Hey guys, my name is Landon and I'm new to the forum. I was recently given a non-working insignia 50" tv and am trying to fix it.

The power supply board number is 569MF0820A.

I started with checking the fuse and it was bad. I put a new one in and it popped immediately when plugged in. I installed another new fuse and unplugged the main boards that plug in to the power board to isolate the problem to the power board. With the power board isolated the fuse still pops. Ive been lurking this forum for a few days now and have just been removing individual components to see what keeps the fuse from popping. After removing several parts and still popping the fuse I finally got to the bridge rectifier. With only it removed the fuse does not pop when power is applied.

I have 120v at the power source and 120v at the middle pins of the bridge rectifier. I thought I found the problem but using a DMM the bridge rectifier seems to test good.

My question is can the bridge rectifier test ok on the bench but still be bad when voltage is applied? If not, what is my next test/step?

I can't buy the fuses locally and I'm down to my last one so I figured I would ask someone instead of sticking with the trial and error approach. Thanks in advance for any help. It's greatly appreciated!

 

Are you testing with a load ?

 

Are you testing with a load ?

Are you talking about the bridge rectifier? No. I'm just checking the resistance back and forth across the diodes. Each middle pin only reads in one direction to the + or - pin next to it. How would I test it under load?

Please pardon my noobness. I have a very general understanding of electronics and this is my first time diving into a circuit board. Everything I've learned in the last few days has been from this forum and YouTube videos.

 

Are you talking about the bridge rectifier? No. I'm just checking the resistance back and forth across the diodes. Each middle pin only reads in one direction to the + or - pin next to it. How would I test it under load?

Please pardon my noobness. I have a very general understanding of electronics and this is my first time diving into a circuit board. Everything I've learned in the last few days has been from this forum and YouTube videos.

Yeah the bridge recifer and do you test the voltage on the pcb with the tv off ??
Sorry for the shortness I am at work driving a Mac truck hauling dirt..

 

Yeah the bridge recifer and do you test the voltage on the pcb with the tv off ??
Sorry for the shortness I am at work driving a Mac truck hauling dirt..

Oh man! Lol. Please be careful as I am in no rush.

I I have the PCB isolated on a bench. It was popping fuses as soon as the power cable was plugged in so I haven't had a chance to turn the tv on.

So as it sits I'm checking voltage with the PCB sitting on a bench and plugged into a 120v outlet.

 

Probably the switching transistor has shorted.

 

Probably the switching transistor has shorted.

I removed all of the transistors and tested them individually. They all seemed to test ok. Plus the board popped a fuse when power was applied with all of the transistors out.

 

With the bridge removed, you have broken the circuit - of course the fuse doesn't blow. Put your black lead on the (-) pad and start probing with the red lead on the (+) traces. The short is near where the resistance is lowest. Check the large electrolytic cap for a short - it looks bulged, but it could be from lousy photos.

 

When you say the (-) pad are you talking about the pad for the (-) pole of the bridge? I will check that in a couple hours when I get home.

As far as the large cap, I believe the bulge is just the photo. None of the caps on the board are swollen at all. There are absolutely no signs of burnt up or bad components and there isn't any smells or odors coming from anything.

 

Something across the rectified output is shorted. You removed all the switching transistors, so you eliminated those. The big cap is also across the output and is a likely suspect.

Yes, the (-) pad is the minus pole of the bridge.

 

Ok, I've checked resistance from the (-) pad where the bridge goes to all of the labeled (+) pads I can find and nothing appears to be shorted out.

Just to clarify, the only thing removed from the PCB at the moment is the bridge.

 

I went ahead and pulled out the mosfet and the large cap and tested them with a DMM using test methods I found online. They both appear to be good.

 

Here's a schematic of your board to help you in your quest.



Here's the same file in .PDF format.

POWER_SUPPLY.pdf

 

Ok. I had to step away from it for a couple days because it was confusing the crap out of me. Lol. I'm trying my best to figure this diagram out. This is where I'm at so far...

I've located the big cap on the wiring diagram. On the board it is labeled as C517 so I found C517 on the diagram but it doesn't seem to be the same schematic. On the diagram it shows the (-) pole of the cap going straight to ground. On the actual board is connected to the (-) pole of the bridge and to several other things so I'm not sure if the diagram is just written that way or if I'm looking at it wrong. It also makes me wonder if I'm looks at the wrong diagram. I dunno.

The next thing I did was put everything back on the board except for the big cap and then applied power to it. The fuse didn't pop. If the fuse doesn't pop with the cap out does that automatically mean a bad cap or could the cap be breaking the circuit of what's popping the fuse? I used a different DMM I had that is supposed to check capacitors but I don't know if I'm using it right. I put the leads on the cap and it switches from nF to uF and just reads OL.

Also, with the cap removed and power applied, black meter lead on any of the 3 poles of the power cable, I'm getting about 100v at the (+) cap pad and about 55v at the (-) cap pad. I am definitely unsure what kind of voltages I should be getting there.

I hope this helps someone point me further in the right direction. Thank y'all so much for the help so far!

 

Also, with the cap removed and power applied, black meter lead on any of the 3 poles of the power cable, I'm getting about 100v at the (+) cap pad and about 55v at the (-) cap pad. I am definitely unsure what kind of voltages I should be getting there.

What are you trying to do, kill yourself? Once you are past the bridge, measure everything on the HOT side referenced to the hot GND (which includes the negative terminal of the big cap - C517). The voltage across C517 should be >170V.

 

What are you trying to do, kill yourself? Once you are past the bridge, measure everything on the HOT side referenced to the hot GND (which includes the negative terminal of the big cap - C517). The voltage across C517 should be >170V.

Don't worry I'm not going to kill myself. I'm pretty experienced with electricity, just not electronics. When I plug it in with the cap removed and measure across C517 I'm getting 169.7v. Does that indicate that the cap is the culprit?

 

Measure across the capacitor for any resistance. Are there any signs of damage on the cap?

 

Measure across the capacitor for any resistance. Are there any signs of damage on the cap?

When I measure resistance across the cap it quickly gets to the 20k ohm range and steadily climbs. It looks fine to me. No bulging, smell, or discoloration.

 

I would order a cap and another pack of fuses.

But SLK001 seems to have been down this road before. You should listen to him.

I was looking at his print, and I was befuddled by the circuitry past the bridge.

I haven't made it over to the cap yet, but I did look up that FAN7930 chip.

The more I read, the smaller I felt. What a gizmo!

 

Yeah I ordered a cap last night. Even if it isn't the problem at least it will be new.

And yes, SLK001 is definitely pointing me at the right direction! Thank you, SLK001!

I used to be an electrical technician at a Lincoln dealership and I thought I was pretty competent reading wiring diagrams until I saw the layout for this power board. It's an entirely different animal! Haha!

Thanks for the input!