Panasonic TCP50C2 No Standby

[Glitcher]

I have this 50" plasma that was said to have gone through a surge and stopped working. There are no blinking lights at all, pressing the power button does nothing, and no standby. I'm thinking PSU but I have a feeling it may be a bad main board also.

I'll start with the PSU, MPF6904A. When plugged in I hear a small slight buzz from one of the transformers, but the sound disappears when I disconnect P7.

Haven't found any shorted FETs on the PSU. However the big cap 450V 300 uf is measuring low at 1.2V and slowly raises a volt every couple of minutes..

Both mains fuses tested good, and Bridge rectifier measures good.

[Glitcher]

Re: Panasonic TCP50C2 No Standby

Was wondering if anyone had the schematic for this PSU?

[Caleb]

Re: Panasonic TCP50C2 No Standby

do you have ac to the bridge? I see a couple mov's next to the fuses, maybe open there?

[Ltank]

Re: Panasonic TCP50C2 No Standby

I would check for a shorted mosfet or a bad cap in the 5vdc standby circuit.
Usually two most common failures

[Glitcher]

Re: Panasonic TCP50C2 No Standby

No obvious dead shorted mosfets, I don't think the problem is in the 5v standby circuit since I'm having low voltages on the primary cap.

Also I'm getting the same low voltages on the bridge. What are "mov's"?

[Caleb]

Re: Panasonic TCP50C2 No Standby

Metal oxide varistor, black round things next to the fuses- designed to limit inrush current and they sometimes go open. I also spy what appears to be a large ceramic resistor to the left of the big cap, also commonly goes open. If you flip the board over and use diode mode on your meter with one test point on the main fuse then work your way towards the bridge when you lose the tone you have a likely suspect.

[Glitcher]

Re: Panasonic TCP50C2 No Standby

oh the varistors, I have checked the three of them already, they aren't open but they seem to have a high resistance, I measured about 1.00 on the 20M scale. Not sure if its suppose to be like that. The large ceramic resistor checks out at 10 ohms.

I also noticed that some of the smaller caps in the primary circuit are not discharging. I left it unplugged for a day and afterwards i decided to work on the board again, I desoldered the smaller caps and noticed alot of them still held 7v. Not sure if it is relevant.

I also find it odd that if this went through a lightning surge, I don't see anything that is physically burnt at all and the fuses are still intact and good. But that surge would easily take out the PSU usually in a way.

[Caleb]

Re: Panasonic TCP50C2 No Standby

as I recall they are supposed to be high resistance and go low when hit with high voltage, are they across the hot and ground to absorb surges or in the path of the current flow? If they are in the path of the current flow they have to be low resistance for the circuit to work.

[Glitcher]

Re: Panasonic TCP50C2 No Standby

I'm not exactly sure, but I would guess they are across hot and ground? And they are all still high resistance so I guess it would be unlikely that they were all hit.

I noticed there are two relays. If one of these were bad would it prevent the flow of circuit? They do seem to be connected to the bridge. I'm unsure of how to test these however.

[Caleb]

Re: Panasonic TCP50C2 No Standby

can we get a picture of the back of the board so we can trace out voltage path? I assume you have less than an ohm across those inductors as well?

[Glitcher]

Re: Panasonic TCP50C2 No Standby

Yes, the inductors are still good and measuring low resistance.

[Glitcher]

Re: Panasonic TCP50C2 No Standby

So I've been having an uneasy feeling with this one. I was hoping someone could tell me that this primary circuit works differently or something like that.

The lack of damage and burnt parts on the PSU (after a surge) was been concerning me. I checked all the usual parts that blow on the psu and they are all good.
And if I remember right, Panasonic Plasmas actually DO NOT have a standby light when it is plugged in... The red light only comes on when it powers on.

I don't have the schematics for this psu and there are no outputs labels, but I probed each pin, most were at zero volts, however did I find one wire that is reading 4.99v... So I'm assuming the standby is working properly DISPITE the low voltage on the primary cap (I guess this is used for something else???).

So I'm concluding this is a main board problem... (It was connected thru HDMI to a dvd box, and the dvd box is not working either along with other electronics in the house after the surge) Seems like the most logical explanation for this.

[tom66]

Re: Panasonic TCP50C2 No Standby

There are two primary caps, the largest one will only be energised after the TV has powered on. The smaller 450V cap should be active all the time.

[Glitcher]

Re: Panasonic TCP50C2 No Standby

Ahh, that's what I was looking for, now everything is making more sense. The smaller 450v cap reads 169.5V. I'm fairly confident that the PSU is okay now. Looks like the main board is fried and isn't turning on the TV then.

[ivoh]

Re: Panasonic TCP50C2 No Standby

Ahh, that's what I was looking for, now everything is making more sense. The smaller 450v cap reads 169.5V. I'm fairly confident that the PSU is okay now. Looks like the main board is fried and isn't turning on the TV then.

trace the 5v on the main board? maybe there is a blown smd fuse? although im not familiar with this model.

[Glitcher]

Re: Panasonic TCP50C2 No Standby

Just replaced the Main board, and the tv is up and running normal again!