Semiconductor Sourcing and Fake Devices (2004 )

Vintage amplifiers incorporate a variety of different types and makes of semiconductor and often exact replacement types are no longer available. What we do in this case is to use more up-to- date devices with similar or superior specification. Direct replacement  vintage audio power output transistors are particularly difficult to source. If our usual suppliers no longer stock an item, we source directly from main distributors in Europe and the USA. Due to the world wide surplus of fake semiconductor devices currently circulating, we take great care  to ensure the devices we use are full spec. This means that every batch we source are tested and physically examined* prior to acceptance and use. A typical example of  a fake power transistor,in this case a 2SB817 audio power output device, was obtained online. We could tell it was fake from the lettering and markings. We removed the plastic casing to show the internal structure. The one on the top right is the genuine Sanyo 2SB817 with a die area five times greater than the fake device. Note, the copper heat pad on the Sanyo device is twice as thick too! During DC testing, the fake device, mounted on a 2oC/W heatsink, failed in less than 20 minutes with a c/e short while operating with a collector current of 2 amps and VCE of 10V. The die size is also important for stability in some amplifiers. For example, the Quad 3O3 has long wire runs to the RCA output transistors type 38494. The 38494 use RCA's Homotaxial process. Unlike modern epitaxial power transistors, homataxial types have large dies and high capacitance. For example the collector base capacitance of the 38494 at 10V is typically 1200pF.  The epitaxial equivalent is around 200pF. So fitting epitaxial types in place of the original homotaxial variety, can cause RF instability and overheating. At Amplabs we receive quite a few amplifiers for service with similar statements to,’t 'it hasn’t sounded as good as it did before it was serviced by a local dealer’. Only to find on inspection, that one channel is singing merrily at an ultrasonic frequency. This creates a ‘muzzy’ sound and the amplifier runs a lot hotter than normal.