[Elecraft] KX1 + KXB3080, Q7 voltages

[Don Wilhelm]

Kevin,

That is an Ooops that has not yet been caught thanks for spotting it.  The
1k resistor designation in the KXB3080 manual should be R28 instead of R2,
and by the way, there is another error on that schematic excerpt too - the
value of C56 should be 68 pf instead of the 6 pf shown.  It looks like the
digits were truncated in the schematic (or it was typed on an old typewriter
with a missing '8' key <G>).

73,
Don W3FPR

> -----Original Message-----
>
> Don,
>
> Yes, you are correct. Lifting one end of R28, Q7 still had a negative
> voltage at its base during transmit. I'll change out Q7.
>
> I'd like confirmation weather the KX1 circuit diagram on page 18 of the
> KBX3080 manual is correct or not. It shows Q7's base connecting to R2, a
> 1Kohm resistor.
>
> Thanks Don,
> 73,
> Kevin, KF0JX
>
>
> Don Wilhelm said the following on 11/01/2006 09:55 AM:
> > Kevin,
> >
> > There is no change in the base circuit of Q7 - that is true whether the
> > KXB30 or the KXB3080 is installed.  You should be tracking down
> the source
> > of the negative voltage at Q7 during transmit.
> >
> > Yes, the voltage at the base of Q7 should be about 0.7 volts
> during transmit
> > (long explaination of why follows:) - The source of the 6 volts during
> > transmit is at pin 7 of U3, then it goes through R26 and then
> through R28 on
> > its way to the base of Q7.
> >
> > When the 6T signal becomes active (it should during transmit),
> among other
> > things it is applied to the base of Q7 to saturate Q7 - effectively
> > producing a short between the collector and emitter of Q7 - the
> reason is to
> > protect the receiver input from the transmit RF.  When a transistor is
> > saturated the base to emitter voltage will be close to 0.7
> volts - if it is
> > anything else, either the transistor is bad or something else strange is
> > wrong.
> >
> > There is not a negative voltage produced intentionally in the
> KX1 - I would
> > guess you have a bad Q7 which is acting more like a diode and
> rectifying the
> > RF on its collector to produce a negative voltage at the base.
> If you want
> > to do a test to confirm my suspicion, lift one end of R28 and
> try a short
> > transmit while measuring the base voltage of Q7 - I would guess
> that it is
> > producing a negative voltage all by itself, if so, replace it
> (and put R28
> > back in place).
> >
> > 73,
> > Don W3FPR
> >