LG42PT353K plasma troubleshooting

[kebl2825]

Hi. In possession of an LG plasma, not working, and a shiny new multimeter. Am a newbie but keen to learn and hopefully rescue this TV, if possible, and sensible. Only previous experience since school is the occasional resoldering.

Model LG42PT353K-ZA.BEKLLJP
Manufactured 2011, Poland.

Was working without fault till a audible pop/bang and picture failed. I wasn't using the TV at the time so don't have any more detail about the moment of failure.

Now when try to use, the standby red light is lit, and powering on with remote results in a click, and a whirr, but a click 'off' again moments later back to standby.
There is no picture during this.

I have read tom66's excellent guide to troubleshotting plasmas and hence starting this thread.

One of the first things I'm asked to establish is if the sound is working or not. To be honest I'm not sure how I would do that? How can I test if there's sound given the TV doesn't fully power up, and there is no picture? (As in, even if TV stayed 'on' how would I know I should be hearing anything?) The TV has been broken, unplugged and idle for months now.

I've opened up the case and checked all recognisable fuses and they're all working. The only component that looks like it may have failed is a 150 microF capacitor on the top right of the power board, near the test points. The 3 sections are slightly raised to the touch, and discoloured. Photo attached. Should swapping out this component be my first course of action? (I only spotted this capacitor while writing this post- have waffled a bit if it is the only problem).

Any help appreciated, even if it's just to say don't bother, but I hope not. I'd like to be able to diagnose the problem properly even if it's not worth fixing.

[Hamie]

Re: LG42PT353K plasma troubleshooting

Firstly disconnect the y-sus and z-sus from the power supply.
Then turn on the TV.
Check to see if you have VS and VA voltages on the power supply.

[kebl2825]

Re: LG42PT353K plasma troubleshooting

Thanks for responding. I should have mentioned I'll only occasionally have time to look at this.

So, I disconnected the cable from power to z-sus (y-sus doesn't connect directly to power but a similar spec cable runs to y-sus from z-sus across top of panel) and powered on.
The symptoms changed. Now the TV stayed 'on' or 'standby' with a click either way, and the power board made a quiet but constant noise while on (like an insect chirruping).

Va 54.9V
Vs 202.0V

I then tried just the z-sus connected (the cable from z-sus to y-susu disconnected). Now power stayed on, there was a blinking light on control board and an increasing pitch whirr from somewhere on y-sus. Both voltages came out the same as above again.

[ReeceyBurger123]

Re: LG42PT353K plasma troubleshooting

That means that you have a short on the ysus then, does your meter have a diode test with a speaker its easier to check for shorts with that function ?

[Hamie]

Re: LG42PT353K plasma troubleshooting

Ok check z-bias voltage on the z-sus
It should match the voltage also located on the panel sticker.

Next disconnect the buffers from the y-sus.
Turn the TV on and if it doesn't cycle check VSC and -VY voltages.

[kebl2825]

Re: LG42PT353K plasma troubleshooting

With the y-sus still disconnected I tested at z-bias resistor, which was switching between 125.1V and 125.2V, so almost matching 125V on the sticker.

I'll disconnect the buffers next, will need to unscrew the buffer board from chassis.
What do you mean by 'cycle' though? As in it seems to switch on and stay on?

The panel sticker has N.A. for VSC, if I'm reading it correctly (photo attached).
I also can't see a VSC or VSCAN written anywhere.

ReeceyBurger123, yes I have a multimeter with a diode/continuity test setting with beep. I'm not sure the procedure to test for shorts though.

[tom66]

Re: LG42PT353K plasma troubleshooting

Your -Vy is -200V. Vsc is not printed but is probably around 140-160V, that is a typical range. Vzbias is 125V.

[kebl2825]

Re: LG42PT353K plasma troubleshooting

Okay, disconnected buffers from y-sus and symptoms same as when buffers connected.

Click, whirr, click off when all connected.

I'm checking VY when everything's connected, right?
It's awkward to test the VY, as I'm trying to switch on at same time as holding the test probes and watching the meter.
The values constantly change, but are only in the mV range.
Got up to about -300mv very briefly but falls back as TV clicks off, and slowly decreasing magnitude towards 0.

Also I still don't know where to test for VSC on the board. I'll attach a photo.
Would the reserve side of board be marked for this? I guess it would be less awkward to test on that side too.

[Hamie]

Re: LG42PT353K plasma troubleshooting

You don't need to check -VY as the TV is still cycling.
The fault lies in the y-sus.

Take a picture of this board

[Hamie]

Re: LG42PT353K plasma troubleshooting

Edit: you have already done this.

[Hamie]

Re: LG42PT353K plasma troubleshooting

The 2 black heatsinks.

Get your meter in diode mode and touch the legs of all the diodes fets etc attached to the heatsink. If you get a short reading remove the parts and check them out of circuit.

[kebl2825]

Re: LG42PT353K plasma troubleshooting

Hi. Just to update.

I eventually removed one heatsink, with great difficulty and after purchasing a new soldering iron (I previously was still using my Grandad's iron from 1970s with the original, and non-replaceable tip). The solder just wouldn't get hot enough to wick/suck away, and I'm certain my desoldering technique needs work!
I'm very chuffed with my new iron though, it is amazing compared to how I've struggled for years when I was younger fixing guitars and the like with the old one. But still the solder wouldn't come away easily till 390-400degC which just seems far too high (is it?). I guess these boards aren't manufactured with any concern for manual tinkering later.

In the process of removing the first few transistors and diodes I snapped the drain pin of one, and the next think I may have burnt out just from the desoldering process itself.

I'm just back from holiday and now in the middle of moving house for myself and various family members so it wasn't the best time for this sort of tinkering.

I gave in a bit and swapped in a replacement Y-sustain board (£45) and the TV now works, good as ever. I slightly tweaked a couple of voltages to match the chassis label too before packing up, though they were all within 0.5V already.

I can use this duff Y-sustain board to learn on (when I'm done renovating my new place), and then, or if/when something else goes bang I'll be back I'm sure.

Have already learned a lot, enjoyed it, and saved a few quid.
Thanks everyone!

[Hamie]

Re: LG42PT353K plasma troubleshooting

Nice to hear you got the TV fixed.

When removing parts form a heatsink you need to remember the heatsink draws all the heat away from the iron making it difficult to clear the pins.

2 methods to use that require no additional tools are.

1. Unscrew the part form the heatsink you want to desolder, gently bent it forward so it nolonger has contact with the heatsink.

2. Heat up the board in the oven to a safe temperature 150c. Then quickly desolder any parts that you wish to inspect.