Panasonic P50ST50H - No power - HELP please

[BEpsilon]

Hello everyone

As you can see from my profile, I am a total newbie when it comes to TV troubleshooting or repairs.

Patient is a Panasonic TH-P50ST50H Plasma TV, which was purchased new back in 2013. This TV runs on 220-240V AC (Hong Kong market).


I was doing some cable management and cleaning over the Christmas holidays (Merry Christmas everyone), so I decided to disconnect power to TV/STB right at the power bar (surge protected).

As luck would have it, the TV can no longer turn on when I restore power to everything. TV was running absolutely fine prior. Sigh.


I truly hope to rescue it as I really like the picture quality, even though it's no GT/VT series.
I can no longer buy a new plasma, and too poor to afford a new OLED TVs (from Panasonic/Sony) of equivalent quality/size right now.

Here are the initial findings:

• Power bar confirmed OK - AC voltage good!
• Power cord/plug to the TV is OK.
• Pressing power button on the side of the TV has no effect.
• Pressing power button on remote has no effect.
• LED indicator (bottom left front of TV) is completely OFF.
• No relay click (I recall there is always relay click at power on)
• No magic smoke or burning stink. No noise.

At this point, I guess it's dead due to power supply failure?

I have taken off the rear metal cover, then disconnected all connectors off the power supply board (TNPA-5567-AD), to quickly check for any visibly damaged/burned/exploded components or bulged capacitors.
There is nothing of that sort, fortunately.

The two fuses, one at the bottom of the center heatink and one next to the set of 3 high-voltage capacitors, are all good, according to my multi-meter.

What is the proper course of action please?


Your help is greatly appreciated. Thank you very much!

[andyb1712]

Re: Panasonic P50ST50H - No power - HELP please

Check the 145degC thermal fuse/10R resistor to the left of the lower large capacitor. If good, it should measure 10 ohms. If it is open circuit then you won't get any AC supply to the PSU.

There may be other components failed that caused this is blow, so check for shorts on transistors etc under the heatsinks before replacing it.

[budwich]

Re: Panasonic P50ST50H - No power - HELP please

when you said "power cord / plug" to tv is good... where did you check the voltage? At the outgoing connector shown in your picture of the back of the set ... after the power cord receptacle.

[BEpsilon]

Re: Panasonic P50ST50H - No power - HELP please

Check the 145degC thermal fuse/10R resistor to the left of the lower large capacitor. If good, it should measure 10 ohms. If it is open circuit then you won't get any AC supply to the PSU.

Thanks for your advice. I just checked that white component with a multi-meter. Results are in the attached image.

There may be other components failed that caused this is blow, so check for shorts on transistors etc under the heatsinks before replacing it.

I have not removed any of the heatsinks yet. Lets hope the failed component(s) won't be SMD ones...


As an update, since this is "badcaps" forum, I measured the voltage at the positive legs of most of the electrolytic capacitors.
Just a quick check to see if part of whole power supply works I guess. Results are in the attached image.

Thanks

[BEpsilon]

Re: Panasonic P50ST50H - No power - HELP please

when you said "power cord / plug" to tv is good... where did you check the voltage? At the outgoing connector shown in your picture of the back of the set ... after the power cord receptacle.

I checked the C13 plug at the end of the power cord that came with the TV. This rules out a damaged cord.

I then checked the two-pin AC input connector on the power supply board. This connector (mounted at an angle) is downstream from the AC filter module. I see full AC mains voltage on the live and neutral pins.

[R_J]

Re: Panasonic P50ST50H - No power - HELP please

You need to get the standby voltage first, with your meter on HOT GROUND (use - of C130) check the voltage on both sides of R224 & R148 (or check on J4) If you have +200 volts on J4. Un plug the power supply and check the resistance of R207 0.3Ω (below the printing HOT LIVE CIRCUIT). If it is open, check for shorted mosfet Q110

[BEpsilon]

Re: Panasonic P50ST50H - No power - HELP please

You need to get the standby voltage first, with your meter on HOT GROUND (use - of C130) check the voltage on both sides of R224 & R148 (or check on J4)

Understood.

