Is any "short" between D-S render the piece bad? I've got a bunch of old N & P ones that test good in that they work as they should, ie closes when gate is energized, but in the "off" or "open" condition they're still conducting 1.5-2V. I've searched for an answer but every vid or tutorial shows they're only bad if it's a dead short, not this condition. Forgive me if this seems obvious, but these are obsolete units (2SK405) & I'd hate to bleep-can them without being sure.
Thanks in advance.
Thanks in advance.
FET's by their very nature have an extremely high static input impedance and this allows the gate to float and acquire charge causing the device to conduct (or appear low resistance or nearly short circuit) if the gate floats during testing.
If your FET's work OK in real life then I suspect this is what is happening when you test them.
If your FET's work OK in real life then I suspect this is what is happening when you test them.
Just to be sure, short the gate to the source momentarily whilst checking drain to source. If the FET is OK, it will be open circuit one way and possibly a diode the other.
I have been repairing power amplifiers for a local shop to me and became their technical support the very same day their old technician wrote off a Crown VZ2400 amplifier telling the customer all of the output FETs were short circuit!
As Mooly says, the gates charge easily and can give false reading but in the worst case destroy the gates.
Hope that helps.
I have been repairing power amplifiers for a local shop to me and became their technical support the very same day their old technician wrote off a Crown VZ2400 amplifier telling the customer all of the output FETs were short circuit!
As Mooly says, the gates charge easily and can give false reading but in the worst case destroy the gates.
Hope that helps.
OK, I'll have to test further. This is the first mosfet amp I've worked on so I'm on a learning curve. It uses five pairs per channel, & it had a few that were definitely bad. Upon further investigation I found that almost all of them were failing the diode test, as are the replacements I acquired, but are passing as "good" on my component tester. It's a lot of soldering to swap them out so I'm trying to be sure instead of stressing the PB's with a lot of work. Thank you for your reply.
Wow, great test. I tried a few & it worked. While getting some voltage across D-S in "open" state, they went to "OL" with a quick touch G-S, then to 0.001v with another touch G-S.
Man, you just made my day, lol. I was ready to resign this amp to paper weight status.
Thanks to you both for the quick replies.
The gate of an fet should never be left open circuit, as it could easily be damaged by static charge.
When out of circuit, always either store fets in conductive foam, or connect the gate to the source,
directly or through a resistor. The soldering iron tip must also be grounded with fets.
When out of circuit, always either store fets in conductive foam, or connect the gate to the source,
directly or through a resistor. The soldering iron tip must also be grounded with fets.
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Noted....... thanks for the tip. I read where someone suggested you wrap copper wiring around the G-S while soldering if you don't have a grounded iron, which I don't.
Something like this would work while soldering, if there is a resistor on the board that is
already connected from G-S, to protect the gate afterward.
https://www.amazon.com/Toothless-Al...W5FBT51D1PD&psc=1&refRID=Y0D4QS591W5FBT51D1PD
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