Trying to troubleshoot a DC computer fan

Hello,

I just bought a new case / CPU cooler fan which im using for a different purpose. For no apparent reason, it simply stopped working. I took it apart and checked power on the circuit and its definitely getting power. So I went to the electromagnet control chip which has these markings on it:

FTC
s310.4wi

I looked at the data sheet for it, but could not specifically find the data sheet that includes the 4wi portion of the packaging ... no big deal, I thought ... but of the few different S310 data sheets that I looked at, I noticed that Vcc on pin 1 can usually vary from 3 to 20 volts. However, I'm getting 2.5 volts on Vcc, which I think is my problem. I am using a 12 volt power source, the same one Ive been using to test the fan. I have tried more than one power source since the fan quit.

In an effort to troubleshoot the possible low input voltage, I traced Vcc over to a cap then to a resistor etc. I thought maybe the cap went out, so I pulled it and tested it, and it rates within 10% of printed spec.

My problem is, I don't know where to go from here. I can see no obvious burn marks on the circuit board, nor can I see any obvious damage to the rest of the circuit. I did initially take my heat gun from my soldering re-working station and I re-melted all of the solder on the board just to make sure there weren't any cold solder points...

I'm at a loss. I cannot get the inductor coils to show any kind of signal at all. I don't have an oscilloscope, or I'd look at the two output pins on the IC to see if they were doing anything. I did put my volt meter (A Fluke 115) into AC voltage mode and I read 0v at both outputs of the S310, which adds up since the coils are not lighting up.

Any thoughts on this? Here is a picture of the circuit board if that helps anyone:



Thanks to anyone who chimes in.

Hey michael, personally I wouldn't bother repairing computer fans since they're so "cheap", but then again, there are some models that are pricier and perhaps good to repair. But, as for that 4-wired component, this datasheet I found:
http://www.feeling-tech.com.tw/km-master/ezcatfiles/cust/img/img/26/fs310lfv04.pdf
in typical application circuit, there is diode before vcc pin, so that would explain voltage difference.
BUT, what sort of fan is it? I'd presume It's 4-wire (vcc,gnd, tacho, PWM)
Normally these require only vcc+gnd, but some models need PWM also (annoying).
EDIT:above is quite irrelevant since it did work for a while at least.

Michael Sims said:

AC voltage mode and I read 0v at both outputs of the S310

Click to expand...

You're better off using DC setting, I'd think. You can test hall-sensor with magnet, outputs give different output depending on polarity, see datasheet for idea.
Also, check that there is continuity between coil com to l1/l2, in case there is looming open circuit there.

That's all I could think for now, since you alrealy re-melted solder joints.