LG 42lc7d low standy voltage

**newtothis** · 03-05-2015, 07:05 AM

Re: LG 42lc7d low standy voltage

Originally posted by topfli8979

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.

If you hadn't noticed I was going to point this out to you.

**topfli8979** · 03-05-2015, 10:06 AM

Re: LG 42lc7d low standy voltage

I have tried forcing the backlights on by jumpering rl-on to standby 5v, no dice. I did notice right before the main board shuts down, there is a click in the mainboard, up by the connectors. Im sure the click is in the main and not the power supply.

**budm** · 03-05-2015, 10:08 AM

Re: LG 42lc7d low standy voltage

"I have tried forcing the backlights on by jumpering rl-on to standby 5v" What is the 'RL-ON'?
INV CTRL is the BL-ON pin which is OK per post 6:
ACD 5v
Pwr 1.9v
standby 5.12v
inv cont 3.3v = OK
error 12mv
burst dim 50mv
standby 5.12v
a-dim 63mv
3.4v on 2.4v

It s the BURST DIM (PWM DIM), or A-DIM (analog dim) that does not look right which I have suggest you to remove the BURST DIM control line to see what happen.

**topfli8979** · 03-05-2015, 10:26 AM

Re: LG 42lc7d low standy voltage

Rl-on is not for backlights I guess, although I don't know what is it. I was able to jumper 5v standby to 5v inverter on main and did get a flash of screen and saw no signal for a millisecond. Will try removing dim line.

Attached Files

if you find these attachements useful please consider making a small donation to the site

**topfli8979** · 03-05-2015, 10:49 AM

Re: LG 42lc7d low standy voltage

Burst dim line removed, nothing changes still powers off. Error line removed tv stays powered on though.

**budm** · 03-05-2015, 11:10 AM

Re: LG 42lc7d low standy voltage

The 'RL-ON (Relay On = PS-ON))' as labeled on the main board which is connected to the 'PWR' (same as PS-ON/RL-ON pin of the power supply board).

**topfli8979** · 03-05-2015, 11:23 AM

Re: LG 42lc7d low standy voltage

Ok thanks, makes sense. Its the error line that is holding everything back I beleive. Just gotta figure out why.

**CapLeaker** · 03-05-2015, 11:34 AM

Re: LG 42lc7d low standy voltage

the inv - on/off should be the backlight

**budm** · 03-05-2015, 11:35 AM

Re: LG 42lc7d low standy voltage

"error 12mv" I cannot tell if the ERROR signal should be HIGH or LO for normal operation.
So the backlights did flash on and then go black may be due to detection of the fault some where in the inverter circuits or bad lamps/wiring.

**topfli8979** · 03-06-2015, 07:42 PM

Re: LG 42lc7d low standy voltage

Thanks budm, I canput 5v to error line. And tv still stays powered up just no backlights still. Can someone tell me what these ' black 4 leg'components are and how to test them? The 3 jumping from primary to secondary. I'm getting "0L" on two of the three. The middle component is at 25+-ohms.

Attached Files

if you find these attachements useful please consider making a small donation to the site

**budm** · 03-06-2015, 08:13 PM

Re: LG 42lc7d low standy voltage

The 4-pin part is OPTO-ISOLATOR, one side has LED another side has phototransistor which is commonly used in TV SMPS power supply.
They are used for feedback or switch that require to have isolation between the two circuits, in this case its the primary and the secondary side.

**topfli8979** · 03-07-2015, 11:39 PM

Re: LG 42lc7d low standy voltage

Thanks for the info Bud, I was able to test the anode and cathode pins in diode mode and found the bottom opto isolator to be dead. I have replaced it and have all my voltages back now. But still no backlights, that error line is the only one showing millivolts still. So close!

**CapLeaker** · 03-08-2015, 04:42 AM

Re: LG 42lc7d low standy voltage

Measure inv on/off pin, to see if the main board is turning on the backlight first place

**topfli8979** · 03-08-2015, 12:58 PM

Re: LG 42lc7d low standy voltage

Inverter pin is 3.3v.

**budm** · 03-08-2015, 01:18 PM

Re: LG 42lc7d low standy voltage

That Inverter On Voltage is correct as shown in the earlier thread.
So the Burst-Dim and the A-Dim are still in mV range?

**topfli8979** · 03-08-2015, 01:25 PM

Re: LG 42lc7d low standy voltage

After replacing opto-isolator burst dim is 3.4v and a-dim is 3.2v now.

Error line is only one showing mv

**budm** · 03-08-2015, 01:35 PM

Re: LG 42lc7d low standy voltage

OK, that is good. If there is error, I would expect the lamps to flash on and then the detection circuit will detect the fault first and go into protection mode.
So I think there is something wrong in the inverter section that does not even try to light up the lamp firstand then goes into protection mode.

**budm** · 03-08-2015, 01:46 PM

Re: LG 42lc7d low standy voltage

BTW, in the earlier post I was looking for >360VDC on the main cap and you report the reading of 340VDC but then you find out that there is problem with your meter, so what DCV do you get now with the new meter?
I will have to trace out the board to see how the inverter section works.

**topfli8979** · 03-08-2015, 02:10 PM

Re: LG 42lc7d low standy voltage

It is still 340v.

**budm** · 03-08-2015, 04:47 PM

Re: LG 42lc7d low standy voltage

BTW, the fuse for the inverter circuit next to the black main filter caps, it is OK, right? Not sure if it was mention already or not.
340V is to me is on the low side.

LG 42lc7d low standy voltage

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Related Topics