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Rather than sell the parts under their original number, they have them
labeled with a part number of their own. Often, a number of OEM parts
could use the same replacment part. (Transistors, and a lot of the
japanese linear ICs often have many equivalent parts). As a part of
their operation, they publish replacement guides with lookup tables that
give their part number for the original.
There used to be several of these companies, each with their own set
of parts with different numbers. (NTE decided to copy ECG and use the
same basic numbers). Now there a just a couple, NTE and Thomson with
their SK line of parts.
The replacment guide catalog comes in useful where, if you know that
two different parts have the same function, you can look up both in the
replacement guide catalog and see if they are replaced by the same
'universal replacment' part. (The replacment guide is also on the
web, and as a PC program with a database lookup).
If you can find the original part, there's not much advantage to buying
these parts, as they go through a couple more middlemen and therefore
cost many times what you'd have to pay at a distributor that sells a
semiconductor manufacturers products. But in many cases, you can't get
the part because they don't sell it here, or it's obselete. They're also
the only semiconductors that a small local electronics distributor will
sell, as the big semiconductor companies won't do business with them