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Old 04-09-2018, 05:24 AM   #1
RukyCon
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Default Trying to repair a cheap 12v power supply

Helloooo, i'm trying to repair this cheap 12v "5A" power supply i was given a while ago and even though i said i was not going to repair it due to how cheap and crummy it was, i was board and so i decided to pull the power supply out of my scrap pile (or what was left of it since i started taking it apart) and put new parts into it. And even after fixing most of it up it still has the same problem as it did before fixing it. The problem appears to be the circuit is pulsing on every second or two almost like it's in short circuit protection mode except there's no reason for it to be at all. The output voltage does rise up to 12v under no load so it does not appear to be shorted.

If anyone knows what's the cause of this problem then i would like to know, thanks.
(PS: The main reason i repairing this power supply is so i know how to fix similar issues in future repairs)
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File Type: jpg 20180409_034949[1].jpg (1.67 MB, 66 views)
File Type: jpg 20180409_035028[1].jpg (1.61 MB, 57 views)
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Old 04-09-2018, 09:22 AM   #2
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Default Re: Trying to repair a cheap 12v power supply

Could we see straight shot, high res pics from top and bottom of the board?

Does the PSU work for like 30 seconds to a few minutes and then shut down? Or is it going in its knees after you put lets say a 1A load on it? Maybe the protection circuit is kicking in.
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Old 04-09-2018, 10:09 AM   #3
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Default Re: Trying to repair a cheap 12v power supply

Even when the power supply is under no load it still does not want to work, it does this all the time.

Here are some better photos but be warned, my soldering is not the best looking but it does work 99.89% of the time so i don't think it's part of the problem and like i said in post one 'it still has the same problem as it did before fixing it'.

Also, any bodge jobs you see were not done by me.
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File Type: jpg 20180409_085624[1].jpg (2.06 MB, 79 views)
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Old 04-09-2018, 10:59 AM   #4
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Default Re: Trying to repair a cheap 12v power supply

Seems like the common failure mode: bad caps...which caps did you try to replace? Any one that feeds back could be at fault.

And yes, nice bodge jobs
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Old 04-09-2018, 11:25 AM   #5
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Default Re: Trying to repair a cheap 12v power supply

Did you replace start-up/running small lytics cap on the primary side?
https://www.badcaps.net/forum/attach...5&d=1523289989

The pins of the common mode filter choke L1 in the primary look like bad solder works.
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Old 04-09-2018, 04:36 PM   #6
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Default Re: Trying to repair a cheap 12v power supply

I replaced all of the capacitors with brand new ones i have on hand which were also pretested before being put in. (they're mostly test capacitors to make sure this thing worked)

Quote:
Originally Posted by budm View Post
The pins of the common mode filter choke L1 in the primary look like bad solder works.
Well i don't think it's at fault in this situation because the unit does try to start up.

Last edited by RukyCon; 04-09-2018 at 04:42 PM..
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Old 04-09-2018, 05:21 PM   #7
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Default Re: Trying to repair a cheap 12v power supply

When you say it rises to 12V, does it overshoot?

I can't read the controller chip and what its behavior is when problem situations arise...
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Old 04-09-2018, 05:30 PM   #8
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Default Re: Trying to repair a cheap 12v power supply

Quote:
Originally Posted by eccerr0r View Post
When you say it rises to 12V, does it overshoot?
It does not overshoot at all, it goes up to and stays at 12v under no load at all, if you were to put a load on it then the voltage would drop below 6v.

Quote:
Originally Posted by eccerr0r View Post
I can't read the controller chip and what its behavior is when problem situations arise...
I'm guessing you're talking about the six-pin chip on the bottom of the board, i'n case you are, the number on it is 73F32S (a number on the lower half of it being 1340).

Last edited by RukyCon; 04-09-2018 at 05:33 PM..
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Old 04-09-2018, 05:41 PM   #9
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Default Re: Trying to repair a cheap 12v power supply

Quote:
Originally Posted by ruky con View Post
It does not overshoot at all, it goes up to and stays at 12v under no load at all, if you were to put a load on it then the voltage would drop below 6v.
Ah please double check budm's suggestion of the dodgy L1 soldering then, make sure your primary side isn't too resistive! Also carefully measure the primary and make sure the voltage isn't drooping.

