Picked up a 50 inch Onn Roku TV someone threw out. Tested components on power supply (with multimeter).
Fuse, bridge rectifier and MOSFET are bad.(Bridge rectifier=outer-most legs are shorted in both directions. mosfet=drain&source shorted.)
When it comes to SMPS circuits, my knowledge is limited. I've only worked on one in my life. It was from a Haier TV I used to have. I managed to fix it by replacing the mosfet, fuse and capacitor. (Wasn't positive whether the capacitor was actually bad. But I replaced it anyway.)

I don't know how to word what I'm trying to ask...but, I'll try...
For those of you with more experience working on SMPSs...
Based off your prior experience, are there any other components you suspect could be bad? In terms of the specific components that failed, I mean. (Fuse, bridge rectifier & MOSFET).
I know enough about SMPSs to realize they all work in basically the same way. I imagine, when this specific combo of components fail, there's a pattern to what they take out in the rest of the circuit.
I don't have enough knowledge about how they work to know if there's anything farther down-stream this fault is likely to take out.
Does that make sense?

I asked this question, (just worded differently), on another site. But they closed my post, saying it wasn't a question.

What I'm trying to ask, IS a question. I just don't know how to word it very well.

Hopefully I did a better job explaining it this time.

Oh yeah...apparently, the circuit diagram nor service manual (for this TV) are publicly available. So, the best I can do is post photos of what the board looks like. It's model # is E021m553-f1.

I also noticed the solder between the two diodes (black chips on bottom of board, right beside mosfet) has bridged.

Another area on the bottom of the board has a resistor and capacitor bridged with solder. (Which, I'll fix when I replace the components.) I wish I could post a better photo, that you could zoom in on.

Anyway, can you guys tell me if you've came across power boards with this specific combo of blown components? And whether there were any other components that required replacing? Thanks!!

 

Your understanding about switcher supplies is certainly right, in that they are a lot more complicated than they look. They are both fairly high power and high frequency systems, for starters. And every connection, as a result, has an effect. That means that the routing of the PCB trace matters, as it's resistance and inductance matter. So very suddenly it gets a lot more complex.
Certainly even the basic concept is more complex than a linear supply. But the pain is in all of those details that do matter.
The question about "what else might have failed" is an excellent question, and a very wise one to ask.
First, I suggest unsoldering the bridge and checking it out of circuit, because something else may also be shorted, which took out the bridge.
The fact is that much of the time when one part fails there is a cause outside of the part that failed. And that cause might not be obvious.

 

See the thick white silkscreen that appears both at the top and the bottom?
That is the boundary between the HV and LV sections of the circuit.

 

The secondary side diodes are a common fault. They could cause the MOSFET to fail, which in turn could cause the bridge rectifier to fail followed by the fuse.

 

The secondary side diodes are a common fault. They could cause the MOSFET to fail, which in turn could cause the bridge rectifier to fail followed by the fuse.

Thank You!

I wish there was a circuit diagram for this supply. I'm not sure if the diodes are connected in series or what. It looks like they are. If so, the bridged solder between them might not be a big deal. (I mean, if they're already connected together that way in the circuit anyway.)

Your understanding about switcher supplies is certainly right, in that they are a lot more complicated than they look. They are both fairly high power and high frequency systems, for starters. And every connection, as a result, has an effect. That means that the routing of the PCB trace matters, as it's resistance and inductance matter. So very suddenly it gets a lot more complex.
Certainly even the basic concept is more complex than a linear supply. But the pain is in all of those details that do matter.
The question about "what else might have failed" is an excellent question, and a very wise one to ask.
First, I suggest unsoldering the bridge and checking it out of circuit, because something else may also be shorted, which took out the bridge.
The fact is that much of the time when one part fails there is a cause outside of the part that failed. And that cause might not be obvious.

Thanks for the tips! Great advice, saying to measure the bridge rectifier out of circuit. I didn't even consider the possibility of other components just making the BR appear shorted.

Hopefully, with you guys' help, I can actually repair this thing!!

 

Thank You!
I wish there was a circuit diagram for this supply.

It is just two bog-standard flyback circuits, with a diode-capacitor-resistor snubber. There will be two standard flyback controller ICs somewhere, probably UC3842

 

First, the snubber components can fail and that can cause the MOSFET to fail, so you should check those components.
Second, sometimes when the MOSFET fails it can cause the components feeding the gate to fail, so you should check those too.

 

With 3 opto-couplers, this unit has multiple outputs. Not simple.
Solder pads do not bridge by themselves.

