NEC px-42xm3a plasma TV screen/fan power issue

[F14t12]

I'd like to report a plasma screen tv problem and get a few ideas how to fix it- I really like the image quality and would hate to have to junk it. I have a feeling someone may have experience that could help steer me in the right direction, I appreciate your time.

So far i've removed the back panel, cleared out any dust and superficially examined easy to see boards for apparent damage or leaky capacitors.

Here it is:

Normal tv operation:
Green light comes on immediately when pressing in the power switch, momentarily followed by a clicking noise that seems to power on the tv screen (fans begin to spin, can hear a whirring noise and voila you have picture).

Faulty operation that got gradually worse:
After pushing in the power switch, the green light turns on, sometimes cuts off for a moment then returns. However, theres no clicking noise or screen activity, fans don't cue on and theres no picture. When the problem began to reveal itself in the past two weeks, the click that accompanies the screen powering is just delayed by about 5 seconds after pushing in the power switch. However, now that its basically broken, the green light will generally stay on about 20 seconds, of course with no click or fans, before turning red which indicates standby. Its like the tv knows the screen is not powering properly and enters standby, which i think is a clue. You can turn the light green again with the remote but it wont actually click and power everything on. It just sits for a bit before turning solid red/standby again after 20 secs or so.

Last week the tv would power on every other night. The last 4 days now i just get the faulty operation explained above.

Let me know if I can help with more info or pics to post. Thank you much.

[R_J]

Re: NEC px-42xm3a plasma TV screen/fan power issue

The fault seems to be with the power supply, post some good quality, in focus pictures of the power supply and also one showing all the boards.
Check your standby voltage and see if it drops when you try and power up the tv. O will guess that D92 is the standby supply diode and C92 will be the standby (5v?) capacitor

[F14t12]

Re: NEC px-42xm3a plasma TV screen/fan power issue

Wow! Thanks for the quick response. I have located d92 and c92 and am attaching some photos here. Unfortunately, I wont be able to access the capacitor leads until later this evening, but I am excited to give this a shot. To make sure I proceed wisely, can you relay the sequence of events you would use to check the standby voltage? Thanks again!

Apologies for the lighting of these photos. I really appreciate the guidance.

[R_J]

Re: NEC px-42xm3a plasma TV screen/fan power issue

Check the standby voltage on the cathode (white mark end) D92, That connects straight to the + of C92. The standby voltage is there all the time, it also supplies the voltage for the main relay (D101 is across the coil of the relay) , so if that voltage drops when you press the power button, the relay won't operate.
IC21 is the standby smps ic, so if the standby voltage is dropping, I would first suspect C26,C31 & C3? (with the silicone on it)

[F14t12]

Re: NEC px-42xm3a plasma TV screen/fan power issue

I am detecting a voltage drop across d92- I've attached photos in case I'm doing something wrong. First photo shows power is off. Second, power is turned on. Immediate drop that only increases when i turn power switch back off.

Should also mention I let it sit upon powering on- when it silently went to standby mode after 15 sec the voltage I didn't see a noticeable difference.

Do you think it would be worth replacing the caps you mentioned above? Could it be possible that heat over time has created a bad joint I could fix by pulling board, draining caps and reheating with solder? I appreciate your patience I'm very new to this type of work! Advice is greatly appreciated.

[R_J]

Re: NEC px-42xm3a plasma TV screen/fan power issue

Don't measure the voltage ACROSS the diode? You need to connect the meter negative lead to CHASSIS GROUND and the positive lead to the diode cathode (white band)

[F14t12]

Re: NEC px-42xm3a plasma TV screen/fan power issue

Thanks for letting me know, my understanding of circuits stinks, I need to think more- but it makes sense that it should be grounded for the read. Attaching another single photo- I actually did not detect a difference between switch on and off in this configuration. Hovers between 7.11 and 7.12.

[R_J]

Re: NEC px-42xm3a plasma TV screen/fan power issue

Have you removed the power supply and checked for poor solder connections? I would check the connections on and around the relay and standby circuit (around ic21) also check the solder connections on all the plug connectors.
It looks like the relay drive transistor is Q91, it's collector connects to the relay coil, its base is connected to pin3 of that plugin module CN01B pin3, and I can't trace the line any further.
There should be a power supply on/off line from the main board to the power supply that turns that relay on and off via Q91, none of the connections are labeled so without a service manual it could be hard to trace the problem on my end

[F14t12]

Re: NEC px-42xm3a plasma TV screen/fan power issue

Thanks for the great advice. I hit a lot of the solder points on the power supply near the relay, bit the problem persisted. There was a good deal for a Power supply replacement online, I swapped it out with the original board last night

At first I was excited to hear the fans power on and the relay click when pressing in the power button along with the green light, but a new issue seems to have came up. When the fans cue and the relay clicks, the backlight flashes for a moment before turning off again. The power button also turns from green to nothing for about 2 sec, before the green returns on solid for about 20 sec before turning solid red again (goes into standby). Over this time theres no fans clicks or backlight present. If I toggle out of standby with the remote it repeats the same process described above. Of note, the backlight only flashes, it doesn't show a genuine screen i can toggle through any menus with. Also, the intensity of the power light seems to be slightly brighter than what i remember with the new power supply in, but maybe I'm just imagining that.

Thanks for the advice, it seems like this is some improvement towards resolving the issue. New power supply was advertised as working and pulled from a broken screen unit. I see some caps are a different color but everything appears identical upon first glance.

Any other advice you can think of to try?

[F14t12]

Re: NEC px-42xm3a plasma TV screen/fan power issue

Another clue! There is a blinking red light on a chip on the power board that repeats four flashes (short long long short) next to D223. Attaching photos.

Board itself marked kpc 2294v-0 ps-250

The light begins flashing as soon as I push in the power switch and stays flashing even when unit is turned off. Only goes away when unplugging.

[F14t12]

Re: NEC px-42xm3a plasma TV screen/fan power issue

I just spoke with NEC tech support, and a 3 or 4 cycle blinking red light where I described should indicate that the panel is dead. I suppose this means that the demise of the panel and the original power supply was interlinked. Thanks for the help rj.

[R_J]

Re: NEC px-42xm3a plasma TV screen/fan power issue

There are no backlights on a plasma tv, what you are seeing is the pre charge of the panel. which is usually a good sign, I doubt the panel is defective, NEC "tech support" ? If there is a problem anywhere they will just say the panel is dead, If it was dead you would not get the precharge flash.
You may want to remove the Y-sus and the Z-sus board and inspect them for poor solder connections as well. NOTE: INSTALL ALL MOUNTING SCREWS for the Y & Z boards BEFORE turing on the tv.
The power supply monitors different voltages and if there is a problem, it will shut down, If the tv worked but was intermitint the panel itself should be ok. Also check all the interconnecting cables, a loose connection could also cause the tv to shut down.

[F14t12]

Re: NEC px-42xm3a plasma TV screen/fan power issue

I will crack in deeper this weekend and post back new findings. Certainly, the replacement power supply is an improvement as the toasted one wouldn’t power the precharge, relay, or fans. It also wasn’t showing the flashing light at d223. It might be slow going, but it’s definitely progress. Just trying to get max life out of it to avoid creating unnecessary waste if possible.

Thank you!