Fake Motorola Transistors

I have been repairing a power amp and running into a lot of problems. I replaced the MJ15025 output transistors and still had outputs shorting with no load on the output, and for no apparent reason. After numerous transistor changes I got the amp up and running with good bias and no offset, and a few minutes later POP another shorted output. Some were shorted base to collector, some collector to emitter. The outputs weren't even warm when this occured, and .68 ohm emitter resistors only had 3 or 4 mV across them. After running out of MJ 15025s that I just purchased, i put in some MJ 15023s that I had in stock for a number of years. Whaddya know, life is good again. I'm wondering if I got some of those "counterfeit" transistors I have heard people talk about. Anybody run into this before?

Yes the MJ series is one of the most counterfeited types of transistors. Usually with rebadged 2N3055 etc. Certainly makes it hard to do a repair when your not sure if the new part is what it says it is :-(



Friend of mine bought a batch of these supposed OSD chips which where expensive chips and they didn't work. When he sanded down the top of one he found the microchip logo just above and to the left of the ST logo which proved that the chip was actually a PIC chip. Even the distributor was unaware they were counterfeit until he was shown this jpg.

Joe

Another story with Motorola
(alias ON semiconductor, alias TESLA Roznov, the factory,
which build man who lived (and has born) three houses near me )
BF256, the usual small FET, was used by my friend in some
school project. Usual situation, schematics O.K, but prototype not
living.
Then we search on google, and we looked up datasheet.
With picture, there was drain on part schemo pic, with gate legend.
The second datasheet we looked up had two variations
of pins on one TO92 part!!!
Our final and maybe best solution was to use meter and
measure FET like two diodes.
We did pin permutation as measuring say, and all was O.K.
One measurement can be better than 1000 bad datasheets.

Some tips for measurement transistors like two diodes:
In the (modern) transistor is emitter strongly dotted.
then base - emitter junction have bigger forward voltage drop
than colector - base, if you measure it by diode - checker.
Modern high transconductancy FETS are nearly symetrical,
you can not measure gate drain or gate source differency.
low transconductancy FETs can be asymetrical, and you measure
higher voltage drop at source - gate diode (as source is small in the area)

have a fun with Motorola s greetings from czech.
xvlk