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Old 11-21-2021, 02:06 PM   #1
Ierwin
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Default Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

Hello everyone,

Ive been a member of this forum since 2017 and always found so much useful informaiton that I never even had to ask questions myself. Now I have a TX-P50STW50 Panasonic Plasma TV on my desk that I just can not make sense of.
As a little disclaimer: English is not my first language so please bear with me in case I do not always use the correct technical terms.

So the previous owner of this set told me that the TV started clicking and restarting every now and then. These events got more frequent until it became completely dead. No reaction whatsoever.

When I first got it suspected some of the caps that are close to the heatsinks to have leaked. They were not buldged or anything but they had black spots on the top that in retrospect may have just been black marker. But since I had them in the drawer I replaced them.

After refitting the Power Board and about one minute after I had plugged everything in, the relay started clicking, but the tv wouldnt start up. I unplugged all other boards from the power supply and started to look for the voltages (there is very good documation in the service manual: https://elektrotanya.com/panasonic_t.../download.html).

So on the Plug P6 (see Manual Page 48) there was 5V on Pin 6 (STBY). I disconnected all boards from the PSU and connected these 5V to Panel_Main_On, STB5V_ON and F_STB_ON using 1000Ohm Resistors each.
This resulted in exactly the same continous clicking.

I then went ahead and looked for teared soldering connections and found a couple that looked suspicious. I resoldered some when I made a (probably) big mistake.
Of course I had discharged all the bigger capicators before I started to work in the PSU with a soldering gun. But I must have missed one of the smaller ones that still had a charge.
So I was trying to resolder the central Pin of Q110 and I was working from the top of the board as it is exposed there. When I applied the solder I accidently touched the heatsink and caused a little spark. I am almost certain that the short must have been between the central pin of Q110 and the Heatsink.

So this is where I am now. Ever since this incident the clocking is gone. The PSU seems completely dead, no 5 Volts no nothing.
Ive been trying to understand the 5 Volts generation circuit all day but just can't make sense of it. I am pretty sure that whatever is causing the 5V to fail atm is just caused by my dumb mistake and not even the root cause of the tv breakdown, but thats the first step that needs fixing.

So I traced all over the place. To my understanding IC501 is some kind of linear voltage regulator that I think is one of the most important part of the 5v generation. Pin 1 should be VDD and is simply connected to Pin 9 of the Transformator T200 over a Diode.
However there is no Voltage on this Pin, so whatever the problem is, must be on the primary side of T200.
Im having a hard time understanding in which order these components are supposed to come up. For example the IC200 "STBY CONTROL" does not have Vcc either.
I think I must have killed something in the part that generates the primary current on T200. There are several SMD transistors involved here and the only one I think I have confirmed working is Q200. This has 5V on one pin and apparently is switching to Gnd and I think this is what it is supposed to do.

I would really appreciate if someone whith more knowledge could take a look at the shematics and help me trace this error.

Best regards

Kolja

Last edited by Ierwin; 11-21-2021 at 02:33 PM..
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Old 11-26-2021, 04:33 AM   #2
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

I made some progress yesterday so I want to give an update.
A little breakthrough was made when I finally flipped to page 2 of the datasheet of the IC 34A65 (IC200 "STBY Control").
When I saw the example circuit I finally realized why I had been tracing in circles all across the noard for hours and hours.
The Standby Cotrol IC is driven by high voltage!
So basically the rectified main goes right into this IC over some resistors.
Once I understoof that I quickly found that one of the Resistors (R224, 2.2Ohm 2W) had failed. I have no Idea how since it shows no sign of burn or whatsoever, but it has unlimited resistance so its definetly gone.

I replaced the Resistor and suddenly everything came to live.
5V on the mainboard connector as well (YEAH!)

I was at my parents place and only had the Powerboard with me, so I once again connected 5V to Panel_Main_On, STB5V_ON and F_STB_ON using 1KOhm Resistors. After some initial Clicking and Clacking the relay just made a single "Click" when I connected main.
I think at that point the Powerboard was actually fully working and hat it been inside the TV I think it would have started.

But I had to get the board back home first. And back here its stupid thing doesnt want to play along anymore.
No 5V on the connector.
IC501 (linear voltage regulator) is getting 5.5V on the input side but does not output anything.
Could it be gone? Could it have been slowly failing all this time? Or maybe there is a short that pulls down the output voltage? Is 5.5V even enough for the voltage regulator to properly work or should I be looking in front of it?
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Old 11-26-2021, 01:28 PM   #3
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

So RL100 makes no sense. Appears to short input. 5.5v into regulator is not a good voltage. Need pictures. Also # off ic200
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Old 11-26-2021, 06:05 PM   #4
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

So what voltage are you getting on pin 8 of ic200? If no voltage and 2.2ohm is burnt again check Q110 for shorts.

