help repairing SMPS 30A 13.8v chinese power supply

[george79]

Hello,
im asking for help or a schematic for this chinese copy power supply, im posting photos of the board inside, it a copy that sold in hundred names and different boxes, but the internals are the same, output 28-30A at 13,8v or adjusted voltage.
My problem is little weird, i have repaired a lot of switching power supplies with success, but here i see something for the first time.
When i turn ON the power supply, it makes a try for a second to start and after that a clicking sound heard from somewhere in the board. And of course no output voltage or working. But when i connect with a touch the base and collector of the Q3 transistor, it starts working as it should!!!
It get me crazy, and i cant get any info like a schematic to search it. There is a lot of SMD parts on the module board, which is build around a TL494. Board have 13pins, so i cant find info for this too, because i can found only for 10 pin boards

Thank you for your time
George

[george79]

thats the big trouble with the chinese SMPS, no schematics and help at all 

The main board is simple to understood and work with it, my question is on module board, because it is SMD parts and really difficult to extract a schematic fro it, have someone any info on it? or somewhere to find a new one etc??

Thank you
George

[AcM]

Hi,

what do you mean with

when I connect with a touch the base and collector of the Q3 transistor, it starts working as it should

[george79]

connect the 2 pins of the transistor for a second, and then the power supply start working as it should and working all the day all normal.. It is something like the starting pulse not appeared or something that i cannot figure out.

[MathWizard]

Look up the datasheet of the controller IC, if it's written on there, and see if any of it's protections are being tripped on start-up.

I think I have a computer PSU somewhere that had a cycling on startup problem. I can't wait to have a go at that again.But the worst thing is I don;t have a good way to test those high power PSU's, besides my own high power PC

[timenutgoblin]

When i turn ON the power supply, it makes a try for a second to start and after that a clicking sound heard from somewhere in the board. And of course no output voltage or working. But when i connect with a touch the base and collector of the Q3 transistor, it starts working as it should!!!

The way you're describing this sounds like (capacitive) coupling acting between your hand (and the tool you're using) and the (BJT?) transistor. Check the solder joints of Q3 and surrounding components. Also, check the power supply components (capacitors) surrounding the TL494 PWM controller.

Do you have a picture of the underside of the PSU?

If the PSU is rated at 28 - 30 amperes, what if you connect a 12V lamp to the output of the PSU? Does connecting a resistive load to the output influence the PSU behaviour?

[inse]

Please put some effort into providing decent photos of top and bottom of each board.

[george79]

Thank you for the tip,
i have connected a 12v/25W lamp to the output, then i turn on the power supply, clicking sound heard as before, BUT when i try to start it connected the two pins of the transistor, it tries to start, when i remove the jumper cable it is off again, and clicking sound heard. When i remove the lamp from the output, and i touch the two terminals of the transistor it starts and have a constant 13,8V. IF i connect again the lamp, it stops and herd the clicking sound.

So i think a bad cap somewhere?

[Swake]

Unsolder the caps at the output and measure them (with a good esr meter)