[Elecraft] dead transistors at the K2

[David Woolley (E.L)]

afu-dg3oo at web.de wrote:

> I am using here an K2, serialnumber about 3000 and more. Yesterday i
+ tried to tune. The effect was: no power output at each qrg. Than i
+ solder out the Transmittransistors 2SC1969 and checked with an ohmmeter
+ the routes Base - Collector, Base - Emitter and Emitter - Collector. In
+ one way Emitter - Collector i measured at each transistor an resistor
+ about 30 ohms.
+
> Now i do the same with drivertransistor 2SC2166 and i got the same
+ effect. They all died! Now i checked the predrivertransistor. oh well,
+ he is living.

That sounds unlikely unless you applied much more than the maximum 
allowed supply voltage.

I'm not convinced that the transistors are actually dead.  If I 
understand you correctly, you get a high resistance between collector 
and emitter with one test polarity and about 30 ohms with the other 
polarity.  My guess is that the 30 ohms happens when the emitter is 
negative with respect to the collector and represents what the meter 
displays as a result of the emitter base junction breaking down because 
its maximum reverse bias has been exceeded.  The voltage at which 
emitter base junctions zener is quite small.

 From what I've heard the most common cause of an indication of no power 
output is electromagnetic pulse damage to the power sense diodes.  This 
failure actually causes maximum output, but the indicated output is zero.
+

> But i dont know, how i killed the transistors. the cause or reason ist unknown. 
+
> Then i bought in the local elektronikshop for little money new
+ transistors with the logo from mitsubishi. Then i called via phone the
+ elecraft distributor in germany and he told me, that most of these
+ transistors are fakes.

I'm not clear about this.  Did it work with these.  If it did, that 
would indicate that the originals really were damaged, or at least some 
of them where.  If its still didn't work, that would indicate that the 
originals were probably OK.


-- 
David Woolley
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