LG Power supply board has no standby voltage.

[br777]

Hi guys,

I have an LG Power board EAX64905601 from a 55LA6230 LCD.
Originally this had a backlight issue, where it would flash on for a second then no backlight.
From a fair bit of research I found that it was likely to be the LED array that was the issue. (LED + voltages fine, no LED - voltage)
I checked Connector P201 and all voltages were normal there, so I thought power supply was ok.

After I stripped the TV down (took power supply board off to lay it face up to get LCD panel off) and did some investigating on the LEDs I wanted to see if they would light up, so I stood the TV up on its stand and re-installed the power supply board and connected it to the main board. (TCON not connected - no screen still). At this point there was nothing at all after turning it on.
I checked the LED pins again - 0v. I checked P201, and there is no voltage on any pins there, including the 3.5V.

Any suggestions on where to look next would be much appreciated. Perhaps I knocked something while taking it off?

Thanks
(there is a burn near F100, it looks as if this fuse has been replaced and that is from it being soldered - I got this TV used with a backlight issue, clearly it has had a problem before)

[dick_barton]

Re: LG Power supply board has no standby voltage.

The fuse F100 has been replaced and not soldered in very well. Have you check with your meter that it is not open circuit? Set off, meter set to ohm's and should read 0 with probes either side of it.
I see there's a second fuse solder onto the board so do the same test on that too.

If they both check out OK then with the set on, carefully measure the DC voltage across C610 with your meter probes on each leg. Capacitor is rated at 450V so measured voltage should be around 400V

[br777]

Re: LG Power supply board has no standby voltage.

Hi, thanks for your input.

I have re-soldered the fuse F100, and checked for continuity whilst in the circuit. It seems to be fine. With power on, there is no voltage either side of this fuse though, perhaps it will ramp up when the set gets switched on? (When i can somehow get voltage to the main board)

The cap C610 has 364v across it

Fuse F101 is testing ok, and has 122v either side of it.

I have seen other troubleshooting posts suggesting have s look at D501. With the black probe on the nuetral pin and the other probe on D501 it gets 124v at both the anode and cathode.

Most places on the primary side have over 100v. Nothing at all anywhere on secondary side.

I shall keep probing...

Just re-tested F100 with DMM set on AC volts. 283V .

[br777]

Re: LG Power supply board has no standby voltage.

I have tested the diodes with the multi meter. D255 and 256 seemed to have current both ways, but when snipped on one side out of the circuit they were fine.

[dick_barton]

Re: LG Power supply board has no standby voltage.

I believe that the mains voltage in New Zealand is 230V ac? and if so the voltage across the capacitor in standby should be around 325V and the boost voltage out of standby close to 400V.

Was the 364V the out of standby voltage and if so it looks as though it's low.

[budm]

Re: LG Power supply board has no standby voltage.

So right now you do have dc Voltage on the main cap but you do not have 3.5VDC Standby Voltage? If that is the case then you need to work in the standby power supply section first.

[br777]

Re: LG Power supply board has no standby voltage.

Hi,
Yes the 364 v was from standby state. As far as I can see I can't test any other way as there is no standby voltage to the main board for a power on trigger.
I don't hear any relay click when I plug in the power cord. Does this relay activate transformers to reduce the voltage to the secondary side?

I tested the pins on what I think is a transformer (yellow on right hand side, bridging the primary to secondary to line) and it has around 110v (from memory) on the primary side, but nothing on the secondary side. This is with the black probe on the main earth point near the AC input plug.

Would this be a suitable earth return for the secondary side of the board? Perhaps i will check again with the board mounted to the chassis and using a chassis ground as a common reference.

@Budm.. yes that is correct. I can't seem to get any voltage on the secondary side.

I feel it must be something obvious as it worked fine in powering the set up before I dismantled it to work on the LEDs. After putting the buses back in the tv, nothing. I suspect that I not have fractured a component connection through the board, but can't see anything. I will next try some continuity checks between the component legs and the solder in the other side to be sure.

[budm]

Re: LG Power supply board has no standby voltage.

So the second fuse (F101?), the white body with purple stripes, right next to the main filter cap is OK?

