Panasonic TH-42PZ80B - Completely dead (no power LED or clicks)

[tom66]

Just picked this one up on eBay. Was mis-listed as 37" plasma (going to be 720p), so I thought it wouldn't be worth it -- but upon getting the model number I found it to be a 42" 1080p. £50 plus fuel, not bad (to Manchester from Leeds and back.)

It has no power at all, no standby LED.

Same as my 46" TH-46PZ80B (except a little smaller.)

Check the 5V standby from PSU: missing.
So checked the output from standby transformer: that's OK, 5.1V.

Traced the diagrams and found that the 5V standby is only switched on if no faults are detected. Tried jumping it and got 5V standby but it didn't proceed further (no clicks -- but didn't check power LED, silly of me I guess.) One detected fault is AC_DET, which measures 0.11V, on the MC552 module. I think it should be high (~5V.) However, Panasonic have not provided schematics for MC552 (only for the power supply as a whole), and do not indicate what AC_DET should measure.

Has anyone encountered this issue? Anyone who has a TV with a working EXT2MM702MF power supply who can measure AC_DET? It's on pin 16 of MC552.

I'm on the verge of tearing apart my 46" to measure AC_DET :P. I'm going to try and avoid that, for now.

When I get this working I face the hard decision: Do I put the 46" in the lounge and 42" in the basement, or the other way around?

[tom66]

Re: Panasonic TH-42PZ80B - Completely dead (no power LED or clicks)

OK, I got a little bored so decided to trace out MC552. With a little knowledge about common circuits it's not too hard. Only know two values of components because they were big enough -- the rest is a mystery, but the architecture of the circuit is easy to figure out.

AC line being applied through approx ~1.5Meg resistance and a 43V zener diode to AC-3 and AC-4 (one live, one neutral.)

For AC-3 (operation identical for AC-4): Pulse appears on AC-3, goes through diode D1 and is reduced by resistor R2 (loads down external network.) Rise time limited by C1 and base protected by Z1 (may be a normal diode, can't tell.) Resulting signal (around ~0.7V) couples into the base of the NPN (NMOS?) transistor Q1 turning it on. Transistor pulls down collector through resistor R3, causing a current to flow through C2 and R4, causing a low pulse to appear at junction between R4 and R5. (Normally high by R5.) C2 and R4 forms RC time constant, probably tuned to last a few cycles of AC mains so loss is detected shortly after power is removed. Signal (probably ~0.7V below BIAS which is 9V nominal) goes into PNP transistor Q3. The negative base-emitter voltage turns on Q3 (PNP), conducting current into the opto through R9. This current through the opto's LED turns on its transistor, connecting 5V standby to the AC-DET.

...if you read that all, congrats. Basically: AC-DET should be active high, 5V. If Panasonic had specified this on the schematic, this would have been all unnecessary. But good brain training!

What's confusing me is: My 46" will power on and the standby LED will stay lit with no AC power until the cap on the primary for the standby discharges (56uF 450V), which takes about a minute. If you try to power it on without AC power it'll briefly flash the LED (two red fast, then green) then the LED will go dark until plugged in. And yet, the 42" won't power up without an AC det signal at all.

My theory: The power control micom is looking for that initial "AC is present" pulse. If it doesn't see it, it never turns on. I guess if AC power is lost during operation it turns off the main supplies and sees fit to turn the standby off, but if the AC power is lost during standby it leaves it on. Perhaps some kind of safety discharge for the standby supply? Or to handle brief power interruptions? Very odd. It would also be helpful if Panasonic provided a basic flowchart of what the power control micom does.

Luckily, I should be able to bodge this one. I'll need to look into the consequences, if any, of forcing AC-DET high, if I can't repair this board... and I don't know what's wrong with it yet! I don't forsee any, except perhaps a blink-code error if you unplug it while running. (I'd expect 2, 4 or 10 blinks as the mains power disappears.) Perhaps the panel will dim before it goes off completely.