Negative probe of multi-meter clipped to the negative terminal of C130 (bottom one of the 3 high-voltage 450V 100uF capacitors).

TV plugged into power outlet.

On R224, both left (towards capacitors) side and right (towards heatsink) side are +311.X to +312.X V DC.

On R148, left side is approx. +311.3 V DC and right side is approx. +1.88 V DC.

On J4, reading is approx. +311.5 V DC.

If you have +200 volts on J4. Un plug the power supply and check the resistance of R207 0.3Ω (below the printing HOT LIVE CIRCUIT). If it is open, check for shorted mosfet Q110

I indeed have more than +200 V DC on J4, so I unplugged the TV, then checked R207 (greenish; vertically-mounted) like you suggested. Resistance is precisely 0.5 ohm. (NOT 0.3 ohm as labelled on the component )

I guess that means the 2W resistor is OK? Should I still check the Q110 MOSFET under the heatsink?


Thanks

[R_J]

Re: Panasonic P50ST50H - No power - HELP please

That voltage on the (right side) of R148 does'nt look right. That should be the HV supply for ic200, and should be around 30~40VDC. Check the value of R148 (3.3k) & R147 (1.0k), I don't know the number on ic200, can you see the number, I think it could be a NCP1234. If that is the ic, check the resistance between pin8 and pin4
The ic markings could be 34A65, NCP1234AD65R2G (the second line is date code info)
I suspect that ic200 is defective
R207 will be ok, it is marked 0.3Ω, and checked 0.5Ω I suspect the 0.2Ω difference is in your meter leads, this is normal but needs to be considered when checking low value resistors. The mosfet is likely ok.

[BEpsilon]

Re: Panasonic P50ST50H - No power - HELP please

I will go and disassemble the P board from the TV and check for you.
Please standby. Thanks.

The mosfet is likely ok.

Prior to reading your reply, I was checking the MOSFET with my multi-meter... the readings I see are weird to me (not sure if I am wrong or not).

Obviously, Q110 is under a heatsink, so I cannot identify its specs. All I can see are the 3 terminals - Gate, Drain, Source (as labelled on the board).

I am new to this, so I browsed a few webpages on how to test MOSFETs with a multi-meter. The following are the steps I tried:

Diode testing mode is selected. Bridge/Short both Gate and Drain with a conductor for a second or so, in order to discharge internal capacitance of the MOSFET.

Positive probe on Drain. Negative probe on Source. Meter shows 0 (no connection). Seems normal so far.

With negative probe still on Source, use positive probe to touch Gate for a second, then move probe back to Drain.

According to the relevant webpages and YouTube videos, I should now see a connection between Source and Drain. The fact is my meter still shows 0 (no connection).

[R_J]

Re: Panasonic P50ST50H - No power - HELP please

Don't worry about the mosfet, If it was shorted either R224 or R207 would be open, You can't turn it on in with a meter in circuit. Like I said, I think that ic200 is bad

[BEpsilon]

Re: Panasonic P50ST50H - No power - HELP please

The ic markings could be 34A65, NCP1234AD65R2G (the second line is date code info)

Wow! Man that's amazing.

Indeed, IC200 is a "34A65", as shown in the attached photo (sorry about the flash).

Image of NCP1234AD65R2G on Digikey looks identical to what is on the board.

I will go check the resistors as you suggested.

[BEpsilon]

Re: Panasonic P50ST50H - No power - HELP please

Check the value of R148 (3.3k) & R147 (1.0k), I don't know the number on ic200, can you see the number, I think it could be a NCP1234. If that is the ic, check the resistance between pin8 and pin4

Thanks for waiting:


R147 = 0.999 kΩ

R148 = 24.36 MΩ

IC200 (34A65, NCP1234), Pin8(HV) & Pin4(Gnd) = 375.2 kΩ (Positive probe on Pin4, Negative probe on Pin8)
IC200 (34A65, NCP1234), Pin8(HV) & Pin4(Gnd) = 24.8X MΩ (Positive probe on Pin8, Negative probe on Pin4)

[R_J]

Re: Panasonic P50ST50H - No power - HELP please

Maybe it is just that R148 went open? I would think if the ic was shorted, the smaller R147 1K would have opened up first. Replace R148 (3.3k/2w) it might be all thats wrong.

