Panasonic LED TV TX-L37D28 don't switch on

[gerry1973]

Hersteller: Panasonic

Typenbezeichnung: LED TV

Inverter: PS-309WW

Chassi: LA02

kurze Fehlerbeschreibung (2-3 Worte): TX-L37D28 don't switch on

Meine Messgeräte::
Analog/Digital Voltmeter

Schaltbild vorhanden?:
Ja, als PDF



Hello,
I have a LED TV Panasonic code TX-L37D28E, the P-board is a PS-309WW, the A-Board is a TNPH0851-1A (TXN/A10QDB), the problem is:
when I turn on the power switch, the front led remains red (no diagnostic blinking) and the P-board relays (which enable 12 and 17Vdc to the A-board) continuously switching between on and off, TV remains OFF .
I have measured all the voltages indicated in the service manual (during 12V and 17 V availability) and all of them are in the expected range.
I have checked the "power on" sequence, it seems the CPU in the A-Board command correctly the closure of the P-Board relays (relay command) but, for a problem, the P-Board generates the SOS signal which request a turn off to the CPU, then CPU removes the relay command and the sequence, after about 1/2 seconds, restart.
What I don't undertsand is why the P-Board send an SOS signal to the A-Board; is it possible is due to overcurrent?
I don't know what other points have to be checked to try to understand if the problem is in the P-board or A-board and, eventually, how to solve the issue.
Thank you very much to anybody is able to help me, let me know if more data are needed!

 

[HermannS]

I'd suspect the backlight. If there was an overcurrent, the P-board wouldn't wait for the A-board to shut down.
I am a bit confused that the SOS does not result in a blinking code of the standby LED.

I don't have the svc manual at hand. Does it contain the schematics of the P-Board?

 

[gerry1973]

thank you;
the p-board schemes aren't in the service manual, but I have found similar one (I have N0AE4JJ00004 but I have found N0AE4JJ00006 and N0AE4JJ00015 schemes, which are very similar to my board).
I haven't checked the voltages for the back light, I will do it asap to verify this possible point of failure.

I confirm no blink codes, the front red led remains ON and the power relays in the P-board continuously opens and close.

Thank you very much!

 

[HermannS]

Hm, if the LEDs were the culprit, I would expect that they flash at least for a quick moment. Well, one can never be sure how the protection circuits work in detail. Relays have become rare in power supplies. In your case, it cannot be a primary relay, which cuts off the power completely. The stand-by would be affected by that. Hence, they cut off the power of a specific primary circuit. I have never seen relays on secondary circuits.

 

[gerry1973]

yes, correct, the relays separes the stand-by circuits from the power curcuits, as in the "p-board 220V" image.
The continuously blinking relays are the S101, S102 and S103. I have re-checked, the red light remains ON, no any kind of blinking; the S101 and S102 remains closed only for half a second, then reopen, so after a second reclosed, and so on.
I'm checking for the exact sequence of the relays.

 

[HermannS]

I see. S103 is bridging the NTC resistor, which reduces inrush current. So, S101/2 come first and then S103 during startup.
The standby circuit is therefore not behind any relays and that's why the red light stays on all the time.

 

[gerry1973]

Hello, I have measured the voltages on the power supply for the rear light and it is ok, so I don't know how to proceed, which other controls and check have to be done.

For the red/green led lights, the managing of the failure blinks is delegated to the A-board, as in the image (outputs from the CPU), so no relays dedicated for this function.



Anyway, in another message in this forum, for a similar problem, user thommyX (LG Thomas) indicate the problem as a possible BGA issue, to heat the board at 215° for 5 minutes (at his own risk...).
Do you think it could be useful or it is better try something else before burn the board?

Thank you!

 

[HermannS]

Ah, so the backlight IS turning on? I just realized that we haven't discussed that properly.

You mentioned the SOS signal. This is supposed to be pulled up from the P-Board, not the A-Board, right? A missing blink is an indicator for a fault on the A-Board, however.
Not a clear situation what we have here. Cause and effect are still in the dark. Have you tried to figure out where the SOS signal is coming (is generated) from on the P-Board?

 

[gerry1973]

I'm sorry, probably I have not listed correct informations (my english is not so good....)
Backlight will not turns on because the command from the A-board to turn on the lamp is missing (the power on sequence will not arrive to this step, it seems it stops in the voltage/currents control check). What I have checked yesterday is the correct presence of the supply voltage on the output of the power supply line, before the PWM which drives the backlight leds (the 174V output in the image).
Also I confirm I have checked all the voltages (as outputs) of the DC/DC converters which are present in the A-Board, all of them are in the correct range; this check has been done during the very short period in which the 12Vdc from the P-board is available (so when S101 and S102 are close for short moment).
I'm going to test the exact sequence of switching of the S101, S102 and S103, to better understand which signals are involved in this process (I have only a tester, the sequence is so quick, so it is difficult without an hoscilloscope).
Thank you very much for the support!

 

[HermannS]

Ah, the mist is slowly clearing up. When the LED_ON is never set high then the boot process gets aborted. This TV is stuck in a boot loop. It tries to start, crashes, and repeats. It is never getting to its regular checks where it might emit error blink codes. It crashes in an early stage of the boot process. We are honing in on a A-Board fault here. In such cases, reading the log output is the diagnostic means of choice. I can't tell you how this works on Panasonic's, unfortunately. Some UART output is required on the board, a little UART (serial port)-USB converter, and a terminal program on a computer.

 

[gerry1973]

thank you very much for your support, I think you are right, the start-up process stops before arrive to error codes.
I haven't any kind of experience on UART for TV, I will open another thread on this point, I hope someone in the forum has some experience on this with Panasonic boards.
I have found this port on the A-board, with anything connected; could it be the serial port?

 

[HermannS]

That looks like the port for the factory programming where they flash the board. Some boards actually use the VGA port as UART, as funny as it sounds.

 

[gerry1973]

I have made another test; when I un-connect the 12V line (cut as image) the CPU runs up to the blinking codes, then I obtain 4 blinks (which correctly indicates 12V missing), so I think the software in the CPU works correctly; there is something in the middle which opens the relays before let the CPU arrives to failure codes.
I'm searching for the signal which opens the relays.

 

[HermannS]

That's interesting. Have you tried unplugging the TCON board?

 

[gerry1973]

Yes, nothing changes removing TCon.
Anyway, I will made other checks in the next days and give feedback, I'm thinking to try to force the closure of the relays and see what happen (if blinking codes starts or not), I hope nothing burns!
Let's see...