LG 32LD450 power LED flashes red, no picture, no sound

[DakotaKid]

Just picked this set up today. When it is plugged in the LED power light is steady red. I think it is supposed to turn blue or green when it is powered on, however, it pulsates red when I power it on. There is no picture, no sound, no video. Flashlight test shows no video as far as I can tell. No clicks or relays sounds when powering on or off.

I cannot locate a FSM for this set but power supply is suspect. 3.3V is present at the pins on plug at upper right, including one that is labeled "PWR ON". No voltage at 12V or 24V pins and no voltage to inverter board (no voltage on 24V output pins at plug on upper left of picture which runs to inverter). Fuses on power supply test good at 0 ohms. Voltage readings on power supply at two large 450V caps shows 70 VDC on positive leg of both caps, also 70 VDC at the diode just below them in the picture. According to the 32LD350 interconnect pdf attached I think this should be 388 VDC. No visible damage to any cap on the boards.

What should I test or look for to find the problem with my 12V and 24V outputs? Should I test the Rectifier/Diodes/FETs for shorts first before doing anything else? Anything else I should check now?

edit: good news is that it looks like there are common parts with LG 42LD350-UA, found schematics at
edit2: found a great interconnect diagram that I think will be helpful as well

Thanks.
DK

[DakotaKid]

Re: LG 32LD450 power LED flashes red, no picture, no sound

Here is what i have so far. After finding a correct ground, the voltage at the bridge rectifier and the large caps is 157VDC, not 70VDC as I said before. The rectifier (RS405M) reads as follows:
no power applied-
+ to EL13 = 21.4 ohms
+ to EL11 = 20.6 ohms
EL11 to EL13 = infinity
+ to - = 17.5 ohms

with power applied:
- to + = 157VDC
- to EL11 = 56VDC
- to EL13 = 55VDC

Thru diode D801 = 157VDC

Q101 and Q102 are Toshiba TK6A60D FETs.
With no power the resistance is as follows:
Q101: G-D=800Kohm, G-S=7Kohm, D-S=750Kohm
Q102: G-D=820Kohm, G-S=8.3Kohm, D-S=860Kohm

When power supply is on, VDC at those FETs is:
Q101: G=3.3mV, D=157VDC, S=3mV
Q102: G=0, D= 3mV, S=0
Q101 source and Q102 drain are connected on same circuit trace.

So, I think I have found the voltage dead end but I am not knowledgeable enough to understand why.

Help anyone?

[budm]

Re: LG 32LD450 power LED flashes red, no picture, no sound

So you need to put on a jumper on P201 as instructed, then you will use the primary ground (the negative leg of the 450V caps, the two caps are connected in parallel), this will be your ground ref point for any voltage measurement in the primary side of the circuit, the secondary side (cold side) ground will be the chassis ground. This will be applied to any TV/ MONITOR you will work on.
When you jump the connector as instructed, the voltage on the 450v should go up to 380~400VDC due to PFC Voltage booster kicks on, this has to work first before looking any where else.
The bad part is that there is no diagram of the power supply.

[DakotaKid]

Re: LG 32LD450 power LED flashes red, no picture, no sound

Thanks Bud. I actually thought I HAD jumped the PS to turn it on but after your response I thought I'd better double check. As it turns out, my jumper wasn't contacting the PWR_ON pin so I wasn't jumpered after all. The good news is that when I did jumper it the voltage came right up to 388VDC at the caps after the "booster".
Q101: G=199V, D=377V, S=194V
Q102: G=50V, D=Q101s=194V, S=0V (gnd)

I now have 12V and 24V output when the power supply is in this state. So, does this mean I have a problem on the mainboard getting a signal to the PS to turn it on? Should I be looking for a short or similar problem over on the mainboard?

Thanks again for the help. It is much appreciated.

[budm]

Re: LG 32LD450 power LED flashes red, no picture, no sound

The problem is on the main board, it is the one that sends out>3vdc to that PWR ON pin which I am a little confused since in post one you state that you do have 3.3v at the PWR-ON pin. If you hook up everything back to normal, then check other signals that are generated by the main board, like Invon pin which should be around 3v also. If you do not have that either, then you have more than one bad control lines which can be due to bad LDO regulators, or blown fuses if any on the main board. The bottom line will be lots of troubleshooting of the main board will be required.

[DakotaKid]

Re: LG 32LD450 power LED flashes red, no picture, no sound

I think I have cleared up the confusion on the PWR ON pin and have fixed the TV. When I had the PS jumpered and was doing additional testing late last night I saw the voltages dropping off and coming back as the board shifted around. Turns out pin 1 on that connector, the PWR ON pin, was loose and making intermittent contact. So, when I thought I had it jumpered earlier last night, it probably wasn't making contact then later it did. And before that when I measured PWR ON at the cable coming from the mainboard I probably did have signal but it wasn't making it to the PS board due to the loose pin. Anyway, I resoldered pin 1 on the PS board, put the boards back in and it fired right up, inverters and picture and sound and everything. The standby light went from red to blue instead of red to pulsating red like it had before. I think someone had taken the plugs on and off of the PS and mainboard enough that they caused this stress on the pins from being none too careful. I have the TV on now and will let it run all day to make sure it's really fixed.

budm...thanks again for the help and insight.

[budm]

Re: LG 32LD450 power LED flashes red, no picture, no sound

Good to have easy fix.

[budm]

Re: LG 32LD450 power LED flashes red, no picture, no sound

Please block out this a-hole clibin009