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[The_Almighty_Bacon_Lord]

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[danadak]

Any burn marks on PCB ? Or discoloration, a clue. Look around heat sunk
components for temperature effects.

Is PSU cycling a thermal limit, eg. the sound coming from magnetics.


Regards, Dana.

[P90]

I would try it again with NEW quality brand low ESR caps. Caps age, and just because they came from one working circuit does not guaranty that they will work in another.
The bubbling sound is coming from the diet coke on your desk... lol.

[pyroesp]

Have you tried looking at SMPS repair videos ?

From what you describe on your pictures it looks like the input filtering is fine, the filter cap charges up, but no voltage is being generated at the output.
You might want to check the IC that generates the pulses for the transistors.

[mariush]

Check each diode and make sure one isn't shorted or open.
Check the small capacitors on the primary side, they're part of the start-up circuitry and if they've gone bad they may not give enough power to the power supply controller IC.

I see you have voltage on the big capacitor, this means  your fuses are good and the bridge rectifier is working, so voltage reaches the capacitor. From there electricity should go to the transformer
That DIP8 chip that says A6159 is the switching voltage controller : http://www.irf.com/product-info/datasheets/data/iris-a6159.pdf
There should be 170v on 5th pin , and a lower voltage on pin 3 at least 3v and at most I think 35v but usually it's around 10-20v (pin 1 is at the bottom of the text on the left, pin 2 is the next one to right pin 5 is the right most pin at the top and so on)
Check for continuity between the drain and source/ocp pins of the chip (see datasheet for pinout), just in case the internal mosfet is shorted.

IF one of the ccfl tubes is broken or the wire got desoldered, the backlight circuit will turn off in about 1-2 seconds to protect itself. Same if one of the tubes is too old and draws too much current. But you should be able to make the monitor work without the backlight connected (just point a flashlight to the lcd display to see the image)

[xavier60]

It has the same yellow glue that I first saw over 30 years ago. Anywhere it turns to brown, it becomes leaky and corrosive.
 The standby supply must run properly before anything else will work. It is the 7 pin DIP IC driving the small transformer. Check for charged capacitors before working on it. You should use an isolation transformer if you are going to work on the live side while powered.

Edit, do not run a SMPS with missing output capacitors or the regulation loop opened. It can cause more damage.

[P90]

It has the same yellow glue that I first saw over 30 years ago. Anywhere it turns to brown, it becomes leaky and corrosive.
 The standby supply must run properly before anything else will work. It is the 7 pin DIP IC driving the small transformer. Check for charged capacitors before working on it. You should use an isolation transformer if you are going to work on on the live side while powered.

Isolation x-formers are for sissies... it takes the excitement out of working on live electronics...     lol.           :--D

[P90]

It has the same yellow glue that I first saw over 30 years ago. Anywhere it turns to brown, it becomes leaky and corrosive.
 The standby supply must run properly before anything else will work. It is the 7 pin DIP IC driving the small transformer. Check for charged capacitors before working on it. You should use an isolation transformer if you are going to work on the live side while powered.

Edit, do not run a SMPS with missing output capacitors or the regulation loop opened. It can cause more damage.

Should I remove the yellow glue? Would that help at all?

Which small transformer? The one closer to mains, or the one closer to the large transformer?

I don't have an isolation transformer. I searched online, but they all cost $50+, so i'd rather not. Plus, If I'm more cautious, it's more fun

Lastly, which capacitors should be the charged ones near the 7 pin ic?

He is saying to be careful as some capacitors are charged, and you can be shocked.
I suggest you watch some YouTube videos on repaing switching power supplies and LCD TV's, you'll pick up lots of info.

[xavier60]

It has the same yellow glue that I first saw over 30 years ago. Anywhere it turns to brown, it becomes leaky and corrosive.
 The standby supply must run properly before anything else will work. It is the 7 pin DIP IC driving the small transformer. Check for charged capacitors before working on it. You should use an isolation transformer if you are going to work on the live side while powered.

