How To Test An FET (2N5458)

BigBird0000 · by **BigBird0000** » Mon Sep 22, 2014 12:52 am

This is slightly off topic, but I didn't know where else to post it.

I have a B&K 747. When I power up with no tubes in, the meter pins to the right.

The troubleshooting manual shows misadjusted R10, or bad Q! (FET, 2N5458).

Does anyone know how to test an FET, so I can check the one in there, and if it is bad, so I know I am installing a good one?

VA Bigdog · by **VA Bigdog** » Mon Sep 22, 2014 2:44 am

Using an ohmmeter set to the R 100 scale, measure the resistance between the drain and the source; then reverse the ohmmeter leads and take another reading. Both readings should be equal (in the 100- to 10,000-ohm range), regardless of the meter lead polarity. Connect the positive meter lead to the gate. Using the negative lead, measure the resistance between the gate and the drain; then measure the resistance between the gate and the source. Both readings should indicate a low resistance and be approximately the same. Disconnect the positive lead from the gate and connect the negative lead to the gate. Using the positive lead, measure the resistance between the gate to the drain; then measure the resistance between the gate and the source. Both readings should show infinity.

: 2N5458.JPG (4.64 KiB) Viewed 427 times

BigBird0000 · by **BigBird0000** » Mon Sep 22, 2014 5:43 am

Thanks BigDog - that helps!
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BigBird