supply is fine, no need to mess with ZD503. Main board is not sending the control signal to turn on the power supply.