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Old 11-21-2021, 02:06 PM   #1
Ierwin
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Join Date: Jul 2017
City & State: Braunschweig
My Country: Germany
I'm a: Knowledge Seeker
Posts: 57
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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