[BEpsilon]

Re: Panasonic P50ST50H - No power - HELP please

Maybe it is just that R148 went open? I would think if the ic was shorted, the smaller R147 1K would have opened up first. Replace R148 (3.3k/2w) it might be all thats wrong.

Thank you very much for all your guidance so far.

So, at this point, just replace that single 3.3k 2-watt resistor?

Should I replace any electrolytic capacitors (even-though they're Rubycons and visibly in good shape)?
There are two 35V 47uF in the area, so I thought I would replace them with 50V 47uF? May be Panasonic FC/FM capacitors?

[R_J]

Re: Panasonic P50ST50H - No power - HELP please

The caps on these Panasonic boards are ok, no need to replace them. I would replace the resistor and see what happens. These power supplies are very reliable, I have never had to replace this (R148) resistor before.

[BEpsilon]

Re: Panasonic P50ST50H - No power - HELP please

The caps on these Panasonic boards are ok, no need to replace them. I would replace the resistor and see what happens.

I will go visit a local electronics components store to get a new resistor.
Lets hope a new resistor is all it needs.
I will let you (and everyone reading this) know the outcome.
Thanks.

These power supplies are very reliable, I have never had to replace this (R148) resistor before.

Assuming the resistor is all it needs, I wonder if there is any preventive maintenance a professional like yourself would recommend me do please?

From my research so far, I have read about loose mounting screws causing arcing and component failure eventually.
All the board mounting screws I see have split washers (aka lock washers). All of them are nice and tight on my first check.

Aside from that, are there any other components on any of the boards (P, A, SC, SD, SU, SS) on this ST50 that I should just replace now before putting the TV back in service?

Really want to keep enjoying this nice (but extinct) plasma, until I can afford something better (something like the Panasonic HZ2000 OLED being shown at CES this year ).

Any advice is greatly appreciated. Thanks.

[BEpsilon]

Re: Panasonic P50ST50H - No power - HELP please

I would replace the resistor and see what happens.

3.3k 2W Resistor has been replaced.

As one can see from the attached image, the old resistor has no resistance whatsoever. I wonder if the resistor was damaged due to external factor(s) or preexisting defect.

Now when AC power is provided, but no other boards attached, I can see +4.5 V DC at Pin06(STB5V) of Connector P6.
+4.5 V DC is also on J62 ("5V" printed on the board). There is +15.75 V on J76, which is right next to J62.

With all the other boards back with the P-board, the relay clicks immediately when TV is plugged in.

At this point, with AC power input active, but TV not on, my power meter shows the TV is drawing 0.08 A at the plug (220 V AC).

When I press the On/Off button on the TV, I can hear a relay click again. TV is drawing [only] 0.15 A at the plug.

For a TV that is specified with a nominal power consumption of 420 Watt, that amp reading is not normal obviously.

Unfortunately, the TV still has no picture, not even the Panasonic Smart Viera logo. LED is not lit, so no error code to see.



More problem-solving for the weekend I guess.

[BEpsilon]

Re: Panasonic P50ST50H - No power - HELP please

OK, I am not sure what happened...

I was following the flow-chart on P.30 of the Troubleshooting Guide (attached).

I disconnected A1 on the A-board as instructed. TV still not on, so it seemed I need to replace the A-board. However, that is now unnecessary.

As I reconnect A1 and everything else just to re-check the whole troubleshooting procedure, I was very surprised to see the front LED in solid red when I switch the TV on. There is still no image or sound.

As I thought the TV might be in standby/sleep, I press the power button on the remote control. Lo and behold, I see "Smart Viera" being displayed on the screen, and then a notification about no antenna signal.


I guess it's safe to conclude that the resistor is the only component required.

Thank you very much everyone, especially R_J!





PS: Now I can proceed to fix another Panasonic device, SE-HDX2 amplifier (bookshelf-type).

[frica]

Re: Panasonic P50ST50H - No power - HELP please

+1