Panasonic th42pz80u: backlight working but no display

[Tetranitrate]

So my neighbor just put a Panasonic th-42pz80u plasma TV out on the curb. Me and my buddy grabbed it and started trouble shooting. It will power on with a backlight, but no image.

I have attached the service manual. According to one of the troubleshooting flowcharts, the problem is with the D-board. Both the SC board and the SS board LEDs are green.

Does anyone have any suggestions for what to test or issues to look for?

[tom66]

Re: Panasonic th42pz80u: backlight working but no display

A plasma screen doesn't have a backlight; it emits its own light from each pixel.

The SC board and SS board are responsible for the dim glow when the display has no video (by limitation of design), so most likely those are working fine. The LEDs indicate the energy recovery circuit (part of the SC and SS boards) is working fine, as well.

I would suspect the D-board, or A-board if you can't get audio.

[tom66]

Re: Panasonic th42pz80u: backlight working but no display

Service manual(s):
http://tgohome.com/ServiceManuals/?d...h=Submit+Query

[DavidOH]

Re: Panasonic th42pz80u: backlight working but no display

Should be getting a blink code for the power led. If you can't tell if it's blinking use the smaller manual tom66 posted. On page 8 is the test for 1 blink. It will tell you if it is the A or D board. I had the same set and couldn't tell if it was blinking so that's what I did. It was the A board. Got one from shopjimmy and it's been good going on 2 years.

[tom66]

Re: Panasonic th42pz80u: backlight working but no display

If it's powering on normally but has abnormal picture it won't throw a blink code.

[Salink]

Re: Panasonic th42pz80u: backlight working but no display

I'm working with Tetranitrate on this. The power LED won't show a blink code. I can use the power button to turn the TV on, but have to hold it to turn it off. Sometimes the TV will turn on again by itself after I've turned it off. I read in other forms that disconnecting A5, A6, and A7 and having the screen show all white is a sign that there's a problem with the D board. Our screen does glow white after A7 is removed.

We also don't seem to be getting any audio from any source. Since there is no OSD, we tried cycling the input button to see if we can get audio, but either audio doesn't work, or we can't cycle through the inputs.

[Tetranitrate]

Re: Panasonic th42pz80u: backlight working but no display

A plasma screen doesn't have a backlight; it emits its own light from each pixel.

::

Yeah, I realized that earlier today when I was thinking about how plasma TVs work. (on a side note I had even taken a course called "Intro to Plasma Engineering" as part of my EE degree ... I am glad my professor didn't hear me say that)

As Sal said, the red power on LED is not throwing any blink codes.

We are not getting any audio. To test the audio, I tried both the RCA inputs on the front and the back. We used a remote from a different Panasonic TV to cycle through the inputs (the remote was able to turn on and off the TV, so we know it works with the TV).

An interesting thing that we just noticed is that the TV seems to be "resetting" itself about every two minutes. After two minutes, the relays on the power supply can be heard clicking, the fans will stop spinning, and the screen will go dark. It will start back up right away (about 1 or 2 seconds) without us doing anything.

Would this be indicative of a power supply issue?

[DavidOH]

Re: Panasonic th42pz80u: backlight working but no display

I had the same set with no picture and no blink code that I could tell. I used this technical guide


http://www.scribd.com/doc/46124232/T...GPH11DU-Chasis


I guessed that I was getting one blink and did the first part of the one blink test. It didn't change so I said yes and when on with the second part. That's when it said the A board was bad.

[Salink]

Re: Panasonic th42pz80u: backlight working but no display

Why did you assume that it was one blink and did that fix the problem? Also when you didn't have video, were you able to get the on screen display?

[tom66]

Re: Panasonic th42pz80u: backlight working but no display

Wow, where can I take a course in plasma engineering?
Everything I know about plasma displays is learned from books, patents and service guides.

Anyway, I think it is an A Board issue. This unit combines the A Board and D Board into one. It's most likely clicking off because it's not detecting a certain condition after a few minutes, like the main processor isn't responding.

[Salink]

Re: Panasonic th42pz80u: backlight working but no display

I've searched through these forms for similar problems. You guys seemed to have helped a few people with the same problem, but most of those guys didn't post results. It does seem to be the A board, like you guys suggested. I'm going to see if I can get a replacement tomorrow and I'll let you know how it goes. How important are revision numbers on boards like these? I've read that Panasonics are touchy when it comes to that.

[Tetranitrate]

Re: Panasonic th42pz80u: backlight working but no display

Wow, where can I take a course in plasma engineering?

Brooklyn Poly , which (as of a few years ago) was bought by NYU, so it is now kinda the NYU school of Engineering. The course was a ton of fun; we only discussed displays in a couple of classes. A couple of other cool concepts discussed were plasma ion engines along with the creation of a leading edge of plasma around high velocity aircrafts to reduce the shock of the air pressure.

The A board and D board are separate in this TV. The D board is underneath the metal plate that the A board is connected to.

