Panasonic TNP8EP101 - no standby

[yohnsee]

Hello everyone,

a long time ago I replaced the PSU of a TX-L42 series LCD Tv. Now I have the bad PSU and I want to repair it, but have no luck so far.
I attach the schematic and the part where only the power supply unit is visible.
I have spent many hours figuring out, why mine won't start, but so far I have no answer. All the time I was looking at IC803 (A6259H) and its surroundings, but found no faulty component. Only thing I can't test is the transformer (T802) but I don't think it is faulty. I have ordered 2 replacement ICs for IC803, but when I wanted to start the circuit with the new one, it made the same nothing as the one before. So I would say that IC is fine. I measured every resistor, every diode, nothing seems off. I may be overlooking something obvious, but I can't put my finger on it.
Only strange thing is the Vcc line (PIN 5) of this IC. When I connect the mains, it slowly ramps up (in about 3 seconds) to 9.5V, and stays there. This IC needs around 15V to start, and as I read the datasheet, it should produce this startup voltage itself.

[R_J]

Re: Panasonic TNP8EP101 - no standby

The ic will produce a small startup voltage with the internal startup cell, once the ic pulses, the transformer t802 will produce the run voltage, so check l807, r851, D826 and C851

[yohnsee]

Re: Panasonic TNP8EP101 - no standby

The ic will produce a small startup voltage with the internal startup cell, once the ic pulses, the transformer t802 will produce the run voltage, so check l807, r851, D826 and C851

Hello R_J, and thank you for your comment.
These were the parts I first tested, but now I tested them again. Nothing is bad, shorted, even the cap has its nominal value.
I soldered a DIP8 socket on the board, so I can quickly change the IC. I even plugged the board in line voltage without this IC. And it confirmed your theory, without this IC there is absolutely no voltage present on the VCC line. I wonder why it climbs up to 9.5V only?

[R_J]

Re: Panasonic TNP8EP101 - no standby

Get the datasheet for the ic. It is not a theory. The ic internally takes voltage from (7,8 D) and supplies vcc at about 1.1ma. to start the ic. The ic will start which should cause a voltage to be developed in the secondaries (only for a few pulses) this will should be enough to induce about 15~20volts High frequency a/c across pins 1&2 of T802.
Check that R838 is good, if IC803 was bad, shorted, it is very likely the resistor opened up. You need to disconnect one end to check it because sometimes D844 can short and it will look like the resistor is good.

Check that there is no short across D822 or D846, if there are no shorts, check if there is any voltage at all on C861

[yohnsee]

Re: Panasonic TNP8EP101 - no standby

Get the datasheet for the ic. It is not a theory. The ic internally takes voltage from (7,8 D) and supplies vcc at about 1.1ma. to start the ic. The ic will start which should cause a voltage to be developed in the secondaries (only for a few pulses) this will should be enough to induce about 15~20volts High frequency a/c across pins 1&2 of T802.
Check that R838 is good, if IC803 was bad, shorted, it is very likely the resistor opened up. You need to disconnect one end to check it because sometimes D844 can short and it will look like the resistor is good.

Check that there is no short across D822 or D846, if there are no shorts, check if there is any voltage at all on C861

I tested every component you mentioned above twice, also by soldering them out and measuring out-of-circuit. Everything is OK, and I am starting to pull my hair out. Normally it is not a big deal to repair a SMPS with schematic, but this time it is.

[R_J]

Re: Panasonic TNP8EP101 - no standby

Are you getting any voltage at all on the secondary side on D822, even a volt or two?
If the ic is trying to start it should produce something.
The other thing could be causing the problem is that Q810 and Q813 circuits are preventing the ic from starting, What is the voltage on pin 4 of the ic?
I believe for the circuit to work, Q810 should be turned on, which would turn off Q813 allowing the feedback voltage to control the ic, If Q810 off, Q813 will be turned on effectively grounding pin 4

[yohnsee]

Re: Panasonic TNP8EP101 - no standby

Are you getting any voltage at all on the secondary side on D822, even a volt or two?
If the ic is trying to start it should produce something

No, I have nothing, not even a tenth of a volt. I used my oscilloscope to see if my multimeter is too slow for this measurement.

[R_J]

Re: Panasonic TNP8EP101 - no standby

What about the voltage on pin 4?
if there is nothing at all on the secondary the ic is being held in the off state so it does not try and start, not even one pulse.

[yohnsee]

Re: Panasonic TNP8EP101 - no standby

Update on the case (also, good news everyone)!

I found the bad part. The little bastard (disguised as D825) was measured 30K Ohms in one direction, 68K Ohms in the other direction. As I changed nearly every single diode and resistor in the standby part, I also removed D826 (better be safe than sorry) and soldered a good-looking 1A schottky there.
And now I have standby (5.7V). Thank you again R_J, you helped a lot (again).
After these seemingly hard repairs I feel a little pride, that I could also do this.
Why is it always these pesky diodes? Last time a zener was messing with me, now this? Unbelievable.

[R_J]

Re: Panasonic TNP8EP101 - no standby

Good to here its working

[alltvrepairs]

Re: Panasonic TNP8EP101 - no standby

Used this thread today after so many years

In my case for no standby voltage 0v, Pin 5 IC 803 12v. Found 1MR R887 Open circuit.

Pin 4 was 0v. With main board disconnected and supply on, Pin 4 rose to 0.35v when tv in standby. Pin 5 supply rose to 18v.