[br777]

Re: LG Power supply board has no standby voltage.

Yes, that has 122 v DC on both sides of it.

[budm]

Re: LG Power supply board has no standby voltage.

Yes, that has 122 v DC on both sides of it.

That cannot be right, one end of that fuses is connected to the positive leg of the main filter cap which you indicated to have 365VDC between the two legs.
More likely you are using the wrong ground reference point for your meter, you are working in the hot side of the circuit so the negative leg of the main cap is the GND ref for the meter.
But the fuse is OK due to the fact that you are getting the same Voltage readings on both ends of the fuse.
You now need to look into the standby power section next.
Need to get the P/N of that SMPS IC501 for the standby power supply, there are 3 small startup/running caps that need to be checked.
Also check 510, R511: they should read < 1 Ohms.

[br777]

Re: LG Power supply board has no standby voltage.

Thanks for your help. I will test those components and see what i find. Meanwhile, here is a photo of IC501, not sure which of the numbers is the actual part number.

[R_J]

Re: LG Power supply board has no standby voltage.

The number to use is ICE3BR4765JZ

[br777]

Re: LG Power supply board has no standby voltage.

Still no luck in fixing this one so far....

I was getting some strange readings from my old DMM for the resistors so bought a new one, hence some of the readings are different to previous posts. I suspect one of the leads was no good on the old one and it affected the readings.

R510 ansd R511 both measure 0.8 ohms.

I have checked all of the diodes with no issues at all. They all have current flow one way, and none the other. The 3 on the secondary side that appeared to have current both ways. I took out of the circuit to test and they are ok.

Both fuses have been checked for continuity, as well as checking voltage on both sides with power applied.

C610 has 318 V across it (new multimeter reads different to previous), as does the fuse F101

All of the capacitors on the Primary side have been taken out of the circuit and tested for capacitance. They are all ok. (perhaps i should have just replaced them given i took them out anyway)

IC501: with power on and using the negative leg of the main cap as a ground ref:
Pin 1 = 0
Pin 2 = 0
Pin 3 = 0
Pin 4 = 0
Pin 5 = 318V
Pin 7 = 1.8v
Pin 8 = GND

From the datasheet for this, it looks like Pin 7 (VCC) should be between 10 and 25V. Would this be where the problem is?

[budm]

Re: LG Power supply board has no standby voltage.

No power applied to the board then check the resistance between VCC pin and the GND pin of that SMPS IC.

[br777]

Re: LG Power supply board has no standby voltage.

I get 345kOhms

the datasheet i was referring to is this one. it is slightly different to the one in your link

Edit

Are you referring to IC101 in your previous post?
IC101 has 18 pins rather than 12 as in the one in your link

[budm]

Re: LG Power supply board has no standby voltage.

12 PINS? There are only 6 pins for that ICE IC501 for the standby power supply.

[br777]

Re: LG Power supply board has no standby voltage.

Hi, I got confused as to what IC you were referring to initially, as the one in the pdf you attached does not correspond to what I have on this board. The pdf I attached in post 15 is the datasheet for IC501. I have checked the resistance between VCC and GND of IC501 and it is 340 kOhms

[budm]

Re: LG Power supply board has no standby voltage.

Sorry for the confusion, the PDF I post does not have the same body package.
But any how, the 340K Ohms is very high so that is not the problem.
At this point if you look at the application circuit diagram, it can be that the IC has bad startup cell to kick start the IC, usually once the IC get the kick start Voltage from the Startup cell, then the running VCC will be generated by the AUX winding of the transformer.
So I would check the filter cap on the VCC pin, the Optpcoupler, the rectifier in the cold side, or it can be bad IC itself.
You can power and test the board by itself without having it connected to the main board.

[br777]

Re: LG Power supply board has no standby voltage.

HI Budm, I have found a circuit schematic that is the same as the power board I have here.
Thanks for that info on how the IC works, I was looking at the circuit and tracing it back through components to try and figure out where the VCC voltage is generated. I will have another closer look at that startup circuit and see how that goes.

[Markus1]

Re: LG Power supply board has no standby voltage.

I will have another closer look at that startup circuit and see how that goes.

Hi br777, anything further on that issue? Any progress?