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Jorgen Lund-Nielsen

wrote:

Hello group,

I am repairing a failed JRL-2000F HF linear amplifier. It employs 48
mosfet (2SK408 and 2SK409). The symptom was power unit shutting down
because it detected a short circuit in the amplifier boards. I knew
all mosfet had been previously replaced.
I started disassembling the power amplifier boards looking for the
short (which looked like a diode drop on a multimeter, from positive
to negative only). I first removed all heatsink to exclude any short
on the tabs to ground. Nothing, the "short" was still there... then I
tried to power a board with a 60A power supply, thinking that the
failing component would release its smoke, well, no smoke at all,
fortunately not even from my poor power supply.
Then I removed one of the mosfets and I checked it off circuit and
noticed I could measure a diode drop from drain to source, right, I
checked on the datasheet and they're supposed to have this diode....
WAIT! what direction is this diode? Yes, you guessed, the diode was in
the WRONG way. What I have is a lot of FAKE 2SK408 and 2SK409. Looking
at the datasheet, pinout for the 2SK408 is G-S-D and for the 2SK409 is
D-S-G, the fake ones have all G-D-S pinout, no matter if they have the
K408 or K409 marking, all have the hitachi logo printed on the case,
they look indeed the right thing and they're indeed a kind of a
mosfet, I could open the channel with a few VGS volts.
Did anyone ever heard of fake components? Unfortunately I don't know
where the owner of the amplifier got the replacement "new" mosfets.

Regards

Francesco IZ8DWF

Faked parts are not uncommon, take a look he

http://sound.westhost.com/counterfeit.htm

Jorgen
dj0ud