Testing mosfets in an Hifi Amplifier

[drewt86]

I spent the better part of a day tracking down issues in a Hafler 9505 that came in over the weekend. Most of the problems were pretty easy to spot. Burned up bias resistor on the output mosfets and a shorted driver transistor. The driver section of the board suffered from some pretty bad burn on the board so I decided to bring the bias down from 400ma to 300ma. I also whacked some TO92 package heatsinks on to the them to give the board a break from sinking all the heat. Today when I turned it on to test it out, my fuses went in the output stage. I'm starting to suspect I have a shorted output mosfet in the array. These things are hard as hell to find so I hope they're not fried.

What are some techniques used to test mosfets? Is it possible to test these in circuit? Do they normal fail short or open?

Thanks in advance.

I've attached the schems for the 9500 as well since it's identical from the driver section on to the 9505 and the schems provided on Hafler's site are practically illegible.

[coromonadalix]

https://www.utm.edu/staff/leeb/mostest.htm

https://www.diyaudio.com/forums/solid-state/96744-test-mosfet.html

hope it help

[drewt86]

thanks for the reply. I love Allan's youtube channel. I actually came across this video. For whatever reason, I'm not getting the same results he's able to get in the video on any of these mosfets. I assumed they're something different about what they've used in this board.