K2 pwr amps

[MARY HAYS CARNEY]

  I have tracked down some of my problems with the K2 transmitter, namely R49 that was not soldered.  Now I'm faced with the problem of very low output rf power.  I see unbalanced rf voltages at the bases of Q7 and Q8; I.e 0.20 v (rms) at the base of Q7 and 6.6 v (rms) at the base of Q8.  I have rewound T2, T3 and T4 and checked that the resistors and rfc chokes in the base circuits of Q7 and Q8 are correct.  Also the dc voltages at  the base of Q7 and Q8 are both 0.6 v. The dc resistances measured at these transistors are somewhat lower that indicated by the manual: 450 ohms from base(+) to either collector(-) or base(-).  With reverse polarity, the base to collector is
   > 3000 ohms and the base to emitter resistance is abt 750 ohms.

   I suspect that Q7 and/or Q8 are bad.  During construction I may have cooked one.  If so, should both be replaced, I.e. does Elecraft supply them as a matched pair?
  j carney
  KB3FBR
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[Don Wilhelm-4]

Yes, it would appear that either Q7 or Q8 is bad.  Elecraft has a K2 PA
transistor kit available.  Even if only one is bad, I suggest that you
be prepared to replace both transistors.

As an initial step, you may want to remove Q7 and Q8 and then recheck
the resistance from each pad to ground to be certain you do not have
some other fault.  You can check the base to emitter resistance and the
base to collector of each transistor while out of circuit.  Because the
bases are tied together by RFC8 and RFC9, it is difficult to tell which
one is bad using in-circuit DC measurements, although I suspect Q7 based
on your RF voltage measurements.  Similarly, the two collectors are tied
together for DC by both T3 and the 1-2 winding of T4.

Remove the transistors, then check the board and check the transistors
out of circuit.  After that you should have a better idea of what had
failed.  Replace one or both output transistors as needed.

The transistors are not normally matched because the K2 uses them
conservatively - the KPA100 PA transistors are a different story - those
are sold in matched sets.

73,
Don W3FPR

MARY HAYS CARNEY wrote:
>   I have tracked down some of my problems with the K2 transmitter, namely R49 that was not soldered.  Now I'm faced with the problem of very low output rf power.  I see unbalanced rf voltages at the bases of Q7 and Q8; I.e 0.20 v (rms) at the base of Q7 and 6.6 v (rms) at the base of Q8.  I have rewound T2, T3 and T4 and checked that the resistors and rfc chokes in the base circuits of Q7 and Q8 are correct.  Also the dc voltages at  the base of Q7 and Q8 are both 0.6 v. The dc resistances measured at these transistors are somewhat lower that indicated by the manual: 450 ohms from base(+) to either collector(-) or base(-).  With reverse polarity, the base to collector is
>    > 3000 ohms and the base to emitter resistance is abt 750 ohms.
>
>    I suspect that Q7 and/or Q8 are bad.  During construction I may have cooked one.  If so, should both be replaced, I.e. does Elecraft supply them as a matched pair?
>   j carney
>   KB3FBR
>
>  
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Elecraft web page: http://www.elecraft.com