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Old 04-26-2022, 11:04 PM   #1
HonkBeep04
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Default Panasonic TC-P65ST50 plasma in protection mode after incident

Hello everyone! I picked up this 65" ST50 plasma TV for $80 on Facebook marketplace two weeks ago. I have the basic electronic knowledge of a teenage computer geek, having assembled laptops and PCs and messed with soldering and software.

This afternoon I decided to disassemble it to mitigate a buzzing noise. This is a well known issue for this TV ever since they were new. The popular solution is to replace the loose internal screws for ones with lock washers, preventing any future buzzing. I figured that I could just fully loosen and tighten all of the screws securing the sustain boards to make it go away. After doing so and putting on only the wall mount screws to secure the rear panel, I turned the TV on. I was disappointed because the effect of tightening the screws seemed to be minimal compared to beforehand.

In the spur of the moment, I thought I could stick something into the vent grate to stabilize the board and/or feel the vibration coming off. My tool of choice was a metal allen key and I stuck it in and touches one of the hollow metal arches marked on the circuit board for high voltage. Of course, there was a fat spark and a loud bang, and the panel immediately went totally black. I cannot say there was any fault but my own here; my only hope is that I learn a hard lesson not only of proper conduct but also how to fix the mess I've made.

I took it back apart but I couldn't find any visually bad components or tested bad fuses. I figure anyway that a failure that immediate wouldn't blow a fuse or cause a capacitor to fail in a visible way. I put the TV back together to start doing research.

When it's first plugged in, a relay clicks regardless of what else goes on. When the power button is pressed, a relay clicks but then clicks off again after a few seconds. The power light blinks in sets of three repeatedly. I found one source that instructed reset it by unplugging the TV, holding down the power button, and then plugging it in and holding it. When I did that, the relay still clicked when it was plugged in, but the power light and the other relay didn't come on until it was released.

I found this link showing that the 3-blink alert on a concurrent model Panasonic meant a failure of the A-board.
https://www.reddit.com/r/TVRepair/co...changeability/
However, every other piece of information I can see seems to point to the power supply board. The hope would be to repair some superficial component, but really I just want to see if this will be worthwhile to fix. Thank you to anyone who read this far and for any information provided. I am grateful for the wealth of information available on this forum.
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Old 04-27-2022, 10:43 AM   #2
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

You turned something that could become a problem, into a very difficult or costly repair.
If you poked into the SC board, the 3 blinks signal is from the power supply, because the VSUS output is shorted, your SC board has shorted components, which are hard to repair and this board is rare.
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Old 04-27-2022, 11:25 AM   #3
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

Thank you for the reply. I found an SC board on Ebay for a price I would be willing to pay to fix the TV. Perhaps I will peruse some guides to see if there is a way to confirm the failure.
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Old 04-27-2022, 11:41 AM   #4
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

open the back and unplug the cable going from the power supply to the SC, if the blink code changes the SC is the problem.

The service manual says that 3 blinks is from main board, but you need to try this test before.
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Old 04-27-2022, 08:05 PM   #5
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

I took the back off again and found that what I touched was the SS board. I unplugged it and the blink code changed to 8. I then plugged it in and unplugged the SC board, and the blink code went back to 3.

I would normally assume that the blink code not changing when unplugging a bad component would mean that the component is indeed bad, but I think I understand what you are saying. With the power going to SS, the main board can detect a fault, whereas with power cut it detects only a lack of communication and thus sends a different code. Assuming my understanding is correct, if the board were good then the code would not change because the problem is more severe with another board.
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Old 04-27-2022, 11:06 PM   #6
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

This tv may power up without the SS, unplug all cables going to it.