The TV is not fully clicking off, it is resetting itself, so I doubt that it is a type of standby mode.

[tom66]

Re: Panasonic th42pz80u: backlight working but no display

Here is where a scope would be handy, to see if the A Board is sending signals to the D Board. I will take punt that it is not.

P.S. How is the Vizio you fixed with the bad ceramic cap going?

[retiredcaps]

Re: Panasonic th42pz80u: backlight working but no display

You guys seemed to have helped a few people with the same problem, but most of those guys didn't post results.

I have nothing to technically contribute, but the above comment is a sore point recently with many regulars who volunteer their precious time to help others and never hear back or have the thread stop midway through.

[DavidOH]

Re: Panasonic th42pz80u: backlight working but no display

Why did you assume that it was one blink and did that fix the problem? Also when you didn't have video, were you able to get the on screen display?

At the time I'd fixed a Viewsonic monitor(replaced board) and done R&R on my PC. I didn"t have a meter to check volts or anything else. I only knew you could change boards. This thread inspired me to try.
http://www.avsforum.com/t/1001318/ma...not-turning-on

The 42pz80u(craigslist $50) was my first try at fixing a TV. At the time I knew of AVSfourm but not Badcaps. At plugin it would prestart and go into standby fine. Turn it on and the power led would come on then nothing. I would have to unplug it to turn it off. I didn't have anything to lose so I assumed 1 blink and went from there. Yours sounds the same, but I could be wrong. I'm just a DIY.

I'm learning to do this as a hobby. Credit for all the info I've found and tried goes to those that do this for a living or have gone to school for it and passed the info on to others. Badcaps and the great people on here has become my first stop for info. This is the longest post I ever made.(Sorry)

[Salink]

Re: Panasonic th42pz80u: backlight working but no display

I can't find an A board online, so I borrowed an NI ELVIS oscilloscope from school and will try to pinpoint the problem.

[Salink]

Re: Panasonic th42pz80u: backlight working but no display

I have a question for you guys. On page 44 of the service manual tom66 posted above, it says:

The result obtained on previous generations when D3 was disconnected, can
now be obtained by disconnecting A6/P6 and A7/P7.
When these connectors are removed, the A, K, GH, and S boards are disabled.
Normally Pin 2 of connector A3 is grounded by Q4014-Q4015. When A6 and
A7 are disconnected pin 2 is no longer grounded. This allows for the 3.3V
regulator on the D board to output 3.3V to the Panel CPU (IC9003).
The Panel CPU (IC9003) outputs a command that is provided to 2 different
circuits.
The “Panel Main On” command (3.2V) is connected to pin 11 of connector P25
on the power supply turning on the circuit that generates Vsus, Vda, P15V, and
P5V.
The “Power On/Off” command (3.2) is connected to the Video/Panel Control
and Discharge circuit to output the panel drive control pulses.
This method of isolation is useful when troubleshooting problems where the A,
GH, K, or S board are suspected to be defective.

My pin 2 on A3 is outputting 3.2V regardless of whether A6 and A7 is connected. From just reading this, would it sound like my problem lies with "Panel Main On"?

I also did some testing and checked transistors Q4014 and Q4015. A diode test on Q4014 resulted in a short between the Emitter and Collector, and a 0.7V drop across all other connections on Q4014 and Q4015. Also, when diode testing Q4015, the TV makes the audible shut off sound, but I was too focused on not shorting anything that I couldn't tell if the screen turned off or the SS/SC LEDs went off either.

What are your thoughts guys?

[MisterD]

Re: Panasonic th42pz80u: backlight working but no display

I'm working with Tetranitrate on this. The power LED won't show a blink code. I can use the power button to turn the TV on, but have to hold it to turn it off. Sometimes the TV will turn on again by itself after I've turned it off. I read in other forms that disconnecting A5, A6, and A7 and having the screen show all white is a sign that there's a problem with the D board. Our screen does glow white after A7 is removed.

We also don't seem to be getting any audio from any source. Since there is no OSD, we tried cycling the input button to see if we can get audio, but either audio doesn't work, or we can't cycle through the inputs.

When cycling through the different inputs wait at least 7 seconds between cycles, it takes that long to take affect on some models.

[Dav]

Re: Panasonic th42pz80u: backlight working but no display

Hi everyone .i am new to this .yesterday my tv th-42pz80u suddenly no picture no sound. . Just very light background light is there. No blinking lights . Some time same channel with sound comes for two seconds . But can not change channel . please let me know if I can see which part has problem.
When I switch on the tv power light and click sound comes. When I switched off then power light goes off but click sound comes couple of second later. Thanks in advance

[johnboy1313]

Re: Panasonic th42pz80u: backlight working but no display

I have nothing to technically contribute, but the above comment is a sore point recently with many regulars who volunteer their precious time to help others and never hear back or have the thread stop midway through.

Again, I have nothing to add to this other than how disappointing it is to spend the time searching and then reading through a thread, just to see it died at the end.