Quote:
I'm guessing you're talking about the six-pin chip on the bottom of the board, i'n case you are, the number on it is 73F32S (a number on the lower half of it being 1340).
Well that turned out to be a dead end of sorts... Ugh... I hate surface mount ICs...
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Old 04-09-2018, 05:50 PM   #10
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Default Re: Trying to repair a cheap 12v power supply

Tested it and both sides of the common mode choke tested at less than 1Ω each.
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Old 04-09-2018, 10:50 PM   #11
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Default Re: Trying to repair a cheap 12v power supply

When I see board in this condition, first thing I would do is to clean up those bad soldering works first to eliminate the known bad looking condition. Someone mod the board too? I will not call the soldering on that inductor pin is acceptable.
The resistance between the inductor pin if checked at the pins may look OK, but if measured between the pads where the pins are solder too is another story.
You should also the DC V meter connected to the two legs of the main filter cap (the 400VDC cap) without the load and see what happen with the load connected.
Also check R46, R43 resistance.
Attached Images
File Type: jpg INDUCTOR PIN.jpg (221.2 KB, 27 views)
File Type: jpg MOD BY WHOM.jpg (233.9 KB, 46 views)

Last edited by budm; 04-09-2018 at 10:51 PM..
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Old 04-09-2018, 11:20 PM   #12
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Default Re: Trying to repair a cheap 12v power supply

R43 tests at around 543k, I could not find R46 but looking at the board i'm guessing you meant to say R26 witch tests at 555k.

However, R35 (a 10Ω resistor used for the snubber) is shot (tests at around 1.2MΩ).

As for the soldering, i did not notice that one pin on the common mode choke was not fully soldered (yet somehow still working) but the diode package and the 680uf cap's soldering have their reasons. (the 680uf capacitors reason being a bit better than the diodes reason) But i will try to make it look better (as i was a bit short on time).

BTW i did not solder that SMD cap.
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Old 04-09-2018, 11:44 PM   #13
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Default Re: Trying to repair a cheap 12v power supply

The primary capacitor's voltage sits at 172VDC even when the output is pulsing. (This is with the diode and common mode choke re-soldered)

Last edited by RukyCon; 04-09-2018 at 11:45 PM..
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Old 04-10-2018, 03:03 AM   #14
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Default Re: Trying to repair a cheap 12v power supply

What's the voltage across C8 like, is it also going like crazy?

Now is the time it would be interesting to find the datasheet...
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Old 04-10-2018, 03:29 AM   #15
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Default Re: Trying to repair a cheap 12v power supply

The voltage on C8 slowly goes up to and stays at 10.33V.
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Old 04-10-2018, 03:42 AM   #16
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Default Re: Trying to repair a cheap 12v power supply

Here is a video i quickly recorded of what it's doing.
Warning: Flashing Lights.
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Old 04-11-2018, 09:44 AM   #17
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Default Re: Trying to repair a cheap 12v power supply

I may try to replace D5 and see if that fixes anything.

One thing i noticed while testing C8 is as C8's voltage raised, the pulsing of the output got faster.

Last edited by RukyCon; 04-11-2018 at 09:47 AM..
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Old 04-11-2018, 12:52 PM   #18
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Default Re: Trying to repair a cheap 12v power supply

I was wondering how bad the pulsing is on C8. Also the datasheet for the controller chip would tell if 10.33V was enough... Scoping it would be interesting though be careful, this is on the live side.
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Old 04-11-2018, 06:57 PM   #19
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Default Re: Trying to repair a cheap 12v power supply

It took C8 about 20 or 30 seconds to rise up to 10.33v.

I wonder if it's possible for a diode to test well under little load but stop working properly when it's under heavy load, the reason being the diode i was thinking about replacing tests fine on my DMM.

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Old 04-11-2018, 08:36 PM   #20
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Default Re: Trying to repair a cheap 12v power supply

That's pretty slow...hmm. wonder if there's any leakage paths that's causing it to charge so slow. What voltage does it get to in 5 sec?
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