 

I am no expert with SMPS but I have done like you have.. swap a few new parts in and restore life.
Vizio is one of the more friendly makers...they used to be made in USA?
ebay may have new or used.
k

 

On a practical note there's a fair chance that the device driving the FET will have failed also. Power MOSFETs seem to always fail with the gate shorted to the source and drain, and gate drivers aren't generally short circuit protected so unless the IC's DC supply is limited the IC may fail. Thankfully there's a relatively small pool of SMPS controller ICs in common use.

 

After taking a break from messing with this TV, I just decided to take another look. I've figured out a few things...

To begin with, I took MisterBill2's advice & measured the BR out of circuit. It was fine.

I swapped out the mosfet for one with similar specs & replaced the fuse. After that, the two 12V pins measure 12.1V. That's without shorting the standby and power on pins, BTW. Also, there's no 5V standby voltage (or voltage on any other pins besides the 12V ones).

I was excited when I happened to stumble upon a circuit diagram (that appeared to match my board). But I realized, while it nearly matches, it doesn't quite.

Honestly, I'm super confused by the diagram. There are a few diff board revisions. None of them actually match the circuit diagram though?! The other boards have the exact same components & component numbers. Some components are just moved around a tiny bit on some. Others have extra components in the spaces that are empty on my board revision. I checked online photos & found that my board isn't missing anything.

I traced the standby pin circuit back & can't understand how it's even connected to any other parts.

I'm so confused at this point. Hopefully you guys can help.

Here's the diagram I found. Pin 6 is labeled standby on the board. The diagram doesn't mention it tho. It calls it something else. I drew lines connecting portions & highlighted various components. The components highlighted green have empty spots on my board in those locations. The blue ones are missing from my board & ALL boards, all together?!? What the heck?!?

Can any of you make sense of this?

 

Picked up a 50 inch Onn Roku TV someone threw out. Tested components on power supply (with multimeter).
Fuse, bridge rectifier and MOSFET are bad.(Bridge rectifier=outer-most legs are shorted in both directions. mosfet=drain&source shorted.)
When it comes to SMPS circuits, my knowledge is limited. I've only worked on one in my life. It was from a Haier TV I used to have. I managed to fix it by replacing the mosfet, fuse and capacitor. (Wasn't positive whether the capacitor was actually bad. But I replaced it anyway.)

I don't know how to word what I'm trying to ask...but, I'll try...
For those of you with more experience working on SMPSs...
Based off your prior experience, are there any other components you suspect could be bad? In terms of the specific components that failed, I mean. (Fuse, bridge rectifier & MOSFET).
I know enough about SMPSs to realize they all work in basically the same way. I imagine, when this specific combo of components fail, there's a pattern to what they take out in the rest of the circuit.
I don't have enough knowledge about how they work to know if there's anything farther down-stream this fault is likely to take out.
Does that make sense?

I asked this question, (just worded differently), on another site. But they closed my post, saying it wasn't a question.

What I'm trying to ask, IS a question. I just don't know how to word it very well.

Hopefully I did a better job explaining it this time.

Oh yeah...apparently, the circuit diagram nor service manual (for this TV) are publicly available. So, the best I can do is post photos of what the board looks like. It's model # is E021m553-f1.
View attachment 352762View attachment 352763
I also noticed the solder between the two diodes (black chips on bottom of board, right beside mosfet) has bridged.

Another area on the bottom of the board has a resistor and capacitor bridged with solder. (Which, I'll fix when I replace the components.) I wish I could post a better photo, that you could zoom in on.

Anyway, can you guys tell me if you've came across power boards with this specific combo of blown components? And whether there were any other components that required replacing? Thanks!!

Hello there,

I worked with switchers when I worked in the industry some years ago. I worked on supplies from maybe 1 watt to 30,000 watts, I also worked on smaller ones under 1 watt. This was both design and I had to troubleshoot some of them also. That was in the manufacturing phase though.
Beyond that, units that had run for some time maybe 2 or more years there was one common failure: the electrolytic capacitors used to filter the input and output, especially the output DC. I actually had several power supplies fail because of that, and after replacing those caps the units ran up normally and stayed working without shutting down. I also had a fuel pump relay for a Volvo go because of the electrolytic caps.
One of the main problems I think is the designs are not usually done as well as they should be. They might use a 15v cap for a 12v automobile, even when the alternator charges the battery up to 14.8 volts, which is too close to 15v or even 16v. They would last longer if overrated a little more.

So that is by far the first thing to check. Start with the output DC filter capacitors and there could be several for each output.

After that, transistors can blow sometimes when a large load is applied to the output suddenly. Unfortunately, if there are driver transistors for those transistors they could blow too and it is common to replace them when the main transistors blow. If there is just an IC that could blow out also.

This assumes you checked all the fuses, and one or more could be hidden in the case somewhere that is hard to see.