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Old 02-18-2020, 08:23 PM   #1
clearchris
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Default Troubleshoot swtichmode power supply with DC Bench Power

I can't seem to find any reference to this anywhere, but I vaguely remember that you can troubleshoot a SMPS by hooking the inputs up to DC power. The bench power supply provides isolation, and a much lower voltage, so there's much less opportunity to juice yourself.

Does anyone have any experience with this method?

Right now I'm trying to troubleshoot the power supply to an Element 605 LED TV. No signs of life on the TV at all, no power on any of the outputs, even with a 12v auto bulb hooked up to the 5v line. The fuse is fine, zero signs of bad caps, burns, anything, the board looks pristine. The caps are "Chang" and the TV is six years old (2014 manufacture date) so I'm leaning towards bad caps on this one.

Last edited by clearchris; 02-18-2020 at 08:26 PM..
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Old 02-19-2020, 10:46 AM   #2
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

I did more testing. I have nothing on the 5VSB line. So the supply either has something wrong with the 5v bus or something on the high side. I think I'm going to pull the transformer, test the diodes on the cold side. I don't see any rectifiers or linear power regulators on the cold side, so one fewer thing to fail. If the diodes test good, I'll check the caps, regardless, they are getting replaced probably with panasonics. If I don't find anything wrong there, it's probably time to turn my attention to the hot side.

Thoughts?

Pics of hot attached. I can't attach the cold side pic for some reason, it is below the acceptable file size.
Full pics and pic of full board at: https://drive.google.com/open?id=1M4...6sGJvOdtPRCDyd
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File Type: jpg IMG_20200218_223029-cropped.jpg (912.8 KB, 22 views)

Last edited by clearchris; 02-19-2020 at 10:48 AM..
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Old 02-19-2020, 11:47 AM   #3
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

The chances of the transformer being bad is rare, What is the voltage across the main (large) capacitor in the primary?
Just a note, but shopjimmy has the board for $49.00

Last edited by R_J; 02-19-2020 at 12:13 PM..
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Old 02-19-2020, 07:30 PM   #4
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

Yeah, I know, all three boards together are only $75. But I'd rather rebuild it with better caps than get another one with bad caps.

I don't think the transformer is fried, it looks great. But if you pull the transformer you can test other items on the board without lifting a leg for each of them.

The voltage across the main capacitor is 168v. It's plugged into a 120v outlet.

Last edited by clearchris; 02-19-2020 at 07:33 PM..
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Old 02-19-2020, 09:17 PM   #5
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

On my bench, I'm now getting 5v from 5VSB line. In the 5v section, I pulled the diodes and caps. The diodes tested good, caps tested within 10% of label uf. ESR of caps was below 0.07 for all. Vloss was around 2%. I did notice a ticking, about once a second. When I measured between 5VSB and GND, the ticking stopped.

Can I jump 5VSB and PS-ON (maybe with a resistor?) and see if it fires up?

Last edited by clearchris; 02-19-2020 at 09:19 PM..
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Old 02-19-2020, 09:29 PM   #6
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

Yes you can do that
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Old 02-20-2020, 12:15 AM   #7
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

It's now the seemingly standard scenario that I wish I could fix. Pulsing outputs. So there's probably a short somewhere. The 24v led inverter section has to be turned on via a pin, so I'm pretty sure I can rule that out. So there remains the 5v section and the 12v section. I think I'm going to disable the 12v output at the transformer and see if the 5v section still pulses.

I checked the rest of caps, they were all nearly exactly 10% lower than the label. ESR was fine and vloss was fine. One was a bit over 10% low, it registered at 192uf for a 220uf cap. It was a 35v cap, I temporarily replaced it with a 25v cap.

I will say, this board is an absolute joy to work on. When I desolder components, they just fall right out, no messing around. They must have oversized the holes just a bit.

Last edited by clearchris; 02-20-2020 at 12:16 AM..
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Old 02-20-2020, 03:43 AM   #8
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

Hi

On the HOT side, what voltages do you have?
You checked that the Mosfets are not short
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Old 02-20-2020, 10:18 AM   #9
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

The voltage across the main capacitor is 168v. It's plugged into a 120v outlet. What else should I check on the high side.

I desoldered the mosfets on the hot side, they all tested good with my LCR-4.
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Old 02-20-2020, 11:20 AM   #10
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

Pulsing does'nt always mean there is a short, It could mean the ic is not coming out of standby, IE low vcc, When you force the power supply on, what is the voltage across the main cap? is it still 160v or does it come up.
This is an always on power supply (no seperate standby) so if you are getting +5 volts in standby, most of the components are ok.
It would help to know the ic numbers.
I would replace those two small caps in the primary (next to the large heatsink), They are likely the vcc filter caps for the ic's in the primary

Last edited by R_J; 02-20-2020 at 11:24 AM..
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Old 02-20-2020, 11:23 AM   #11
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

check the startup cap - as usual.
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Old 02-20-2020, 10:27 PM   #12
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

It lives! I replaced the small caps near the heat sink per instructions and it worked. Surprisingly, nothing was very far out of spec, but it worked anyway. Then I went through and replaced all the other caps that I had on hand. I need to put an order in for the rest of the caps, but I have a question.

I normally order panasonics FM series, or some other low ESR capacitor, but for the big cap, all I can find for 83uf 450v is "general purpose" caps. Is this ok, or should I keep looking?

Thanks!

Now I have to figure out where to put this monster tv.
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Old 02-21-2020, 09:48 AM   #13
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

The large 83f/450v is likely ok and will last a long time, but general purpose will be fine, digikey lists most caps as G.P.
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Old 02-21-2020, 11:09 AM   #14
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

Ok, I'll skip it then. Thanks!

I figured I'd do them all because this beast is cumbersome to work on.

I do have to say, I'm surprised at the high build quality though, I thought this was a cheaper tv. The back is all metal and the plastic cover that hides the boards is really thick plastic.
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Old 02-25-2020, 10:24 PM   #15
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

Quote:
Originally Posted by R_J View Post
The large 83f/450v is likely ok and will last a long time
If the PSU has an APFC circuit, I suggest to be careful, as cheap cap brands tend to not stand the abuse of APFC too long (especially CapXon.) Also, if there is an APFC circuit, I suggest minimum 420V caps, and 450V as the norm.

The fact that this PSU does have a 450V main cap will probably help it a bit. But if the cap isn't too expensive, I'd replace it with a known good Japanese brand 105C cap.
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Old 03-07-2020, 09:25 AM   #16
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Default Re: Troubleshoot swtichmode power supply with DC Bench Power

I'll get that cap on order just to have it, but tv has been completely recapped, including all boards except for that one. Fired it up, works great, now I just need to mount it. It's a heavy beast, the back is all metal.
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