LG 50PG20-UA Plasma - ** NO POWER **

[VividElectrode]

TV: 50PG20-UA.AUSLLJR
Power board: EAY41360901

Good morning. I'm looking for a little help in diagnosing (rediagnosing?) this set that I am having trouble with. The problem was no power. No standby light, nothing. Opened it up and found Fuse F801 was blown and Caps C202, C203, C204, C218, and C917 bulging. As well as C201 on the YSUS board. From looking online I found that if fuse F801 continues to blow it is likely that transistors Q801 and Q802 were bad. Just those 2 transistors were about $6 a piece plus shipping from digikey and mouser, so I chose to order the "repair kit" from Shop Jimmy that had all the caps I needed (except the one for the YSUS board, but I had an acceptable replacement in my parts bin) plus the transistors and more.
Here is everything that I replaced using the kit.
Fuses F801 (came with the F101 fuse as well but mine wasnt blown)
Transistors Q601, Q602, Q801, Q802
Caps C202, C203, C204, C207, C218, C917
ICs U101, U801

Also, Thermistors TH101 and TH102 were bad (cracked and missing pieces), so I replaced them as well.

I replaced all of these parts, hooked everything back up, and Fuse F801 blew again immediately upon plugging in the set. I double and triple checked all of my solder points for shorts and don't see any issues there. Obviously there's something else bad but can't for the life of me figure it out. I'm hoping you guys can help me pinpoint this issue. Pics are attached, thank you in advance!!

[Hamie]

Re: LG 50PG20-UA Plasma - ** NO POWER **

Solder in 2 x 100w in series or a 200w service bulb lightbulbs in place of the fuse.

You should always do this when replacing lots of parts as it will prevent damage to your new parts.

Once you have the bulb/s in place, check the voltages across the PFC.

[Hamie]

Re: LG 50PG20-UA Plasma - ** NO POWER **

Have you checked the bridge rectifier, if the thermistor was blown good chance it's shorted.

[VividElectrode]

Re: LG 50PG20-UA Plasma - ** NO POWER **

Solder in 2 x 100w in series or a 200w service bulb lightbulbs in place of the fuse.

You should always do this when replacing lots of parts as it will prevent damage to your new parts.

Once you have the bulb/s in place, check the voltages across the PFC.

Ok I will try this. Any particular type of bulb that you recommend?
And no I have not tested the BR yet, I will do that as well.

[Hamie]

Re: LG 50PG20-UA Plasma - ** NO POWER **

The bridge rectifier should test as 4 diodes.
Any 100w normal bulb, not an energy saver.

[VividElectrode]

Re: LG 50PG20-UA Plasma - ** NO POWER **

ok. I will post results.
thanks

[VividElectrode]

Re: LG 50PG20-UA Plasma - ** NO POWER **

Both bridge rectifiers test ok.

[VividElectrode]

Re: LG 50PG20-UA Plasma - ** NO POWER **

picked up some bulbs. will get them wired in later this morning or early afternoon. Would you minding pointing out which voltages you'd like me to read?

[Hamie]

Re: LG 50PG20-UA Plasma - ** NO POWER **

with the bulbs in place measure the voltage across the PFC capacitor.
then jump the powersupply on by connecting PS-ON to GND and VS-ON to 5VSTBY and again measure the voltage across the PFC.

PFC = Primary Filter Capacitor.

if the PFC doesnt get the correct voltage you know that the problem is on the HOT primary side of the board.

Remember you are dealing with the hot side of the board so take your time.

[VividElectrode]

Re: LG 50PG20-UA Plasma - ** NO POWER **

Ok thank you. I have the bulbs (2x100W wired in series) soldered in in place of the fuse. When I plug it in the bulbs come on bright and stay on, which to me means theres obviously a short since the bulbs are having to draw the power and no the board itself, am I correct in that thinking? Also, I have all of the other boards disconnected at the moment as well.

now, as far as what you've asked of me. I need a little clarification. Can you point out which of the caps is the PFC on this particular power board?
Also, I see a VS-On, and of course ground, but don't see anything specifically labeled PS-On or 5VSTBY or anything that is similar enough for me to be certain is the correct one. Here are pictures of all of the connectors on the board. Which ones should I be looking at for jumping.

