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I'm trying to get a circuit to work (linistepper driver) and I suspect I may have fried the drive transistors (tip122's in this case).

Are there common behaviors of broken transistors?

does c->e short? b->e? open?

Can I measure with my multimeter to get any meaningful information?

is it possible for devices to half break with degraded performance?

I'm looking for generalities. I also have an old stereo which has a broken channel.

I'm not particularly good at desoldering, so it'd be good to have some confidence that I'm looking at the right component.

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1 Answer

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In my experience, power transistors fail shorted. All the pins will measure low resistance to each other.

TIP122 is in TO-220 case. All three leads are in a row, at least in the linistepper. The way to desolder this part is to add solder until all three joints are bridged in a big solder blob. Melt the blob, then pull the part out. Then clean the solder blob up with a solder sucker, then braid. Do not attempt to desolder one pin at a time.

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+1 for the desoldering technique. I was not aware of that. – Earlz Apr 3 '11 at 18:32
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Alternatively, a pair of dikes can usually remove each pin individually. I find that it's hard to get the solder out of the holes when they're attached to planes (like a power transistor should be); I prefer to use braid while the transistor is still on the board and then clip the leads and remove them one by one. If you're lucky, there won't be any solder left. – Kevin Vermeer Apr 3 '11 at 20:45

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