S620 fan "repair"

Some years ago (2004) I bought my second computer, at the time I wanted a very small desktop computer with parallel and serial port (for my electronics projects), a reasonable number of USB ports (3), audio line in/out and phones/mic, DVD-CDRW, good amount of RAM 768MB and a Celeron D.

I found the S620, it was small computer with acceptable performance, just what I wanted! The computer is now Ana's computer, she uses it to browse the web, store and edit her photos, update her blog and working from home. The computer has two shortcomings, one is that I have to reboot after a save to BIOS (the computer locks), the other is the fan noise. The really noticeable problem of the computer is the fan noise, maybe if I could fit a bit more RAM and a slower processor this could be solved. After a full day of work one could really feel that casing was warm and the Fan working hard.

When we came back from our Christmas holidays I noticed that the computer's Fan stopped working. Ana reported that the computer would start (very silently) but would never completely boot Windows. I tried booting linux and then the over temperature event was sent to the console and the computer stopped booting, after this "hint" I placed my hand on the Fan exit and noticed that no air was flowing! Surely it is a fan fault!! or so I thought... I rushed to buy one only and waited... After placing the new fan the system still did not work!!

I removed the Fan and examined it, and tried it with an external power supply... it worked!! Tried the old fan and it also worked! Then I measured the fan's output pins on the Motherboard, and the power pin had a voltage increase as temperature of the processor was increasing (checked using the BIOS monitor). When the fan was connected this voltage would drop and the fan would not start.
I suspected that "somehow" the output driver to the fan entered shutdown thinking it had a heavy load.
I decided to create a NPN(BDX33) emitter follower (a.k.a voltage follower), using the hard drive's power supply to supply more current and about the same voltage to the fan. The transistor was "fused" to the fan case using the soldering iron going through the case and the hole in the transistors plate.

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