Tenma 72-10480 (Korad KA3003D) power supply - noisy fan

[mrchen]

I am trying to make my Tenma 72-10480 power supply more quiet. Without any load the fan is on and can be annoying. I was going to add a little circuit with a NTC thermistor and a Attiny microcontroller to control the fan with PWM depending on the temperature. But when I opened the case I found a thermistor is already there. Nevertheless the fan speed does not change at all when I heated the thermistor with a hair dryer. I suspected a faulty thermisor but it tested fine.

One might think that the fan runs at constant speed and is not controlled, but that's not the case. There is a thermal fuse inside the main power transformer and it is connected to the main board. When I disconnect it (simulating a blown fuse) the fan suddenly starts running faster. Reconnecting it slows the fan down again.

I am at a loss here - why is the temperature of the thermistor is ignored by the fan control logic? Any suggestions / advice would be appreciated. My unit is about 8 months old and it is free from the problem that Dave described. I also wondered if I got a faulty unit. If someone owns the same / similar model and is willing to open it and test the NTC thermistor / fan speed with a hair dryer that  would be appreciated too.

I enclose a photo with the  NTC thermistor marked.

Thank you

[euzer]

With the output turned off, the fan on our 72-10480 idles away, but every 15 to 45 seconds suddenly ramps the fan on to a faster speed for a second or so, which is quite annoying.