WEP 858D rework station - Excessive voltage destroing fan

[Matthew98]

Hey, guys. Sadly, I need your help again. Thought I´m in a need of a hot air gun station for SMD, so I ordered one cheap one from different country. Today I unpacked it and to my surprise, Its not working. I dont want to go to truble of sending it back, so I open it up and start measuring.
Here Is what happened after I tested it : temperature regulation works just fine, but fan spins on minimum RPM untill I crank it all the way up. The fan span quickly, but on the second test it did not work at all. I opened it up, tried to connect the fan on a power supply and its shot. I measured the voltage on the fan connectors and its ove 40V at the maximum settings with load on it (the fan is 24V 0.15A, so I connected 150R resistor to it.) Doesnt sound so good, does it ? I took a look at the main board and the potentiometer for the fan and both buttons were not soldered to the board ! (or only with tin vapors it seemed) I fixed that one up, but no change.
I guess its familiar PCB layout Chinese people were copying for centuries, so I will post some photos down here and ask you guys nicely, any idea what could be the problem ? I will order new fan in the meantime.

[Matthew98]

Found this on the internet. It seems to be very similar.
https://blog.enbewe.de/posts/2018-06-21-saike-858d-original-schematic/

It says the power supply for fan controlling circuit is 40V, so there is where the voltage is going from. The circuit is supposed to identify, when the fan is overloaded. I´m guessing thats where the fault is... From quick read through it, I dont really understand how it does what it does, though...

[Nusa]

You said the fan was shot. Replacing that sounds like the first step. Unless you have another 24V fan suitable for testing purposes.

[Matthew98]

Yes, I will. I´m just a little afraid I will fry the new one as well, as the old one was kind of working at first. It could have been on its way out, or it was the 40 V :/

[amyk]

Use a dummy load. A 120V incandescent bulb would be fine with 40V, for example.

[fzabkar]

ISTM that Q2 could be shorted. In any case the fan control circuit is a linear regulator, so it can operate unloaded.
 

[Matthew98]

OKey, guys. The new fan arrived and there is some news.
The voltage control is not linear AT ALL its about 10V almost all the way to the end and then it jumps up to 25+V at the end. See the photos.
The rework station is usable though now and the temperature control works a treat.
Any idea what could cause this behaviour ?



--------------------------------------------


--------------------------------------------

[amyk]

Check the potentiometer.

[Matthew98]

Check the potentiometer.

... You got it mate  Thank you ! Can´t bealive I didnt check it in the first place.
It is working a treat now.
Thanks again.