LG Electronics Plasma TV Model DU-42PX12X Chassis #AF044A

[PlainBill]

This is another Craigslist special. Symptoms are simple - hit the power button, the power LED goes green (operating), then a few seconds later drops back to red (standby).

I've got both the service manual for this specific model and a LG Electronics Plasma TV training manual that covers several models (but not this one). Both manuals appear to have been translated from Korean into English using a very inferior version of Bablefish. What is worse, the Service Manual seems to deal mainly with the small signal boards and only briefly mentions how to adjust voltages on the power supply for the particular panel. The only information given for the display control, driver, y-sus and z-sus boards is part numbers.

The Training manual DOES have some information. It gives a rudimentary troubleshooting tree, one part of which is very illuminating

PROTECT operation;
When the load voltage is short.
When each voltage doesn't work
(in general)

The troubleshooting tree includes a number of steps like this

P301 Connector Open Check.. X -Board Top Right Change..

The only sense I can make of that is it's telling the technician to disconnect the P301 connector, check if the power supply still goes into protection, and if it does not, replace the Top-right X driver board. After a series of these steps, it says 'replace power supply', which does make a little bit of sense. The only problem is that a constant refrain from my training was to never power up a piece of equipment with a board hooked to the other circuitry, but disconnected from the power supply. (I'd prefer not to see a curl of smoke rising from this thing).

PlainBill

[PlainBill]

Re: LG Electronics Plasma TV Model DU-42PX12X Chassis #AF044A

Sometimes I have to talk (or type) to someone to get my thoughts in order. After posting the above I tried a logical approach.

First step: Check the outputs of the power supply with an ohmmeter. Result: no shorts.

Second step: Check each power supply output while hitting the 'Power' button. There was no response from any of the supplies going to the various panel driver boards.

Third step: Check the links between power supply and digital board. This was more instructive. There are two of them, the Control and standby connector, and the power connector. (These are inaccessible at the power supply end, so I had not checked them before). On the power supply connector, the +3.3V supply appeared to be briefly working. The +6V and +12V supplies remained at 0 volts. On the control connector AC DET (AC Detect?)and 5VST (5V Standby?) remained steady (both of these are signals from the power supply board). HL ON would go high, then drop low about two seconds later (this is a control signal to the power supply). This coincided with a relay clicking. 5V_MNT (5V monitor?, from the power supply) never changed state, nor did VAVS ON (another control signal to the power supply).

At a first guess, it would appear the 5V, 6V, and 12 V section of the power supply is not functioning. This is a DN-42PY10X AF044A MURATA MPF7413. Anybody have any ideas on finding a schematic for this? I'm going to be pulling it later and seeing if I can find a bad solder joint, smoked component, or bad capacitor.

PlainBill

[PlainBill]

Re: LG Electronics Plasma TV Model DU-42PX12X Chassis #AF044A

This is getting seriously weird. I've been unable to find any sort of schematic for this power supply. When I started to troubleshoot it I assumed it should be relatively easy. On first examination, it was quite clean; very little of the dust I would expect for a 4 year old TV. I checked the ESR on all capacitors (mostly Nippon Chemi-con KMQ and KZE, with a few Rubyicon) and all were well within specs.

A close examination with a magnifier failed to find any burned components or bad solder joints, so it was time to check other possibilities. The training manual mentioned that the wire wound resistors in the power supply could be a source of trouble, so I checked them and thought I hit paydirt. One 5 W 10 ohm resistor was open. All other wire wound resistors checked out. It turns out I had a 10 ohm 25 watt resistor in my parts box, so I shoehorned it in, hooked everything up, and shucks - no change. This didn't really surprise me, it was in the Va and Vs supplies, which as far as I can determine don't get switched on until the lower voltage supplies come up.

I quickly discovered there is a BIG difference between troubleshooting the typical monitor power supply and one of these beasts. After searching for a schematic - ANY schematic for a plasma TV, I decided this was going to have to be done the hard way.

The first step was to check for any obviously bad transistors in the driver side of the 4 transformers. All check out when tested with an ohmmeter. Then I tried identifying which transformer is used of each voltage (the supply has 8 outputs ranging in power from 265 watts down to 3 watts). And then I discovered something strange. The 3.4V 2A supply shows a 55 ohm resistance across the output. This is a REALLY small drain (.2 watts), but there are no resistors larger than 1/8 watt in the circuit. All of the other supplies show more than 200 ohms resistance across their outputs.

Any ideas would be appreciated. I won't have time to start pulling components until we get back from vacation. Oh, and links to a schematic for a LG DU-42PX12X plasma tv (or schematics for the power supply for an equivalent size plasma tv) would be much appreciated. The supply is variously described as 3501V00181B Murata MPF7413 PCPF0060 DN-42PY10X AF044A

PlainBill

[Wizard]

Re: LG Electronics Plasma TV Model DU-42PX12X Chassis #AF044A

Look again, does this circuit have 5V to 3.4V switching buck regulator step down with a coil?

Cheers, Wizard

[PlainBill]

Re: LG Electronics Plasma TV Model DU-42PX12X Chassis #AF044A

Look again, does this circuit have 5V to 3.4V switching buck regulator step down with a coil?