Last edited by dskall; 11-26-2021 at 06:27 PM..
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Old 11-29-2021, 03:12 PM   #5
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

Thank you very much for looking into this.
I hope I can provide some more measurements later when I am at the board, but for now I can at least give an update from memory.
Since the 2
2Ohm resistor was replaced ic200 seems to perform well. It's getting 310Volts on the HV input and if I remember correctly is outputting 15V.
What I have done since my last post:
I disconnected the output leg of the 5v voltage regulator from the rest of the circuit.
With this connection open the regulator has 7.2V on the input and 4.6 on the output.

I suspected the regulator to be faulty and used a 78S05cv as a temporary replacement just to see what it does. This also causes the voltage on the input line of the regulator to drop and the output of this regulator is about 3,5Volts.

I suspect that either the input of the regulator is lacking Power so the voltage drops under load or a short on the output side of the 5v regulator.
Actually I think it is the first because I connected an external power supply to the 5V circuit and the circuit was pulling about 250mA (not connected to mains),which seems reasonable an not like a short.

Maybe the error is on the primary side of T200. If I remember correctly Q110 is involved there as an amplifier of the driver output of ic200. Oh by the way ic200 is a 34A65.
I had previously checked Q110 for shorts bit it seemed ok.
If you need more measurements or Pictures let me know
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Old 11-29-2021, 07:28 PM   #6
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

The fact that you are getting voltage on the T200 secondary means the primary circuit, ic200, Q110 are working, but the circuit is current limiting under load.

The schematic is too blurry (f'n Panasonc) but have you replaced the capacitor (C204?) 47f/35v D205 supplies the vcc voltage by charging this capacitor.
Sometimes the diode in the source pin of Q110 can short causing the wrong voltage back to the CS pin of the ic, The diode is across R207 a 0.5Ω so one end of either the resistor or the diode needs to be unsoldered to check the resistor and also the diode.
Also the capacitors on the +5v line may be weak and need to be replaced.

As a side note: RL100 contacts short across R100 when the tv is turned on, R100 is a current limiting resistor for when the tv is in standby and only draws a small amount of current.

Last edited by R_J; 11-29-2021 at 07:49 PM..
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Old 11-29-2021, 07:50 PM   #7
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

As a side note: RL100 contacts short across R100 when the tv is turned on, R100 is a current limiting resistor for when the tv is in standby and only draws a small amount of current.[/QUOTE]

I was looking at block diagram on page 36. Didn't make sense.
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Old 11-29-2021, 08:16 PM   #8
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

Quote:
Originally Posted by dskall View Post
As a side note: RL100 contacts short across R100 when the tv is turned on, R100 is a current limiting resistor for when the tv is in standby and only draws a small amount of current.

I was looking at block diagram on page 36. Didn't make sense.
Using the block diagram and looking from the a/c filter LF101, one line connects to the ac ~ input of the bridge D100, the other line connects to R100 or RL100 pin2 and then to the other ac ~ input of D100 bridge.
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Old 11-30-2021, 04:12 AM   #9
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

Quote:
Originally Posted by R_J View Post
The schematic is too blurry (f'n Panasonc) but have you replaced the capacitor (C204?) 47f/35v D205 supplies the vcc voltage by charging this capacitor.
Sometimes the diode in the source pin of Q110 can short causing the wrong voltage back to the CS pin of the ic, The diode is across R207 a 0.5Ω so one end of either the resistor or the diode needs to be unsoldered to check the resistor and also the diode.
Also the capacitors on the +5v line may be weak and need to be replaced.
I had unsoldered the Diode that is in parallel with R207 when i still hadnt found the failed R224 (2.2Ohm). Both seemed okay. I can not completely and 100% rule out that the Diode was reversed when i put it back. I mean I triple checked but in the end its such a tiny part and there are no readable markings on it... But Im 99% sure its correct.

C204 is a new one. Which caps are your referring to by "capacitors on the +5v line"?
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Old 11-30-2021, 08:43 AM   #10
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

Oh and one more question @R_J:
Am I getting you right that it might be the 34A65 limiting because it senses an error on the CS pin?
If I was able to understand the 34A65 Datasheet, I could measure whats happening on CS and see if it is correct?
This stuff is way above my head so far but I am here to learn, so Im trying to understand.

I attached the table that describes Current sensing of 34A65. I dont know if any of this can be measured without an oscilloscope but I should be able to get my hands on one if needed.
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Old 11-30-2021, 11:04 AM   #11
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

Quote:
Originally Posted by R_J View Post
Using the block diagram and looking from the a/c filter LF101, one line connects to the ac ~ input of the bridge D100, the other line connects to R100 or RL100 pin2 and then to the other ac ~ input of D100 bridge.
Ok, it makes sense when I eliminate the relay and go straight to bridge rectifier. I thought schematics where confusing. Block diagrams are worse.
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Old 11-30-2021, 11:24 AM   #12
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

Quote:
Originally Posted by Ierwin View Post
I had unsoldered the Diode that is in parallel with R207 when i still hadnt found the failed R224 (2.2Ohm). Both seemed okay. I can not completely and 100% rule out that the Diode was reversed when i put it back. I mean I triple checked but in the end its such a tiny part and there are no readable markings on it... But Im 99% sure its correct.