Another thing. I went through and tried to retest all of the diodes and resistors on the board, and I *think* I may have found 2 shorted diodes. They are reading 001ohm both directions. Whereas all the others read a higher resistance one way and inf. the other. Am I right to assume this means these two diodes are shorted, or do I need to pull them from the board to accurately test them.

Thanks!

[VividElectrode]

Re: LG 50PG20-UA Plasma - ** NO POWER **

Oops. Forgot the pics. Here we go.

[newtothis]

Re: LG 50PG20-UA Plasma - ** NO POWER **

PFC caps are the ones with the black tops rather than silver, Just need to test across a single one, Red to + and black to -

any 5v to VS_ON will do

Not sure which is the power on for the PSU though

*Edit* yes the bulbs glowing brightly indicates a short

[Hamie]

Re: LG 50PG20-UA Plasma - ** NO POWER **

Firstly check the voltage on the PFC caps.

If the diodes are in parallel with (for example) a low ohm resistor they will test short.
So you'd have to remove one leg from the circuit.

[VividElectrode]

Re: LG 50PG20-UA Plasma - ** NO POWER **

newtothis, thank you for IDing the PFC caps for me.

Hamie, pulled the diodes to test while I was awaiting a reply. They test fine outside of the circuit. so nevermind those. No as far as jumping to power supply, newtothis said for the VS_ON I can just run any, 5V. easy enough. No for grounding the PS_ON, I need to figure out which connector. I uploaded pics of the connectors a couple posts back.

[paulstef]

Re: LG 50PG20-UA Plasma - ** NO POWER **

I would first check all bigger semiconductors for shorts. FETs, diodes etc. The ones mounted on heatsinks.

Use the diode tester of your multimeter. FETs have a bodydiode from drain to source you should be able to measure with the diode tester too.

[VividElectrode]

Re: LG 50PG20-UA Plasma - ** NO POWER **

Thanks paulstef. I tested both of the bridge rectifiers earlier and those checked out ok. And I replaced Q601, 602, 801, and 802 as part of a repair kit I installed. Same story with the IC on the same heatsink as Q801,802. I have not however tested any of the to-220 FETs yet. I always seem to get strange results when attempting to test FETs, so I will attribute that to me doing it incorrectly. Any advise for me there?

[paulstef]

Re: LG 50PG20-UA Plasma - ** NO POWER **

Thanks paulstef. I tested both of the bridge rectifiers earlier and those checked out ok. And I replaced Q601, 602, 801, and 802 as part of a repair kit I installed. Same story with the IC on the same heatsink as Q801,802. I have not however tested any of the to-220 FETs yet. I always seem to get strange results when attempting to test FETs, so I will attribute that to me doing it incorrectly. Any advise for me there?

Normally the pins on TO220 FETs are from left to right: gate, drain, source,

There is a diode to be measured from source to drain with your diode tester (most are normal N-channel FETs)

Gate to source should have a few kOhm or higher, often there is a resistor mounted from gate to source...

Btw, here is the training manual with some additional info (not that it helps much): http://elektrotanya.com/lg_50pg20_tr.../download.html

[VividElectrode]

Re: LG 50PG20-UA Plasma - ** NO POWER **

Thank you. I will get those tested while I'm waiting to figure out which pin is PS_ON, unless of course I can find it in that manual, that is.

[paulstef]

Re: LG 50PG20-UA Plasma - ** NO POWER **

If the light bulbs are fully lit it means there is quite some current flowing through it. If there was a high current through a small component there would be smoke.

That's why I would check first ALL heatsink mounted bigger semiconductors.

I don''t see why you would need to worry right now about PS_ON.

[VividElectrode]

Re: LG 50PG20-UA Plasma - ** NO POWER **

yea that definitely makes sense. because they are certainly fully lit.