Repairing/Replacing Jags

[dudefise]

We have several bad grey jaguars from last year that Luminary can't replace as the deadline expired... We have been looking to repair them, and assume the problem is ruined MOSFET's. We have found several MOSFETs that read differently than the others. First, there is continuity between some of the tops of the mosfets and one connects all 3 pins, while 2 adjacent ones connect the two outer pins. I am not very experienced in this area, so I greatly appreciate any advice you may have about repairing them!

About the pic: The shorted one is the one thats by the fan connector, marked black on top by sharpie... the two other weird ones are right above it.

[WizenedEE]

I think you forgot to upload the picture.

In general for things packaged like that, the tops are common (ground). They should be shorted together. Okay, I shouldn't say in general, but they are often.

How did they break?

[dudefise]

Sorry about the picture... i guess it was over the size limit, i'll resize when I get home after robotics. Thanks for the info!

[Al Skierkiewicz]

Dude,
The MOSFETs are in four groups of three (12 total) on the old Jags. The gate leads are fed separately through resistors but the drain and source are in parallel. There are two groups where the source leads are tied to power supply and two where the drain leads are tied to power common. Most often the FETs take out other components. Is the transistor isolator melted? Have you found any aluminum dust or shavings? Is any part of the board burned?

[dudefise]

Thanks for the advice!... we are looked for damage to the isolater but there isnt any obvious damage to the board. Where on the board is the isolator? Also, we found metal shavings inside and assume it burned the MOSFET...

[Al Skierkiewicz]

Dude,
The isolator is the star shaped, brown plastic device that surrounds the FETs. It is inteneded to keep the FETs from touching each other. The tabs are tied to one of the leads so they can have voltage on them. If you have metal shavings, assume other damage has been done in addition to the FETs.