Edit, do not run a SMPS with missing output capacitors or the regulation loop opened. It can cause more damage.

Should I remove the yellow glue? Would that help at all?


Which small transformer? The one closer to mains, or the one closer to the large transformer?

I don't have an isolation transformer. I searched online, but they all cost $50+, so i'd rather not. Plus, If I'm more cautious, it's more fun

Lastly, which capacitors should be the charged ones near the 7 pin ic?

Mainly look for where the glue has turned brown. What is the part number of the 7 pin DIP IC?.  You should see tracks connecting it to the small standby transformer.  The standby supply is a Fly-back type. The main supply looks like the Series Resonant type. Research to know how they work.

[xavier60]

The PFC stage boosts the rectified unfiltered mains voltage up to about 390VDC filtered by a large capacitor. The PFC stage usually doesn't work properly until the whole power supply is running and loaded. Confirm that the A6159 IC connects to the standby transformer.
Is the standby supply working? If not, check the output diode first.

[BradC]

It sounds like it's chirping. That is the controller attempts to start up the PSU, a short somewhere (usually on the secondary side) causes the controller to go into protect with over-current and after a time the process repeats.
I've seen this quite a bit with shorted MLCC caps on the output side. Just something else to look at.

[BMack]

It has the same yellow glue that I first saw over 30 years ago. Anywhere it turns to brown, it becomes leaky and corrosive.
 The standby supply must run properly before anything else will work. It is the 7 pin DIP IC driving the small transformer. Check for charged capacitors before working on it. You should use an isolation transformer if you are going to work on the live side while powered.

Edit, do not run a SMPS with missing output capacitors or the regulation loop opened. It can cause more damage.

Should I remove the yellow glue? Would that help at all?


Which small transformer? The one closer to mains, or the one closer to the large transformer?

I don't have an isolation transformer. I searched online, but they all cost $50+, so i'd rather not. Plus, If I'm more cautious, it's more fun

Lastly, which capacitors should be the charged ones near the 7 pin ic?

Mainly look for where the glue has turned brown. What is the part number of the 7 pin DIP IC?.  You should see tracks connecting it to the small standby transformer.  The standby supply is a Fly-back type. The main supply looks like the Series Resonant type. Research to know how they work.

I can confirm what he's saying. It turns a dark brown and will short components and things will start to blow up. If it's not that color, it should be fine but I'd remove it after you get it working.

Check the diodes and check all transistors for shorts. Check caps to see if they've gone short or open, you may have to pull a leg to get it out of circuit.

Is the power supply showing as dead without the main board plugged in?

[P90]

I would assume that's the first thing one would do, is to pull all the connections to the other PCB's to determine if a short is in fact in the SMPS. I've repaired a few with bad capacitors that had also taken out a MOSFET. Also, some have SMD fuses on the power supply board, something else to check...

[BMack]

I would assume that's the first thing one would do, is to pull all the connections to the other PCB's to determine if a short is in fact in the SMPS. I've repaired a few with bad capacitors that had also taken out a MOSFET. Also, some have SMD fuses on the power supply board, something else to check...

One of the first things I learned when I started working in a repair shop is don't assume someone did something they should have done. Sometimes the obvious might not be soo obvious from another perspective and sometimes it simply slips someone's mind.

[P90]

I would assume that's the first thing one would do, is to pull all the connections to the other PCB's to determine if a short is in fact in the SMPS. I've repaired a few with bad capacitors that had also taken out a MOSFET. Also, some have SMD fuses on the power supply board, something else to check...

One of the first things I learned when I started working in a repair shop is don't assume someone did something they should have done. Sometimes the obvious might not be soo obvious from another perspective and sometimes it simply slips someone's mind.

True. My math teacher, may he rest in peace, use to write the word "ASSUME" on the blackboard ( yes, blackboard, I'm that old), circle the words and say, "when you assume, you make an ASS out of U and ME."  God bless his sole.