Cheers, Wizard

Nope. This circuit APPEARS to be a 'traditional' SMPS design. There's a primary with multiple secondaries. I haven't traced what the other secondaries supply YET. The output of this secondary goes to a YG805C04, a Schottky barrier diode rated at 40 Volts, 20 Amps. There is a TA76431S shunt regulator feeding a opto isolator to provide feedback to the SMPS controller, a handful of caps, some 1/8 watt resistors, and an adjustment pot labeled 3.4V (a good clue, right?).

Looking at the board, I would have to guess the other secondaries on this transformer provide +6V (adjustable), +12v (not adjustable), and the supply for the 19V audio supply, which is adjustable). There are a pair of YG811S06 Schottky diodes, a pair of inductors and a pair of PQ1CG2032FZ chopping regulators on the same heatsink. I would say this particular section of the power supply board handles the 19V .6A, the 12V 1A, the 6V 1A, and the 3.4V 2A outputs.

The PFC circuitry is obvious (after someone explained it to me, thanks everyone). I've just located the 5V SB supply. The 5V 4A supply and the Va and Vs supplies are on the other side of the board. That will have to wait until next week. Still, this think looks WAY more complicated than it has to be. There are all these daughter boards scattered around the main board.

Wizard, I've been operating on the assumption (based in part on the labels on the labels on the connector) that the 5V SB comes up whenever power is applies to the TV, the 3.4, 5, 6, 12, and 19 volt supplies come up when the TV is first turned on, then a few seconds later the Va and Vs supplies are turned on by the controller. Does this make sense to you? Or does the 5V supply (which goes to the driver and Sus boards) get turned on with the Va and Vs supply?

PlainBill

[Wizard]

Re: LG Electronics Plasma TV Model DU-42PX12X Chassis #AF044A

5Vsb is first to come up when plugged in.

The other outputs comes all at same time when commanded to turn on.

A group regulated transformer like the 3.4V, adjust 3.4V makes other voltages move up or down. It GONNA must have bucking regulator especially for low voltages that needs to be adjusted. Get a pic of both sides (clear ones)?

The Vs and Va comes through 2 seperate transformers to allow you to adjust interdepently.

Cheers, Wizard

[PlainBill]

Re: LG Electronics Plasma TV Model DU-42PX12X Chassis #AF044A

5Vsb is first to come up when plugged in.

The other outputs comes all at same time when commanded to turn on.

A group regulated transformer like the 3.4V, adjust 3.4V makes other voltages move up or down. It GONNA must have bucking regulator especially for low voltages that needs to be adjusted. Get a pic of both sides (clear ones)?

The Vs and Va comes through 2 seperate transformers to allow you to adjust interdepently.

Cheers, Wizard

Thanks for the help. Pictures will have to wait until next week. Gotta pack my swim trunks for our vacation. Oh, yeah, clothes too, I guess.

Ohhh!! I just remember the scene from 'Nobody's Fool' where Melanie Griffith shows Paul Newman the pair of airline tickets and invites him to join her somewhere warm. When he says he'll have to pack his swimsuit, she says "Where we're going you won't need a swimsuit, just some suntan lotion. I'll do you, then you can do me."

PlainBill

[Wizard]

Re: LG Electronics Plasma TV Model DU-42PX12X Chassis #AF044A

Enjoy your vacation and be mindful. Probably better get vaccined for hep A&B?

Cheers, Wizard

[PlainBill]

Re: LG Electronics Plasma TV Model DU-42PX12X Chassis #AF044A

Enjoy your vacation and be mindful. Probably better get vaccined for hep A&B?

Cheers, Wizard

Vaccinations weren't advised. After all, this is the USA; sometimes the best health care is avoiding medical care. However, a block from the hotel was a bar which advertised 'Totally Nude Billiards'. After a few seconds of reflection I felt it would be best not to investigate. Curiosity and the cat, you know.

I DID learn a new word, however - Mahalo.

I've attached a bunch of pictures of the power supply. Real Soon Now I will attach an addendum indication the destinations of the connectors, and identifying the various pins on P801 - the control / status signals to the digital board.

Mahalo

PlainBill

[Techmedic]

Re: LG Electronics Plasma TV Model DU-42PX12X Chassis #AF044A

PlainBill, did you ever resolve your problem?

[Wizard]

Re: LG Electronics Plasma TV Model DU-42PX12X Chassis #AF044A

Yes, there are 2 bucking voltage regulators, two toroidal coils with 5 pin IC on heatsink along with rectifiers near the voltage outputs (low voltage only).

The Va and Vs I think one is adjustable onboard but other one is on external DC to DC converter/regulator board for the other voltage. I occasionally see this.

The true one board have 2 large transformers adjusted separately for this reason for Va and Vs.

Cheers, Wizard

[PlainBill]

Re: LG Electronics Plasma TV Model DU-42PX12X Chassis #AF044A

PlainBill, did you ever resolve your problem?

Yes, but only by replacing the power supply. I note that there are several of the supplies available for $100 or less on eBay.

I have an isolation transformer still packed in one of a couple of dozen boxes. With that and my scope I might have been able to fix it. Unfortunately, I was unable to locate the proper box. I had already established that the 3.3V supply did start to come up, but none of the others did. I also determined that none of the diodes were shorted.

PlainBill