EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: JohnAdriaan on May 08, 2020, 12:22:03 pm
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Hi all,
I have a new Tenma 72-2645 power supply (rebranded Korad KA3305P) where, as soon as I power it up, the fan starts.
According to this Korad 3305P teardown (https://www.eevblog.com/forum/testgear/inside-the-new-korad-ka3305p-linear-psu/), that shouldn't happen: it's supposed to be silent until it actually starts to deliver current.
But worse, although it's running, it sounds like it's "growling" - it's not a smoothly humming fan. I opened it up and the fan is definitely turning: it's not "twitching" in place like I've seen some fans do. So I recorded the sound (https://1drv.ms/u/s!AqGzSp2Zlpb4hbgDh7Sy3Ra4SGhlUw?e=3egT2G).
I doubt it's the fan itself, since that spins fine. It's more likely to be the fan controller or temperature sensor: it kind of sounds like the fan is being turned on and off many times per second.
But any suggestions are welcome!
[Edit]
In the following picture from the above post,
(http://picturehosting.verhey.org/korad/08-heasink_shield.JPG)
at the top left behind the large cap you can just see a trimpot (trimcap?). I was wondering if this could possibly be an adjustment to the temperature sensitivity? It's the only adjuster that I can see on the whole PCB.
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Really difficult to hear in the recording, but I'd say it's either motor commutation noise from the fan being operated at reduced voltage, or perhaps they are controlling the speed with a very low frequency PWM. Have an oscilloscope handy?
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I had something like this on an Instek PSU. The fan was basically being driven by low frequency PWM squarewave from a 555 timer and drive transistor. I cured it with an RC filter - lowish value resistor and electrolytic cap. It was quiet after that.
https://www.eevblog.com/forum/repair/cured-my-gps2032-psu-rattle/ (https://www.eevblog.com/forum/repair/cured-my-gps2032-psu-rattle/)