Vizio SV421XVT No Power

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I just got this lcd tv today. The first thing I tested was the main fuse. I noticed the fuse had soldered leads going to board and one was broken. I repaired it. Now, I have 5v standby coming out at power supply going to main board. Next, I noticed that the screen didn't come on when I tried to power it up. So, I checked for 5 volt standby at the main board and found a smd fuse (LF5A) to be blown. My thought was that maybe the main fuse problem caused the smd fuse to blow due to power interruptions. I got a regular 5 amp fuse 32v and put in the circuit. It blew as well. I know I have a shorted component, but where? Does anyone have schematic for this tv? Thanks.

[PlainBill]

Re: Vizio SV421XVT No Power

I just got this lcd tv today. The first thing I tested was the main fuse. I noticed the fuse had soldered leads going to board and one was broken. I repaired it. Now, I have 5v standby coming out at power supply going to main board. Next, I noticed that the screen didn't come on when I tried to power it up. So, I checked for 5 volt standby at the main board and found a smd fuse (LF5A) to be blown. My thought was that maybe the main fuse problem caused the smd fuse to blow due to power interruptions. I got a regular 5 amp fuse 32v and put in the circuit. It blew as well. I know I have a shorted component, but where? Does anyone have schematic for this tv? Thanks.

It doesn't affect the problem, but your theory of why the fuse blew doesn't make any sense.

Use the standard troubleshooting technique for this problem. Hook a 5V current limiting power supply to the load side of the LF5A fuse. Increase the voltage and current limit for no more than 3A. This will PROBABLY be at less than 1 volt.

First step - gingerly trace the 5V standby line. You are looking for a component that feels warm. Likely candidates are LDO regulators and SMD capacitors. If you don't find something heating up, it gets more tedious.

Set your DMM to the lowest voltage scale. Place the red lead at the point on the board where the external power supply is connected. With the black lead follow the 5VSB rail noting the voltage at each component connected to the rail. The voltage drop will increase slightly as you get further away from the injection point. At some point it will stop increasing. The short is at that point.

PlainBill

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Re: Vizio SV421XVT No Power

PlainBill, what you said made perfect sense. I thought about using the old light bulb trick but decided against it. Instead, I ordered a new power supply with adjustable voltage and current to do the job. All of the power supplies I have, currently, are regulated and have a fixed voltage and amperage rating. It'll be here in a few days and I will test the main board and report my findings. Thanks PlainBill - you're the man!

[PlainBill]

Re: Vizio SV421XVT No Power

PlainBill, what you said made perfect sense. I thought about using the old light bulb trick but decided against it. Instead, I ordered a new power supply with adjustable voltage and current to do the job. All of the power supplies I have, currently, are regulated and have a fixed voltage and amperage rating. It'll be here in a few days and I will test the main board and report my findings. Thanks PlainBill - you're the man!

Well, you could have probably done it with a 5V supply and a light bulb to limit the current, but it doesn't have real fine control. To point out the obvious, set the bench supply to provide 5.0 volts maximum.

Way back in my youth the test department had a variable supply that would put out 15V at 10 amps or something ridiculous like that. The technique I outlined above was a popular way to locate shorts.

The problem was some techs would sometimes get careless and crank the voltage upto 15 volts, then increase the current. The problem occurred when they located the short (quite often it was a ceramic bypass cap) they would get excited and cut it out. Of course, then the supply would be feeding 15V into a 5V rail and all the little TTL ICs would short.

PlainBill

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Re: Vizio SV421XVT No Power

Just a little update, I finally got the adjustable power supply to test the main board for the short. I set the unit to 2.5V at 1A and connected black to ground and red to the load side of the blown fuse. I found one ic getting extremely hot (part #4953GM). It is a 30v 5A power mosfet. So, I ordered a replacement and a fuse. I will post my findings as soon as the parts arrive.

[PlainBill]

Re: Vizio SV421XVT No Power

Just a little update, I finally got the adjustable power supply to test the main board for the short. I set the unit to 2.5V at 1A and connected black to ground and red to the load side of the blown fuse. I found one ic getting extremely hot (part #4953GM). It is a 30v 5A power mosfet. So, I ordered a replacement and a fuse. I will post my findings as soon as the parts arrive.

Good work. While you are waiting for the IC, check the load side. The short may be 'downstream' from this.

PlainBill

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Re: Vizio SV421XVT No Power

I decided to just replace whole main board since I was unable to obtain a schematic. Trying to trace all the paths of a circuit on a multi-layered board is difficult at best. After placing the new board into the unit, the television is now working. I will keep the older board for parts. Thanks for the help PlainBill.