Nec PX-42VM5A Dead from Standby

[MIPS]

Salvaged the power supply, driver and digital boards from a machine at the office. The salvaged power supply clearly had three failed capacitors so I knew where to check first on mine.
As it turns out like with previous flat panels I've had to work on I have a different revision display because the driver board interfaces with the panel differently, so if that is the problem I'll have to wait for another panel to surface.

I also found that my power supply was in better condition than the salvaged one, so for now I cannot make any better assumptions as to where the fault may lie until I recap at least one of the power supplies.

Also, what is this madness? This is an NEC product but the plasma panel and the power supply are clearly branded by Pioneer.

[MIPS]

Another panel with an identical interior chassis finally surfaced that I was able to verify was fully functional aside from extremely bad burn-in.
Replacing the sustain and the power supply (it probably wasn't the problem but at least I knew this one worked) got me up and going again, though before I put it immediately back into use I will recap the boards as to not have this happen immediately again.

[mr_rye89]

I had a Vizio plasma that I sold a friend do the exact same thing. Replacing the bulgy 'lytics in the PSU fix'd her right up.

[MIPS]

I finally got back to this and after a bit of looking found a service manual. It's strange.


As we left off you apply AC, the power supply comes up, the high voltage comes up and then the high voltage kicks out and you got a panel that runs totally blind with no error blinks. You got audio, you got a solid green light and you have working controls.

As it turns out that's not entirely true because what is happening isn't listed in the docs as a fault. If there's a power issue the front indicator will start cycling green and red rapidly and you have to power cycle it and hold the Input button on the front panel to reset the fault monitor. I never does that. The power supply is a Pioneer PS-250 and has its own smarts for fault monitoring on a daughterboard. When the high voltage drops out it starts blinking an LED code. (one long and one short before repeating)



Only if you unplug connector PD (note how one pin is ALARM and its also the larger DC voltages) will the panel fault out with a power issue. So the power supply is catching something that the rest of the panel doesn't see.
I have tried several salvaged power supplies now and they all do the same. The service manual does not have a schematic for this power supply and a separate schematic so far has not been found. Additionally, troubleshooting information for the high voltage side is missing from the service manual.

[MIPS]

I'm poking at this again.
I need to make room and it's last call on this panel before it's off to E-waste.

Let's summarize:

-Display "ran when parked", meaning the last time I saw it working it was flawless and I was the person who turned it off without issue

-Display was left plugged in and on standby for a period of months

-When turned on you get a green LED + a relay click, the high voltage comes up, then after about two seconds the high-voltage drops, a relay releases somewhere and the green LED briefly goes out, then comes back on.

-The Pioneer manufactured PSU starts cycling a 1-long, 1-short blink error which means it has shutdown part of the PSU for some reason.

-If the panel has a valid video input and audio, even with the above the display is still responsive and has a green light on the bezel, but it's operating completely blind.

-The display never detects the PSU failure and as such the entire display never shuts down because it never detects the fault stored in the PSU.

-NEC's service manual does not list this as a potential fault scenario. It's "impossible"

-NEC's service manual does not cover the plasma display portion (sustain boards, drivers and buffers) with any technical detail, instead referring to another service manual I do not have and cannot locate.

-For a period of time I had access to a large quantity of heavily worn or defective plasma displays of the same make and model

-I salvaged two power supplies and the contents of at least two panels

-I did NOT save the correct boards that fit my revision of the display......

-Internally, the PX-42VM5A has two major board revisions. The plasma display portion is NOT interchangeable between revisions due to different connections/mounting hole placement, so I do not have any buffer, driver or sustain boards I can substitute.

-I can swap in a known good power supply and the high voltage shuts down, so the fault remains in the plasma display section and not the power supply.

-Isolating the high-voltage output of the power supply causes the PSU to not go into a partial shutdown, however the display then detects a power failure and shuts down. This is exactly how it should according to the service manual.

-The DC outputs of the power supply are all within spec. The high voltage however shuts down too quickly for me to measure the loading of the voltage rail

End of summary.

Since high voltage comes up consistently and remains stable until the partial shutdown I can only guess that nothing between the PSU and the panel itself is dead-shorted, such as the monolithic IGBT, the drivers or the buffers, unless power is totally blocked from reaching things further downstream of the PSU until commanded. The only thing I can guess is when the panel tries to "strike" (the panel brings up the cells to their illuminated idle state) is the point where the PSU detects something abnormal. I'm not sure how the initial striking process works on a plasma panel but I am assuming this requires either a boost circuit or inrush protection provided by one of the many high voltage capacitors. In comparison to newer plasma panels there are considerably more of them in this TV.

Why am I dragging my butt rather than recapping? Price.
There's over $40 in capacitors to replace, plus an afternoon of work at the bench. If I'm wrong I'm about to make a very costly mistake. I'm hoping to narrow this down as much as I can before pulling the trigger because if it's anything else in the circuit there is no solution because no replacement parts are available. :/

[Alex KL-1]

Considering that the plasmas are not made anymore, perhaps the extra mile effort is worth?

[JohnCT]

Originally Posted by MIPS

I'm poking at this again.
I need to make room and it's last call on this panel before it's off to E-waste.

Any chance you had removed the rear cover and plugged it in? Pioneers have a trap switch that must be defeated once the rear cover is removed and the AC applied (or turned on depending on model). If that happens, there is a procedure to recover the operation of the TV.

You should be able to see the panel "prime" when turned on. In a dark room, turn the display on and see if you see a dull gray from the panel. It should be there and not flickering while the display is powered up.

If you suspect a wayward cap on the main or any of the boards, preheat the boards with a hairdryer or a heat gun (careful with that). Give a long heat soak getting the suspected board nice and warm over say 10 minutes. If you have a lazy cap, that will give the cap time to soak heat to the core. Warm caps are happy caps.

If the display powers up, power down the display and get an ESR meter and have at it. If you don't have one but have a scope, scope the positive side of all caps and look for a high waveform of noise. Even warmed up caps will show noise if they're weak. If you find one or many (most likely), hit them gently with some cold spray and and see if the waveform doubles - those are most likely the culprits.

John