[mariush]

The blue thing is a capacitor, it's normal to measure weird when you treat it like a resistor.
That dark yellow stuff is supposed to be a sort of semi-elastic material/glue used to keep components in place while the circuit board moved through reflow oven and gets all the parts soldered to the bottom. A second purpose would be to prevent vibrations in parts so your monitor wouldn't make noises when speakers are working (in case of monitors with hdmi and speakers built in)

Problem is - i'm not 100% sure , may be wrong - that dark yellow stuff isn't supposed to be yellow... the normal substance has another color but as it dries up due to lots of heating and cooling cycles, it goes bad and becomes slightly conductive so it can potentially short components (acts like a resistance between leads of various parts so the circuit goes all wonky)

You should remove that stuff, it won't hurt the circuit if it's removed.  Not sure what's the best strategy.. I think maybe heating it up with a hot air gun or maybe a hair dryer from a distance.

[kxenos]

I would check and replace the two small electrolytics near the controller IC (the DIP-16) and then check the feedback circuit. Do you have an oscilloscope? If so post the optocoupler's LED voltage when the PS is powering up

[P90]

The blue thing is a capacitor, it's normal to measure weird when you treat it like a resistor.
That dark yellow stuff is supposed to be a sort of semi-elastic material/glue used to keep components in place while the circuit board moved through reflow oven and gets all the parts soldered to the bottom. A second purpose would be to prevent vibrations in parts so your monitor wouldn't make noises when speakers are working (in case of monitors with hdmi and speakers built in)

Problem is - i'm not 100% sure , may be wrong - that dark yellow stuff isn't supposed to be yellow... the normal substance has another color but as it dries up due to lots of heating and cooling cycles, it goes bad and becomes slightly conductive so it can potentially short components (acts like a resistance between leads of various parts so the circuit goes all wonky)

You should remove that stuff, it won't hurt the circuit if it's removed.  Not sure what's the best strategy.. I think maybe heating it up with a hot air gun or maybe a hair dryer from a distance.

Yes, you can bet that yellow stuff, once it turns dark, has enought resistance to do nasty things in the circuit!

[Marinated]

So I got a picture of the capacitor numbers, but I can't seem to find out what it is, or what it means? http://imgur.com/a/nMzZD
It reads "B 331K 1KV". I have no clue what the B or 331K means, but I guess the 1KV must be the max voltage rating, but that seems pretty absurd. Anyway, what is the farad rating of this thing, and what would I most likely be fine to replace it with? Maybe I can buy a replacement online? I looked and couldn't find anything.

331 indicates 330pF, and K indicates 10% tolerance.

[mariush]

You can find them on stores like Digikey, Newark, Mouser .. closer to you.
Put them in the basket and add other components you may need and make one order.

Scrape off that yellow solidified crap which can cause problems just by being there.

[BradC]

https://www.aliexpress.com/item/10pcs-lot-1KV331K-1KV-331K-high-voltage-ceramic-capacitors/32455796094.html?spm=2114.01010208.3.17.NDzzzx&ws_ab_test=searchweb0_0,searchweb201602_2,searchweb201603_1&btsid=be8f5aad-9d8c-487c-b8e3-06fd5726d191
So I was browsing around, and I found these. These have the exact same numbers on it, so I'm guessing this will work? It's not that expensive either.

Don't do that. You are already fumbling around in the dark, don't add to it by buying cheap, unknown and probably counterfeit components from deep dark asia.
As said by mariush, go to a reputable source and get genuine components so that when you install them you *know* they won't be contributing to your problem.

[petieken]

I bought the exact same monitor about 2 years ago, also no power. The previous owner replaced all the electrolytics, but it didn't work. I repaired it in like 20 minutes, by removing the dried glue/elastomer from between the legs of a cap. The switching ic was happily oscillating again!

Verstuurd vanaf mijn HUAWEI VNS-L31 met Tapatalk

[Brumby]

Congratulations!

When I first saw the title of this thread, I was thinking you were in for a ride, especially as you don't have a lot of experience.

I must commend you on your perseverance.

Not only do you now have a functioning monitor, but you have increased your knowledge.  Enjoy the feeling - and let it inspire you to greater things!!