Plasma 42V7 problem - please help!!!

[Th3_uN1Qu3]

Re: Plasma 42V7 problem - please help!!!

Okay, this here says:

DC 5.2v
Va: 65v Vs: 188v
188 / -75 / 115 / N/A / 165

Seems to be a slightly different order here. Va and Vs test good, i have -74.7v on Vy so that's good too. Vsc reads 40 volts. Can't find Vzbias, and i'd have to pull the main board housing to get to more test points on the Ysus.

I have the service manual but it doesn't seem to have the schematics for the sustain boards.

[tom66]

Re: Plasma 42V7 problem - please help!!!

Schematics? For an LG? Sorry...They're unobtanium.
Looks like you have a Vset too, but on most panels that's equal to Vs.
Can we see a picture of the Z-sustain?

[Th3_uN1Qu3]

Re: Plasma 42V7 problem - please help!!!

Don't have my camera with me now, i'll take pics of the TV tomorrow. This is the board: http://www.ebay.co.uk/itm/42-LG-PLAS...item41732003e1

[tom66]

Re: Plasma 42V7 problem - please help!!!

Nicer pic
http://flattvparts.co.uk/bmz_cache/a....3000x1668.jpg

Measure Vzbias with one probe on ground/chassis, other probe on tab of fet Q3. Voltage across R23 may work too.

[Th3_uN1Qu3]

Re: Plasma 42V7 problem - please help!!!

Tab of Q3 reads 164.4v.

[tom66]

Re: Plasma 42V7 problem - please help!!!

Seems fine then.

[tom66]

Re: Plasma 42V7 problem - please help!!!

You can also check ERC functionality. The voltage across caps like C6, C7, C8 (row of red caps) should measure from 1/3 Vs to 2/3 Vs, and should be nearly the same (I'd say +/-5V) on both boards. The caps are all in parallel you need only check one.

[Th3_uN1Qu3]

Re: Plasma 42V7 problem - please help!!!

Thanks. I've left the shop, will check those caps tomorrow. I would also appreciate some pointers on where to scope.

[tom66]

Re: Plasma 42V7 problem - please help!!!

On the Z-sus, the orange bead near the output.
Y-sus I'd need a picture to tell you for sure but probably one of the metal plates connecting to the Y-buffer.

[Th3_uN1Qu3]

Re: Plasma 42V7 problem - please help!!!

Here's a pic of the Ysus. And here's the waveforms of the Zsus at the orange bead near the two large inductors, i suppose that's the one you were referring to. 100v/div, TV trigger. First pic is 1ms/div, second 2ms/div.

Waveform is the same at the blue cap you asked the OP to check.

[tom66]

Re: Plasma 42V7 problem - please help!!!

The waveform from the Z-sustain looks good. The Vzbias is being applied (lower, static portion) and the panel is being sustained (higher, oscillating portion.)

As for the Y-sustain, I think you should test at B32.

Also, test the voltage across those caps I mentioned previously. One end of them is grounded, so you can just try and find the positive with your meter's COM attached to the chassis.

[Th3_uN1Qu3]

Re: Plasma 42V7 problem - please help!!!

I think i found a problem.

Red caps on Zsus read 92.9v
Red caps on Ysus read 6.08v.

Also, it might be my imagination, but the waveform of the Ysus seems to have dropped in amplitude. I tested it about 20 minutes ago, but didn't take a pic then. It looked something like the OP's waveform.

Now, it's lower in amplitude, and there's a hiss noise at regular intervals. When the noise occurs, the waveform spikes (to the amplitude it should've had?), and i can't catch it. Here's how it looks now, when it's stable. 2ms/div, 50v/div. So... ordering an Ysus already?

[tom66]

Re: Plasma 42V7 problem - please help!!!

Sounds like a bad IPM. Swap the IPM or the Y-sustain.

You need to take a longer shot at 10ms/div using a long exposure on the camera so we can see if there are any ramp or set up waves.

[Th3_uN1Qu3]

Re: Plasma 42V7 problem - please help!!!

There USED TO BE a ramp when i first measured. No longer.

[tom66]

Re: Plasma 42V7 problem - please help!!!

You should check if those transistors are still warm, if not then check the transistors on there for shorts or replace the whole Y-sus. The buffer may be OK, check with DMM using this method, use the middle pad on each buffer board, not the far pad for your return pad. http://www.youtube.com/watch?v=51SZsfH60EY

[Th3_uN1Qu3]

Re: Plasma 42V7 problem - please help!!!

The two transistors are no longer warm since the waveform changed.

I'm going to check the buffers some other day. We phoned the owner before i left the shop, and he can't be bothered to come pick it up (we told him it isn't worth fixing), so it's ours to keep. Hence we'll be taking the cheaper route... I'm going to order an IPM and once it arrives, i'll resume troubleshooting.

[Clydeuk]

Re: Plasma 42V7 problem - please help!!!

I've successfully flashed these 29LV320 ROM's now with an external "willem" type programmer (a cheap chinese copy). I thought it wasn't working but realised I was using the wrong file and also needed to do a temporary sector unprotect. The 42V7 I have does not match any of the LG files, I only got display back after flashing the original back in (good job I saved it first!).

The other one is a 42V8A panel with minor discharge at the top (worse when cold and only noticeable on dark scenes (seems worse on blue), it's also at worst in the top corners. I flashed the update rom into this after trying panel setup, but it has appeared to make no difference. I can tweak out the sparkles but then get dark specks instead. I suspect it's the panel to blame, I'm sure there must be some way to alter things. I might check the caps though I'm not convinced, it made no difference on my other one.