42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

[tieny]

Just got this 42" spectroniq lcd tv. Model: pltv-42hw68
There is no sign of power when plugged in.

Opening up this TV I've concluded some things.
This has been worked on before, fixed, and then resold. The last person who bought this was unforunate to have the tv break down and thus I buy the broken TV. I'm pretty sure the last person who owned it hasn't tampered with it in anyway (not 100% though).

So heres what I got.
While opening the TV, the AC input to the PSU is burnt badly. I looked at a psu on shopjimmy and it shows that there is suppose to be a 3 pin connector for the AC input. The person who actually repaired the tv must of damaged the connector and instead used two wires for the + and - AC input, however there is no ground pin....

I'm guessing there was a surge, but with no ground connection it instead went through the circuit burning the trace and blowing the fuse. The blue cap next to it looks to have some heat wear also.

So here are my questions:
The fuse i pulled out is rated 250V 6A. I'm gonna need to find another fuse, will I be able to use any fuse that is rated lower?

As for the ground pin. How would I wire the ground connection from the PSU for the AC connection?

[tieny]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

So just replaced the fuse. I used a regular 250V 3A glass fuse.

I plugged it in and there was a pop and a huge green spark around this area in the photo...

I checked the fuse I used and it was still good and not open. Looking around the area in the photo I found the cause of the spark. The two grey resistors had popped... I also checked the little capacitor and the diode also but those checked out good.

I decided to check the mosfet there also and it seems to be shorted. I'm not sure if this was the cause for the overload? Or the mosfet got shorted after the overload when I plugged it in?

[tw2005]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

It looks all scary to me.
Incorrect wiring color standards, I don't like how a DC color coded lead has been patched in.

The connections are active(live) WHITE,neutral BLACK, earth GREEN/yellow.
There is no + or - with AC.

I'm not sure if you're suggesting the missing pin is earth but please don't. It may be used if the PSU is configured for another countries mains supply.

I'd post up a full shot of the track side of the board so you get more informed responses from those familiar with this set or US market TVs.

I've taken a snap of the basic wiring diagram from a HAIER HLH406BB TV which uses the same PSU according to info on SJ website.

You can see shows the 3 pins , center is No Connection

[tieny]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

You might be right, since there doesn't seem to be a third input for the middle pin. Tracing the middle pin it is connected to a screw hole so i was thinking it to be ground...

Would that bad mosfet be the cause of this problem? I havent checked the bridge rectifier yet either.

[tw2005]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

You might be right, since there doesn't seem to be a third input for the middle pin. Tracing the middle pin it is connected to a screw hole so i was thinking it to be ground...

Would that bad mosfet be the cause of this problem? I havent checked the bridge rectifier yet either.

Fair enough, photo did not show enough, I was concerned for your safety. To me the wiring looks correct as in the chassis is earthed, the the 2 wires to the board and the PSu probably earthed when bolted to the chassis.

I'd trace from the input and check the major components like the FET and rectifier. I have seen a mistake made not insulating a FET on a heatsink which was live and that was spectactular. Usually big flashes to me suggests shorted mains side.

I've only ever done one Panasonic PSU with a shorted FET. On that, FET,bridge rectifier, inrush resistor and fuses blown.

Hope you fix it.

[tieny]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

I kinda expected something to blow after replacing the fuse. Surprised that it wasn't the fuse that blew first even though I put a lower rated one than the original.

So i started testing the other FETS and i found another one that is shorted in all three pins. Its the same label as the first one i found.

As for the bridge rectifier, there isn't any labels on it at all..
how do you test if its bad?

As for the two resistors it has 5 bands. Orange, white, silver, gold, and yellow. I've been looking online to find the resistance but i can't seem to find one with the 5th yellow band. When neglecting the last yellow band, It says its a 0.39 ohm 5% tolerance. Is that right?

[Mr Bill]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

As for the bridge rectifier, there isn't any labels on it at all..
how do you test if its bad?

The bridge is 2 diodes side by side. You'd test it with your meter in diode mode. 1-2 is one diode and 3-4 is another. 2-3 should show OL or "1".

[tieny]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

The bridge is 2 diodes side by side. You'd test it with your meter in diode mode. 1-2 is one diode and 3-4 is another. 2-3 should show OL or "1".

