LG 42lc7d low standy voltage

[topfli8979]

Working on a lg 42lc7d will not power up, no backlights,main board shuts down after 5 seconds. All voltages are good on ps except for the 5v standby is only 3.2. I have checked all caps with esr meter and all are good except for two measure bad in circuit and fine out of circuit. My picture editor is not working right now, the 2 caps showing short are to left of lower heat sink in first picture towards the bottom edge. Im stumped on this one.

[Alastair E]

Re: LG 42lc7d low standy voltage

Whats the 5v with the mainboard disconnected...?

[budm]

Re: LG 42lc7d low standy voltage

"All voltages are good on ps except for the 5v standby is only 3.2." Are you saying that all the output voltages are the same as shown on the pin labels?
Can you list the pin names of the connector, the pictures are not clear enough to see all the labels for the pins?
Are you sure that the Standby power supply is 5VDC not 3.3V (some TV uses 3.3V for standby Voltage)?
If the 5VSTBY is only 3.2V, the main board then cannot function correctly to send the PS-ON to turn on the main power supply section.

[topfli8979]

Re: LG 42lc7d low standy voltage

I'm sorry I read wrong pin. Ps on is supposed to be 3.4 but only reading 2.4 on meter. Main board is confirmed good on another set. Standby voltage is good.

[CapLeaker]

Re: LG 42lc7d low standy voltage

If the main board is good confirmed on a other set and there is 5V STBY voltage on the PSU, then the PSon should be correct too.
On that PSU take some measurements with the pin names, so we can figure out what is going on.

[topfli8979]

Re: LG 42lc7d low standy voltage

ACD 5v
Pwr 1.9v
standby 5.12v
inv cont 3.3v
error 12mv
burst dim 50mv
standby 5.12v
a-dim 63mv
3.4v on 2.4v

[budm]

Re: LG 42lc7d low standy voltage

I believe PWR is the PS-ON.
Are you getting >360VDC on the two legs of the main filter caps? I f you do then PS-ON is OK.
Burst dim and a-dim do not look right either.

[CapLeaker]

Re: LG 42lc7d low standy voltage

maybe time to force it on

[budm]

Re: LG 42lc7d low standy voltage

maybe time to force it on

If there are >360VDC on the main caps then PS-ON is OK but there may be problem with Dimm Control signal.

[topfli8979]

Re: LG 42lc7d low standy voltage

I have 340v at main caps. Why would those two lower caps show short to ground and low esr if power supply is ok?

[budm]

Re: LG 42lc7d low standy voltage

I have 340v at main caps. Why would those two lower caps show short to ground and low esr if power supply is ok?

340VDC is on a low side but at least we know that the PS-ON is OK otherwise the voltage on the cap will stay at 160~170VDC.
Which lower caps?

[topfli8979]

Re: LG 42lc7d low standy voltage

arrow pointing to both caps on lower side of heat sink

[budm]

Re: LG 42lc7d low standy voltage

Those two caps are the filter caps look to be for the 5V standby power supply, if they are shorted out you will not have STBY Voltage.
What kind of resistance, reading do you get measuring right at the two legs of the caps? <1 Ohms?

[topfli8979]

Re: LG 42lc7d low standy voltage

The bigger cap is all over the place up to 2k ohms, smaller one is at 5 ohm.

[budm]

Re: LG 42lc7d low standy voltage

"smaller one is at 5 ohm." Can you please good clear pictures of the bottom side around that area?
Does it still show 5 Ohms if the cable between the power supply board and the main board is removed?

[topfli8979]

Re: LG 42lc7d low standy voltage

Yes 5 ohms unhooked completely out of the tv.

[budm]

Re: LG 42lc7d low standy voltage

How about the pictures of the bottom side of the board so I can see how the caps are connected. I still have a big doubt about that 5 Ohms reading since you are getting 5VSTBY OK at 5V, so @5 Ohms = 1A and about 5Watts of power dissipation, some thing will get quite hot in that area which I do not see any brown spot in that section at all.

[topfli8979]

Re: LG 42lc7d low standy voltage

After I posted that picture I wondered the same thing, my batteries were on there way out on my meter. Im getting 2500 ohms now that meter is working properly. Sorry bout that.

[topfli8979]

Re: LG 42lc7d low standy voltage

Picture

[CapLeaker]

Re: LG 42lc7d low standy voltage

After I posted that picture I wondered the same thing, my batteries were on there way out on my meter. Im getting 2500 ohms now that meter is working properly. Sorry bout that.

Well... that's a better reading from the DMM. Most DMMs don't correctly if the battery is getting low. Looks like the cap is o.k. then. At 5 Ohms, that cap would have been glowing hot and it should have dragged the output voltage down.

Something still doesn't add up.