It all depends on the circuit where you are working, in principle it has parameters such as the collector voltage and the current that are different. The C3866 could work safely replacing the C4533 but conversely, no. Regards. Ric. -------------------------------------------------- -------------------------------------------------- -------------------------------------------------- --------------------- It depends on the electrical circuit, they are the parameters that you should set. Ex: To replace a transistor in an RF amplifier stage it cannot be set only in the type of polarity and transistor that is how it could be replaced in a simple low frequency switching stage. You have to look at parameters such as the maximum power supported, the current, the CE voltage, the FT, the HFE, etc. You can put in any search engine the phrase Replacement 2Nxxx and it will surely bring you useful information. It is generally sought that the important parameters such as those described above are similar or greater than those of the original transistor but if they are different than these do not go too far. In the power Mosfet the Rds-on parameter is important apart from the supported TensionDS, the maximum current. There are pages on the Web that show some transistor substitutes but with the great variety of them there is impossible that one contains them all. Ex: http://www.reparacionlcd.com/transtandar.php https://www.electronica-pt.com/db/componentes.php?ref=2SC3866 https://alltransistors.com/es/ Regards. Ric.