[Elecraft] PA transistors

[Lyle Johnson]

Hello Vic!

> I have four 2SC2879 transistors, such as are used in the KPA100
> PA.  When I measure the forward voltage drop of the emitter-base
> jucnctions I get:
>
> #1: 0.592v
> #2: 0 v
> #3: no continuity
> #4: no continuity
>
> question 1: I presume this means that only #1 is good?

Well, it means the base-emitter junction of that transistor is still at
least partly good. The transistor may still be bad :-/  The other 3
transistors are clearly zapped.  If it was operating in the same circuit at
the same time as one or more of the others, I'd be suspicious of its
long-term relaibility.

> question 2: Is it necessary to replace both transistors with a
> matched pair in a push-pull circuit like the KPA100?

It's best, unless the circuit it came out of has individual bias current
adjustments for each transistor.

> question 3: Assuming #3 and #4 are bad, what conditions cause this kind of
> failure?

The B-E junction can go if it is overdriven (e.g., too much current).  There
may be other ways it could go as well, such as excessive reverse voltage...
You might check the B-C junctions as well.

73,

Lyle KK7P