Plasma 42V7 problem - please help!!!

[scokljat]

Hello!

Recently I bought nonfunctional LG plasma TV model PDP42V706A2 using 42V7 plasma module. I bought it very cheap and I want to fix it for my parents house. I have good repair skills especially for laptops, LCD's and synthesizers, but this is first plasma I ever put my hands on.

I have read lot of repair info on the web, forums, downloaded and studied LG service manual, so now I have some knowledge about plasma principles, parts, boards, voltages, signals and common failures.

My 42V7 initially suffered from common “no picture – sound ok” problem. I took LG 42V7 service manual and started diagnostics and adjustments step by step. When I disassembled TV and measured all voltages (+5V, Vs, Va, Vsc, -Vy etc…) I discovered that almost all voltages are wrong like someone was playing with trimmers, probably some unskilled serviceman before.

Then I started to measure and adjust voltages one by one, first +5V, Vs +187V, Va +65V on the PSU, then Vsc +120V and –Vy -75V on the Y-SUS board. When all voltages checked OK I started to get (jerky) picture on the screen!! I was very happy. I also experimented with VsetUP and VsetDN on the Y-SUS but I don't have proper oscilloscope to do it like service manual instruct.

Now, most of the time picture is black (no picture) with maybe some light pixel noise and sometimes suddenly picture appear but it's jerky and darker then normal like some connection is loose and then sometimes very rarely picture is displayed 100% normal for a few seconds with full brightness, and then again it starts to loose to jerky then back to black and so on. More then 90% of time picture is black. I recorded a movie that greatly describes this condition.

Video:
http://www.youtube.com/watch?v=2tfF-iK0hYE

Here are my notices and measurements:

- all measurements done with standard TV picture trough HDMI and external DVB-T tuner (not with full white test signal like service manual says because I don't have signal generator)
- all fuses are OK
- caps look ok (no cracks, spills...)
- PSU: +5V, +19V, +6V, +3.4V, +12V - OK!
- Y-SUS: Vsc +120V, -Vy -75V - OK!
- Z-SUS: +128V on FET output - not sure if it's OK???! My Z-SUS board does not have adjustment trimmer for Vzb or R23 resistor to measure Vzb. There are two types of Z-SUS for 42V7 module.
- CTRL board: FET1 IC5 +3.3V - OK! FET2 IC3 +1.8V - OK! Didn't measure oscillator because I don't have 200Mhz oscilloscope (I only have 60Mhz oscilloscope)
- on CRTL board three status LED's are ON (D3, D4, D5), two are OFF (D1, D2)
- small heatsink on CTRL board (over IC9) is cold all the time
- I cleaned all contacts, cables and connectors
- after approx. 30 minutes of operation Z-SUS IPM heatsink gets hot, but Y-SUS IPM heatsink gets barely warm.
- Z-SUS IPM produces more buzzing noise than Y-SUS IPM
- when suddenly picture start to display some light clicks can be heard from Y-SUS board (probably IPM) and when picture appears (after black periods) buzzing noise from Y-SUS IPM is equal like Z-SUS IPM
- I checked all 10uF/350V caps from Y-SUS (took all 8 of them out) they measured barely in 20% tolerance around 8.5uF, one of them was 7.5uF (put them back)
- Y-SUS output diode check - OK!
- Z-SUS FPC output diode check - OK!
- Y-SUS IPM diodes check - OK!
- Z-SUS IPM diodes check - OK!
- sound is ok, MENU displays OK, program and AV input change OK
- I can't adjust VsetUP and VsetDN as I don't have proper oscilloscope
- for now I didn't replaced any caps


Board P/N information:

PSU: JPSU-J006A 1H251WI P/N6709V00010A
YSUS: 6871QYH045C
YDRVTP: 6871QDH094A
YDRVBT: 6871QDH096A
ZSUS: 6871QZH049A
XRLBT: 6871QLH052C
XRRBT: 6871QRH060C
CTRL: 6871QCH066E

Service manual:
https://cdn.badcaps-static.com/pdfs/...fa21a8ae9b.pdf

I see people buy and replace all three boards (Z-SUS, Y-SUS, CTRL) and solve problems, but I don't have budget for that, so I prefer repair on the component level on the affected board or if IPM is bad I'll buy and change only affected board.

All help and information is welcome.

Thanks!!!

Cheers!

Suad

[tom66]

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

Possibly bad BGA chip on the control board. Common issue with this generation. Does it look like this? http://www.youtube.com/watch?v=UAzsasx8od8

With a 60 MHz scope, you should be able to see a slight oscillation from the crystal. On my 100 MHz one, I can view 300 MHz FSB clocks, just about.

[tom66]

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

Nevermind. Just noticed your video. If picture is DIM, sometimes purple-tinted and slow to update, your Z-sus is the fault 95% of the time.

[scokljat]

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

Possibly bad BGA chip on the control board. Common issue with this generation. Does it look like this? http://www.youtube.com/watch?v=UAzsasx8od8

With a 60 MHz scope, you should be able to see a slight oscillation from the crystal. On my 100 MHz one, I can view 300 MHz FSB clocks, just about.

Tom, thank you for quick reply. My TV looks like one from your link (same model), but symptoms are different. On my plasma TV picture is black (or ultra dark) most of the time and then sometimes picture appear but it's flashing and it's jerky, and very rarely picture is OK for second or two... When picture appears there are NO missing lines or black bars, so screen, drivers and TCP cables are OK, I belive.

I tried put some pressure to CTRL BGA chip, and bend a board a little but no difference.

thanks!

