LG 50PA4500 Plasma Troubleshooting

[brassicGamer]

Hi folks. This panel seems to be a 'favourite' on these forums. I've checked through all the existing posts and my issue doesn't match up with the common faults described. I think it might be the logic board.

I acquired it second-hand recently and the previous owner described the panel working fine until a power cut, when it only turned back on briefly. The apparent issue was that, when powered on, the TV would display the manufacturer logo and then go into standby mode. I figured this was the main board shutting it down after a test condition failed.

I checked the capacitors and found that all the 100uF, 100V caps had blown their arses out. Replace them and tried again - same problem. No other visual problems are apparent. (Note: the user was a heavy smoker and I have had to clean the boards and chassis from top to bottom - the same problem existed before and after I cleaned everything.)

I did the usual steps of measuring voltages (all good) and unplugging boards in sequence and none of this made a difference. I then desoldered and checked cap C802 because of the sheer amount of reports that this goes bad, but it was absolutely fine.

Something changed around this point. Now when I plug it in, I don't get the LG logo anymore, just some momentary snow before there is no image (and no sound - there never has been), but the panel is 'on', instead of going into standby like before. The Y-SUS board is making some noise when powered, but not when the unit is off or in standby. Also, the red light on the main board is lit.

I have tried the AUTO_EXT_GEN test pattern feature and no picture is produced, though it does alter the sound coming from the Y-SUS board. There is a green light on the logic board, which is displaying what looks like an error code (one long dash, one short dash, repeated once) but I have no documentation to refer to (the service manual appears not to include such codes).

Any ideas?

p.s. I've noticed a common factor people discuss with this panel is clicking relays - mine appears to be a revision that has no relays.

[Biruslapio]

That Y-SUS is totally blackened by heating, check it and Z-SUS for cold soldering joints.

Disconnect the main board from the PSU, then the TV will auto-start when you plug in AC cable, that way it's easier to rule out the other boards.

For the auto-gen you need to disconnect the LVDS cable, if the panel lights up and display black without pixels fading or misfiring everywhere both sustain boards are ok.

[brassicGamer]

I removed the heatsink nearest the driver board and tested all the MOSFETs and Schottkeys. One of the MOSFETS is bad. I guess I should also test all the others just to make sure.

[brassicGamer]

I found another one and they're actually IGBTs. First time working on a plasma panel so I've not come across these before. Ordered some as I don't have any in stock. They were in series with failed capacitors so I guess they got slammed by unfiltered current over a long period of time until they failed. I'm hoping that replacing these fixes the problem.

[Biruslapio]

All plasma sustain boards use IGBTs, and they're the worst part about the repair, hard to find original parts, so if you run into a RJP63F3 or 30J127 compare it against an original, if the label has nothing in common or is misplaced it's probably fake and may or may not blow up.

[brassicGamer]

Originally Posted by Biruslapio

All plasma sustain boards use IGBTs, and they're the worst part about the repair, hard to find original parts, so if you run into a RJP63F3 or 30J127 compare it against an original, if the label has nothing in common or is misplaced it's probably fake and may or may not blow up.

I found some 30J121s on Mouser, which are same spec but rated for 600V instead of 300V, so hopefully they will do the job.

[lotas]

Originally Posted by brassicGamer

I found some 30J121s on Mouser, which are same spec but rated for 600V instead of 300V, so hopefully they will do the job.

And how are you going to install such a transistor if it has a different case (TO-3P) and it is not insulated (metal)?

[brassicGamer]

Originally Posted by lotas

And how are you going to install such a transistor if it has a different case (TO-3P) and it is not insulated (metal)?

Seems like a good place to ask such a question!

[vinceroger69]

Originally Posted by brassicGamer

Seems like a good place to ask such a question!

not sure if these will work https://www.ebay.co.uk/itm/194364884...sAAOSwyLhhQa3F

[Biruslapio]

It won't, the heatsink and hole spacing are different, replace IGBT with the same model, be caureful of fake parts.

[brassicGamer]

Okay, okay. I got lucky and someone was throwing away a Panasonic Viera panel literally as I was driving past them. It had two RJP30Ks in it, which look like a good match:

Model___Voltage_Amps_Pulsed_Package
30F126__330V___30A__200A__TO-220
RJP30K__360V___40A__300A__TO-220

[Biruslapio]

Ok. Are you replacing 30F126 with RJP30K? What happens is that the transistor's pins in Panasonic boards have less length, they may not reach the holes all the way, solder them well.

[brassicGamer]

Originally Posted by Biruslapio

Ok. Are you replacing 30F126 with RJP30K? What happens is that the transistor's pins in Panasonic boards have less length, they may not reach the holes all the way, solder them well.

It's cool, they're literally the same length. Lucky, I know, but thanks for the heads-up.

[brassicGamer]

Well, it made no zero difference to the symptoms (some snow when it first powers on, then nothing), so the problem must be further back. On the verge of giving up on this but I've put a lot of effort in so far cleaning it up (from the smoking tar). Would be nice if I should just swap in working boards to find out which one is the problem.

[Biruslapio]

When IGBTs fail often their gate resistors and some 2W resistors go open from the current surge.

I thought the boards were blackened from the heat, it's actually smoke, that must smell.

[Biruslapio]

Ok, I'll go into more detail, check these circled components.

On the Y-SUS these 5 pin optocouplers drive the IGBTs, they cannot be shorted, also there are some 2.2, 10 and 100 OHMs SMD resistors next to them that fail, the IGBTs and MOSFETs gate resistors are circled too.

On the top picture the "fusible" resistors are marked and cannot be open.
These procedures are valid for the Z-SUS.

This model has buffer boards failure quite frequently, it's important to test them. During power-up you can measure VSCAN, -VY and ZBIAS to see if they are stable and matching the panel sticker.

[brassicGamer]

Thanks for your time on this, I really appreciate it. I thought I would list the component labels identified in case it further helps others dealing with a similar issue. I found only found two failed components.

Optocouplers (as arranged: left to right, top to bottom):
IC402, IC481, IC401, IC501, IC552, IC553, IC441, IC442

Resistors (27 Ohm):
R420, R417, R485, R482, R413, R415, R416, R414, R449, R448, R453, R452.

Resistors (15 Ohm):
R490, R489, R572.

Resistors (330 Ohm):
R576.

Resistors (2.2k):
R577.

Resistors (39k):
R571.

IC501 is closed, but all the resistors are reading their expected values except for one of the 2.2Ohm through-hole ones (R283). Time to salvage replacements!

[Biruslapio]

I circled what I could see in the image, not all those components are suspicious or fail regularly, the opto model should be 155E and the most suspicious resistors are 1.0, 2.2, 10.0 and 100 OHMs next to the opto.

I have a Y-SUS repair log from 2018 and it went similar to your story, I should've looked at it first.

log:
D402 D2217 SF20A300H not shorted but leaking
Q404 30F126 shorted
Q401 Q402 30J127 shorted
IC501 155E optocoupler driver shorted
C501 shorted, linked to VCC and cathode of IC501
2R2 next to IC501 open

In that TV there was some damage on Z-SUS, look at R221 and components next to it.