Thanks! Just tested it and it seems to be good.

[tom66]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

There are four in a bridge. Two from positive going to each AC line, and two from negative going to each AC line.

[budm]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

"The connections are active(live) WHITE,neutral BLACK, earth GREEN/yellow."
Sorry, but that is not correct:
White (Euro uses Blue) is NEUTRAL which is bonded to ground at the service panel).
Black (Euro uses Brown) is Line (HOT).
Safety ground is Green (or Green with yellow stripe).
Please be real careful about correct Hot and Neutral and and Ground connection, or you may get killed.

[tw2005]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

"The connections are active(live) WHITE,neutral BLACK, earth GREEN/yellow."
Sorry, but that is not correct:
White (Euro uses Blue) is NEUTRAL which is bonded to ground at the service panel).
Black (Euro uses Brown) is Line (HOT).
Safety ground is Green (or Green with yellow stripe).
Please be real careful about correct Hot and Neutral and and Ground connection, or you may get killed.

Sorry, grabbed it from wiki which obviously has me confused now. The top table (Standard wire colours for FLEXIBLE cable.(e.g. Extension cords, power (line) cords and lamp cords))had that but further down I see what you're saying as it has it reversed(Standard wire colours for FIXED cable(e.g. In-, On-, or Behind-the-wall wiring cables))???

http://en.wikipedia.org/wiki/Electrical_wiring

Best I stay out of it then, really bad I've made that mistake. It'd be nice if they had one std around the globe.

Could you explain why the 2 different codes listed?


Apologies

[tieny]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

Yikes, wouldn't want to mix those wires around.

For the 2 blown resistors what value would they have, (Orange, White, Silver, Gold, Yellow)?

When measuring them, one is open, while the other measures around 1.8M ohms.. But I think the generated heat may of cause the major increase resistance before it was about to blow.

An online resistor calculator came out with 0.39 ohms, and from the size I would say 1 watt? Seems low, but is this right? I don't wanna get the wrong values in there.

[martyparish]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

Maybe post a pic of the resistors(if there's anything left of them lol)? Those colors don't sound right. Yellow should not be the last color band.

[tieny]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

Theres a photo in post #2

[martyparish]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

Ah yes, indeed there is Going by the chart, it would appear to be a 39.8 ohm: (using the five band method)

Orange: 3 (digit 1)
White: 9 (digit 2)
Gray: 8 (digit 3)

398

Gold: 0.1 (multiplier)

398 x 0.1 = 39.8

HOWEVER! I think it's using just two "digit" bands plus multiplier and then the last two are tolerance and temperature coefficient.

In this case, your first estimation would be correct!

[budm]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

Is that Gray or Silver? 0.39 Ohm? It looks like the resistor for the MOSFET Source pin (You can trace out the connection), or they are fusible resistors for the B+ feeding the circuits.
@tw2005
"Standard wire colours for FLEXIBLE cable'
The Phase wire is still Black as shown in the table, if you look at NEMA5-15P (US 15Amp max plug, or NEMA5-15R for the receptacle), you will see the terminal screw has coor Brass for the HOT Black wire, the neutral will have Silver (Chrome) screw for the Neutral (White Neutral wire). Same color is also applied o NEMA5-20P or NEMA5-20R.

[tieny]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

Its silver, I can see some shine to the color. You're right its connected to the FETs source pin.

[budm]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

For the resistor to burn up, the MOSFET is more likely to be shorted out, which it can also take out the IC that drive the Gate of the MOSFET and the components connected to the Gate of the MOSFET.

[tieny]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

Ok just replaced the mosfets and the resistors. Flipped the switch for the power and so far so good, no explosion! =D

The standby LED is now on. I turn on the TV and the standby LED will blink a few times and then go off, but I'm still getting no picture..

I just noticed that theres a missing wire for the connection... the 2 pin connection on the psu. So it has been worked on before, doesn't feel too promising =(. Looking around I think it connects to the inverter board as its the only board that also has an empty 2 pin input.

Measuring the +12v, I'm getting 0V on standby and once I press the power button it goes to 0.02V.

[tieny]

Re: 42" Spectroniq pltv-42hw68 LCD TV PSU burnt fuse

I'm missing the 12V, where does the 12V usually come from on the psu?