Hello,

I'm a neophyte in electronics and I have a motherboard (TitaniumIE for a Pentium MMX) that is older than a decade but is still functioning without problem. However, I noticed a transistor (SEC, 651, TIP120) that is discolored (it has a slight brownish/yellowish color instead of being pure black). Moreover, the black anodized aluminum heatsink on it has taken a reddish hue.

Personally, I think this transistor may have overheated and have been damaged. In the case it needs to be replaced, are all TIP120 the same? I ask this because Mouser ( http://ca.mouser.com/Search/Refine.aspx?Keyword=tip120 ) has different types of TIP120 available and I prefer to first ask people who are more knowledgeable than me and avoid potential problems down the road.

Thanks for your help.

 

Yeah, you will be ok with any on the Mouser page.
There different manufacturers, but rated the same. Choose one and go for it.

 

Quote:"I think this transistor may have overheated and have been damaged."
Which begs the question:"What was pulling current to make it hot"
Of coarse this assumes that the transistor circuit/ heat sink arraingement was proper from the get go.

 

If there is no sign of thermal grease, that is a common failure reason. The IC cant transmit the heat to the heat sink so it frys.

 

It could be the part designed to fail after 10 years to force you to upgrade (cynical - me?).
Best get a heatsink for it too and then it will stand a better chance.

 

I'm not sure why this particular transistor was so hot especially since this motherboard has never been overclocked and temperature around here (even in summer) isn't terribly high. However, it's true that the heatsink on it was inadequate; it was just a small piece of metal bent into a U shape without any fins and there was no thermal paste either.

At any rate, I removed the old transistor and ordered a handful of new ones. Since the transistor is located in a corner of the board, I will be able use a larger heatsink that I have salvaged from a broken power supply and with some thermal paste to make sure there is a good contact between them, I think it should be fine.