S

[tom66]

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

With your scope, check Zsus output. 10X probe, 5V/div, 20ms/div. TV trigger works sometimes, if you have it.

[electronicsurplus]

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

I am betting, although rare, the Y-Buffer

can you check for shorts to ground on the connector end there will be many connections which go directly to ground and, some that do not, these are data (signal lines) and voltage lines to fire the plasma (Be careful because there is very high striking voltages coming through them)

Because, on LG when a buffer goes bad it does not take out the Y-Sus, unlike Philips Samsung) and, because of this the Buffers are cheap and easy to find

Jon
Electronic surplus

[tom66]

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

If a buffer goes, you will get random black lines, noise or no picture.

[scokljat]

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

With your scope, check Zsus output. 10X probe, 5V/div, 20ms/div. TV trigger works sometimes, if you have it.

Please, would you show me precisely where to measure Z-SUS output? You could use and mark one of my Z-SUS pictures.

Thank you!

S

[scokljat]

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

I am betting, although rare, the Y-Buffer

can you check for shorts to ground on the connector end there will be many connections which go directly to ground and, some that do not, these are data (signal lines) and voltage lines to fire the plasma (Be careful because there is very high striking voltages coming through them)

Because, on LG when a buffer goes bad it does not take out the Y-Sus, unlike Philips Samsung) and, because of this the Buffers are cheap and easy to find

Jon
Electronic surplus

Which Y-Buffer connector are you referring to precisely? The large one with thin flat cable that outputs signals to plasma screen or ones that connects Y-SUS board and Y-Buffer boards?

[tom66]

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

Please, would you show me precisely where to measure Z-SUS output? You could use and mark one of my Z-SUS pictures.

Thank you!

S

Just on any exposed pads connected to the large flat brown connectors going to the plasma panel itself.

[tom66]

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

Picture of where to test, you could use a hook probe.

[scokljat]

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

With your scope, check Zsus output. 10X probe, 5V/div, 20ms/div. TV trigger works sometimes, if you have it.

I checked Z-SUS output with a 60Mhz scope. As I'm no expert at oscilloscope reading, I recorded a movie and few pics so if you can take a look. When TV starts to show some picture, lite clicks are heard from Y-SUS board and oscilloscope reading on Z-SUS changes a bit at that exact moment... I think it didn't happen during this video. Please if you can guide me step by step through this scope process, as I'm not familiar with scope and this types of signals. I measured voltage on that same point - it's +128V. Is it Vzb?

Video:
http://www.youtube.com/watch?v=yXpuKc4Fveo


Thanks!

S

[tom66]

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

Nice Tek scope...

Vzb should be around 90V for these TVs I think, although when I played around with it, it didn't seem to make much difference. But your Zsus waveform looks pretty good. It is only a square wave really; nothing else.

Check Ysus - there is a test point "Yscw" I think - you can check the sustain output to the buffers there. Ysus wave is much, much more complex.

[scokljat]

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

Hmm, where exactly to find that "Yscw" point on Z-Sus? Should I check it with a scope? I searched service manual for "Yscw" with no results... Take a look on Y-SUS picture, please.

Thanks!

S

[tom66]

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

Just the big metal connectors going to Y buffers One will be ground, the other will be the waveform you need to test.

[scokljat]

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

Just the big metal connectors going to Y buffers One will be ground, the other will be the waveform you need to test.

I found what you say in service manual. It's "Vscw" and it should be tested at bead B39 which is connected to center big metal which goes to buffer boards. Other two upper and lower metal are not connected to ground.

Should I test it with scope across ground and B39 (center metal)? See pics...

Thanks!

S

[scokljat]

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

Just the big metal connectors going to Y buffers One will be ground, the other will be the waveform you need to test.

OK, we have some progress here! ;-)

I measured Y-SUS output wave with scope. It's obvious something is wrong with it! When picture briefly appears, wave on scope jumps. I recorded a video of this condition. Actually, that was longest burst of displayed picture I ever got on this broken plasma.

Look at the movie:
http://www.youtube.com/watch?v=UTkkJBE5FdM

When picture is near black voltage is 135V-155V, but when picture is briefly displayed perfectly voltage jumps to 190V and clicks are heard from Y-SUS and IPM get normal buzzing noise! Something like IPM is switching on and off...

What do you think?

Btw, picture is little bit "slow" I believe because misadjustment of VsetUP and VsetDN.

Thanks!

S

[tom66]

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

Does your scope have a pulse-ramp rate trigger? Try triggering on the slow VSET ramp; it ramps up over several microseconds.

It sounds like your Y-sustain is faulty. This issue can be caused by a walking-wounded IPM.

[scokljat]

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

Does your scope have a pulse-ramp rate trigger? Try triggering on the slow VSET ramp; it ramps up over several microseconds.

It sounds like your Y-sustain is faulty. This issue can be caused by a walking-wounded IPM.

Hmm, Tom, I'm really an oscilloscope dummy ;-)
Could you please precisely describe what to do and where to measure? My oscilloscope is Tektronix TDS210 60Mhz. There is alignment tutorial for VsetUP and VsetDN in service manual that requires triggering with trigger probe to VS-DA point on CTRL board. Did you mean that?

Thanks,

S

[tom66]

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

My work place has the 100 MHz version of that scope. I *think* it only has edge and video triggers. But yours may be slightly different. Press TRIGGER MENU and see what options you get. You can still get a decent trigger without, just requires playing around with the HOLDOFF control, which this scope definitely has (had to use it on one circuit with a data line with varying frequency.)