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ATX 700w corsair psu making noise GS700

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davidut5:
A friend gave me a faulty corsair GS700 700w power supply, it was not opened before. I saw two bulging capacitors on 5vsb and on fan control, replaced all electrolitics on the board with new quality ones(except bulk cap, measured ok). Anyway the power supply makes a lot of hissing, popping noises, especially with no load just green wire to ground. The no load noises are concerning, if i put some load anywhere say 1 amp the concerning noises disappear and remain a rather abnormal hissing.
When tested on a system, when pc is starting up it makes a lot of noise, when hdd is loading os.
The noise comes from main transformer. When the unit is warm, if i press hard on the transformer in the middle the noise almost disappears.
All voltages are good, pc runs fine under load, even tested with some 12v light bulbs and it was fine at 40A on the 12v for about 10minutes.
The pfc/pwm controller is cm6800t, the secondary rectification is with transistors, the 3v3 and 5v are done with dc-dc converters.
Got a little dso pro scope, but dont really know what to see, and where to look for, and most important how its supposed to look like.
Found a lot of similar units online with the noise problem, and with burning 12v toroid inductor coil. On the unit i have, the 12v inductor is getting a little hot with minimal load. This could be a bad design.
Turns out it is easier to repair units that have no life, than the ones that work.
I put a wire around transformer to get some readings, in the pics below.
https://youtu.be/uCE4kdqoj_s

wraper:

--- Quote ---The noise comes from main transformer. When the unit is warm, if i press hard on the transformer in the middle the noise almost disappears.
--- End quote ---
Probably transformer core halves came apart of core is loose within the bobbin. You could either take it apart if its really does not hold together after removing green tape or just fill the gaps with adhesive if it does not come apart. First check if there are no issues with solder joints though.

RoGeorge:
Working without any load means very short PWM pulses, might be quite stressful for a SMPS designed to deliver hundreds of W.  If all the voltages are good, connect it to the MB, and the hissing should go away.

Does it still hiss when connected to the PC?

davidut5:
Tested other 450w,500w,650w psu, just green wire to ground and no noise. The 650w i think has the same controller cm6800t.
When connected to pc it makes the hissing sound, and cliks/pops consistent with hdd accessing os, as i have said. So dynamic load makes it sound worse.
https://youtu.be/vWqsKOgwOJ4

Vovk_Z:
Some ATX PSU types are prone to hissing. I would sell it to somebody who doesn't care about a sound (with a proper describing).

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