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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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fake mosfet!
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 |
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