If it does not work, then you can power up the PSU + motherboard only by unplugging the 3 ribbon cables that goes to the C boards and SC.
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Old 04-28-2022, 09:24 AM   #7
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

I unplugged both the power cable and the ribbon cable going to the A board and plugged in the TV. When turned on, it showed a dim, purple image of the no signal screen. I assume this means the SS board is bad then, I should be ordering one soon.
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Old 04-28-2022, 10:03 AM   #8
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

I've been looking on eBay for replacement boards, and I found by searching the model number that there are varying prices for the same model number (TNPA5648) board with a different "order number" stamped on. Mine is AE and other P65ST50 boards are also AE. Some P60ST50 boards are AF and some are unmarked. The boards from the P60GT50 are usually AB, and I have also seen an A3. I should generally assume that this order number means no interchange, correct? The AE seems to be the most expensive.

Edit: I have just noticed that the GT50 board has more components noticeable like an extra capacitor and metal loop thing.

Last edited by HonkBeep04; 04-28-2022 at 10:08 AM.. Reason: Just noticed extra information
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Old 04-28-2022, 10:34 AM   #9
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

Only the SS board is defective, lucky.
You should use an AE variant then, the others will require some changes in component placing.
The screws repair is done by swapping them with screws of proper length with washers, cleaning the ground pads on the boards and on the TV metal frame itself, look at this blog post for more information https://alpengeist-tvrepair.blogspot...d-7-blink.html

Last edited by Biruslapio; 04-28-2022 at 10:38 AM..
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Old 04-28-2022, 04:48 PM   #10
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

I have the same tv. Got it with the blown power supply capacitor, driver ic,(str y6766) and the fusible. 56 ohm resistor.
I found the 331 2kv capacitor and the str y6766 ic in my spare parts. Only thing I could find for the resistor was a .22 ohm.
I put it all back together and the tv turned on and looked great for about 7 minutes. It blew the controller ic again. Does anybody think the fusible resistor was the cause of the failure?
I can't find a replacement anywhere.
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Old 04-29-2022, 01:13 AM   #11
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

The original resistor was 0.56 and you replaced with 0.22? Is this the one that goes to the drain of the main switching MOSFET? If it is then it's clearly the resistor.
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Old 04-29-2022, 12:59 PM   #12
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

I agree, the source resistor voltage drop is monitored by the ic and changes its operation based on the mosfets current. You say it blew the controller ic again, did the source resistor also go open? You don't say the wattage but digikey has 560mΩ (0.56Ω) resistors, I would try for a non-inductive or use a metal film type

Last edited by R_J; 04-29-2022 at 01:05 PM..
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Old 04-29-2022, 02:33 PM   #13
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

The replacement resistor (.22) did not go open. It is circled in red.
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File Type: jpg sketch-1651264255397.jpg (239.8 KB, 10 views)
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Old 04-29-2022, 02:41 PM   #14
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

Ah correct, the resistor is connected to the Source not the Drain.

It would only go open if the mosfet also blew, since only the driver had problems, you can try to do an association with parallel or series, just need to match very close to the 0.56 ohms and power rating of the original resistor.
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Old 04-29-2022, 03:35 PM   #15
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

I bought a .56 ohm 5w and a new str-y6766. I'll report back when they arrive.
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Old 04-29-2022, 05:47 PM   #16
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

R414 is not the source resistor it is just a fusible resistor in the supply voltage line. The source resistor is R403, I would check that resistor and the diode D411, If the ic shorted, D411 may have shorted and now R403 would be bypassed by a shorted diode. With pin 2, S/OCP connected directly to ground and not through the resistor, the ic can't monitor the current.
I should add that having a lower resistance for R414 should not have caused a problem.

Last edited by R_J; 04-29-2022 at 06:59 PM..
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Old 05-05-2022, 12:58 AM   #17
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Default Re: Panasonic TC-P65ST50 plasma in protection mode after incident

The SS board arrived today. I installed it and all is going well. I also ordered one of the official Panasonic part number screw packs, but when I got them they were way longer then the existing ones. I didn't want to take the chance on breaking the panel, so I put in the old screws. The new board has very little buzzing so I'm pretty sure it was actually caused by the coils going bad, which is another common failure for these TVs later on.

Thanks so much again for your help. You really saved me a big headache!
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