"It's not only from eBay and amateurs that get caught. Plessey
Semiconductors stopped making the SL521 logarithmic amplifier in 1997
(having made it large quantities from 1963!) and a South African
manufacturer needed some in about 2000. They found some from a 'stockist'
in Hong Kong and bought the whole stock of 500 at some outrageous price -
I think it was something like $50 each. The 'stockist' disappeared, the
parts didn't work and were found to be out of spec rebranded 741 op amps -
with a 1998 date code! Of course, the South Africans came back to Plessey,
and were most upset when told that they had been well and truly
duped...It's getting quite common with older devices that scams like this
are happening."
Be careful when sourcing any semiconductor. The counterfeit market has
widened to include JFETS, low-noise BJTs, and high-end op-amps like the
AD797. These components are widely available at low cost on the Asian market
but when making a purchase through many on-line retailers, it's not possible
to trace the supply chain of the component to its origin. For example, some
Toshiba low-noise bipolar transistors (e.g., BC550/BC560) are in fact
re-labeled 2N3904/3906 devices. Unless the purchaser has access to a curve
tracer and a means to perform critical noise and frequency response testing,
one is left to trust the seller for a clean "chain of custody" between the
time of manufacture and point of sale. Because of this, I try to purchase
semiconductors direct from the OEM or through well-established distributors
like Newark, Mouser, Digi-Key, Arrow, AvNet, and Allied.
When a component becomes obsolete and is not available through the big
distributors, I tend look at specialized vendors like RF Parts and Surplus
Sales Of Nebraska. If it's still not available through a trusted vendor,
then there's little choice but to take what you can get -- but try and ask a
lot of questions about the component's source before making the purchase.
Paul, W9AC
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