I just had a brain wave: Perhaps the reason it turns off the standby with no AC power is because it has no way to tell the main board that the power up failed without throwing 2/4/10 blinks. And customers may get worried if their TV blinks 2/4/10 times continuously after power plug is removed. Also, this explains why the TV won't even show standby LED with AC det missing. It performs a power-on self-test when it is first plugged in and the first step of this is to turn on the 15V main supply. If this fails the P board throws 10 blinks (which is a common fault but not the one with this model.) The code is written so that it decides there's no point in trying the first power up without AC. Could be nice if it threw 4 blinks or some other code instead though. The red-red-green is just the A board getting ready to boot, and sending the command to boot to the P board at the end of the green flash. No relays click in this mode, so clearly the micom isn't even attempting to turn them on (there'd be enough energy IMO to turn them on if the cap was nearly fully charged.)

The rest of MC552 is OVP or UVP for the 5V standby, I didn't trace it as it's an independent circuit.

[vinceroger69]

Re: Panasonic TH-42PZ80B - Completely dead (no power LED or clicks)

did you find the problem on this tom just saw this thread.

[tom66]

Re: Panasonic TH-42PZ80B - Completely dead (no power LED or clicks)

Still working on it but I've got a possible fix coming up.

[tom66]

Re: Panasonic TH-42PZ80B - Completely dead (no power LED or clicks)

So, I was working on the PSU today, and I had MC552 out for testing. Decided to check the optos on it with the help of a friend, and they appeared OK (got 0.2V diode drop across output when LED driven with second meter, OL when not.) So, I decided to do some tests, reinstalled MC552, and plugged it in.

Click-Click. Red power LED... Uh, OK. This didn't happen before.

So, cautiously, I hold the CH- down, to turn the TV on. The heatsinks aren't screwed in, so the transistors could overheat. But it powers up, and shows "NO SIGNAL". Wow, umm, that... that's good, I guess.

Take the PSU out and install about 50 million screws... Power it up and run test patterns. Panel looks great, no burn in or stuck lines. And I've been using it since... it hasn't skipped a beat.

My guess is, MC552 had a cold joint on the connection going to the P board, and the removal of the board and reinstallation (new solder applied) has helped it. However, I'm probably going to put this in the kitchen for the other guys to play with for a couple of days, to see if it lasts. If it does... well, that's not bad for £50 + £15 fuel!

Too small for the living room though when put against the 46"!

And to anyone who said you can have too many plasmas... you were right. It's hard to move in this place any more. Luckily, we move in soon, so I'll have some more space.

Also, I have to tweak this one, for ultimate blacks. Let's see what I can get out of this panel.

[vinceroger69]

Re: Panasonic TH-42PZ80B - Completely dead (no power LED or clicks)

another one saved tom these Panasonic plasma's do look nice sets are you still fixing many lcds yourself or concentrating on plasma's more as they seem to go for more money/profit once repaired.

[tom66]

Re: Panasonic TH-42PZ80B - Completely dead (no power LED or clicks)

I'm keeping this one. As well. This one just popped up on eBay so I -had- to grab it, too good to miss.

I do fix quite a lot of LCDs, however, I'm currently taking a break and selling off all the old stock, and looking out for a 32"~37" for my bedroom, and another 32" for a friend. (Good brands, no Vestel crap. Toshiba, LG, Sharp, Panasonic, Samsung etc.)

No idea how I'll fit it all in here!!!

[vinceroger69]

Re: Panasonic TH-42PZ80B - Completely dead (no power LED or clicks)

gumtree seems to have some good bargains on at the minute not sure in your area but mine defently has just need the cash to buy them lol also i am saving to buy a reasonable priced hot air station what model did you go for?

[tom66]

Re: Panasonic TH-42PZ80B - Completely dead (no power LED or clicks)

I bought an AOYUE 968, but NEVER again. Worst purchase ever. Buy an old Metcal or Weller hot air used.