Recently we have noticed many low cost "Toshiba" 2SK170BL and 2SJ74BL in the market. To identify whether they are genuine, the serious method is by putting them on a curve tracer. Otherwise, a reliable way is to use an ohmmeter to measure the resistance between the drain and source. Genuine 2SK170/J74 has a resistance of 50 ohms or below. Fake ones have a resistance over 100 ohms.

It was heard that the fake ones were remarked from 2SK246/J103. But when these FETs are also out of production, people remark 2SK30ATM for 2SK170BL. The 2SK30ATM has much lower admittance (thus gain) than the original 2SK170, so the fake ones cannot function well in a 2SK170 circuit.

A further study recently found that some "2SK170 and 2SJ74" emerged in the market at a super low cost. We have got some and did measurement. The 2SK170 has similar channel resistance as the original 2SK170 but lower transconductance. The 2SJ74 has much higher channel resistance. If these were used in complementary application, it may result in serious distortion as they are not complementary at all.