C204 is a new one. Which caps are your referring to by "capacitors on the +5v line"?
If D204 is indeed backwards, that could be the problem. Can you easily remove either the resistor or the diode to check? you should be able to use your meter in diode test to find the correct direction of the diode.

The caps in the secondary 5v supply seem to be 470f/25v, one is by itself off D505 and the other three are off D501

The CS pin voltage changes at the operating frequency (65/100kHz), You can try checking the voltage with a meter but you need to be very, very careful where you measure it. Try not to probe right at the ic, one slip and it could be a disaster.

You could also try checking the feedback line to the ic, If the feedback is not working the ic will not detect the drop in voltage on the secondary (5v) and not increase the drive to compensate.

Last edited by R_J; 11-30-2021 at 11:41 AM..
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Old 12-01-2021, 02:04 AM   #13
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

I unsoldered R207 and measured it at 0.6Ohm which seems good enough.

Then I tried to check the Diode and Im not quite sure what I am seeing and if it is at all possible to check it in the circuit. I attached a picture of the actual board and he readings.
In diode mode my meter reads 0.737V from T200 towards IC200 and 0.484V in the oposite direction. I assume that means the diode is not reversed and not shorted, right?
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Old 12-01-2021, 10:23 AM   #14
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

The diode is not shorted, but it is hard to say if it is reversed or not, use some alcohol to clean the top of the diode so you can see the letters or marks. in diode mode it should read around .6xx volts one way only. the cathode would be the negative lead of the meter when it reads .6xx v
You don't indicate which way your probes were so the arrows don't mean much. was the negative probe (black) at the point of the arrow?

Last edited by R_J; 12-01-2021 at 10:46 AM..
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Old 12-01-2021, 02:52 PM   #15
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

Oh, you are right, these errors do not make much sense.
They are always from black probe to red Probe. The point of the arrow is where the positive prove was.
I will try to clean up the diode later

*edit*
I was not able to uncover any markings on the diode so I eventually gave in, unsoldered it and measured it outside of the circuit.
I even put another larger diode with a visible marking next to it to make sure I do not do anything stupid with the probes.
I am now 100% sure that the diode is correct.
Oh and yes... I had it wrong before .
But the bad news is: it makes absolutely no difference to the 5V line.

*edit2*
Tried to measure CS on IC200 (I used R228 and pin 5 of T200 so I dont short the pins).
0.001V in AC mode. I have no idea if my meter is capable of measuring AC at the frequency it needs to

*edit3*
And Im reading 0.776V on the Feedback Pin

Last edited by Ierwin; 12-01-2021 at 04:16 PM..
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Old 12-01-2021, 05:48 PM   #16
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

I realized I did not say it in detail in my earlier post:
0.7V is also what can be measured on the output side of the 5v regulator.
I have not traced or understood the feedback circuit yet, so this might be a coincidence
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Old 12-01-2021, 06:23 PM   #17
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

What where you getting on pin6 of ic200. You said you do have voltage on pin8. Did you check stand by with board connected to TV.
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Old 12-02-2021, 12:58 AM   #18
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

305V on pin 8 (HV) and 15V on Pin 6 (VCC).
All these measurements are with the Powerboard disconnected from all other Boards and just the 230V Main connected.
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Old 12-02-2021, 10:51 AM   #19
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

Maybe it's powering down with no load connected.
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Old 12-02-2021, 04:35 PM   #20
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Default Re: Panasonic TX-P50STW50 Power Supply no 5V (was clicking)

I am 100% sure that the 5V should come up without anything connected to the board.
Lets take a step or two back.
Last thursday I had the Powerboard over at may parents place for debugging with my dad who has way more knowledge in eletronics that I have (although he says he never understood modern power supplies).

We had the Powerboard on the desk with nothing but Main connected and finally found the faulty 2.2Ohms resistor. We didnt have the correct resistor at hand so we replaced it with a series of four... I forgot the values, must have been 0.5Ohms. Maybe slightly higher, because I remember we debated if we use 3 or 4.
Once that was in and Power was connected I had 4.6V on the output side of the voltage regulator and on the mainboard connector. And then when I connected 5V to Panel_Main_On, STB5V_ON and F_STB_ON the relay started clicking again.

I have a large box of resistors at my place so I assumed I should have a 2.2Ohm. I unsoldered the 4 resistors in series and took the board home.
After hours of searching through the box I realized I do not have a 2.2 at home either so I used 4 10Ohm in parallel.

Since then there has never been 5V on the output of the regulator.
It frustrates me to no end because I know it has been working and I didnt even change anyhitng (critical) afterwards
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