KX1 build problem

[Richard Fagar]

Thank you in advance for your help.  I built a KX1, though I had it working
on 20-40 as I could receive great and made a few contacts.  No idea what the
power level was.  I added the 30 80 board and finally the tuner.  The tuner
is showing .4 watts topps.  I don't have a wattmeter, however someone showed
how to use a diode - capacitor and multimeter to get a good aproximation.
Again I am getting from .3 to .5 watts.  I have done the power mod, so I
started to look at voltages.  All are good except Q7 which should have a
base voltage in tune of .7 volts.  I am getting a negative .7 .8 volts
depending on band.  Since this transistor seems to be acting as a switch
driven by a line driver, driven by the MCU, I am wondering if it is safe to
lift a leg of the resistor feeding it and see what shows up.  I am fearfull
of dumping high level rf into somewhere it shouldn't be by doing this.
 
Has anyone else had this problem?  Looking for any help or solutions.
 
Thank you,
          Richard  KK4UFY
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[Don Wilhelm]

Richard,

Do not lift the leg of R28, it will not do any good.  You might want to
re-flow the soldering of R28 and see what happens.

I don't understand how you have a negative DC voltage at the base of
Q7.  That base voltage comes from the 6T voltage rail in the KX1 and
then flows through R28.  Measure the voltage at both ends of R28.  
During transmit, one end should be about 6 volts and the other end
should be near 0.7 volts (the base to emitter voltage of a typical
transistor).  In other words, Q7 should be saturated which places the
capacitor CA and (depending on the band) C26 in parallel with the input
of the Low Pass Filter into the LPF circuit while protecting the
receiver front end from the TX RF. If that does not happen, the LPF will
not be tuned properly.

Measure again carefully and if still not in line with the expected
voltages, replace Q7.

On the low RF output, if you have an oscilloscope with a 10X probe or an
RF Probe, I can give you comparison RF voltage readings at each relevant
point in the transmit chain.  If you want to do that, perhaps it is best
we take this off-reflector and do it via private email.

73,
Don W3FPR

On 8/9/2016 6:17 PM, Richard Fagar wrote:

> Thank you in advance for your help.  I built a KX1, though I had it working
> on 20-40 as I could receive great and made a few contacts.  No idea what the
> power level was.  I added the 30 80 board and finally the tuner.  The tuner
> is showing .4 watts topps.  I don't have a wattmeter, however someone showed
> how to use a diode - capacitor and multimeter to get a good aproximation.
> Again I am getting from .3 to .5 watts.  I have done the power mod, so I
> started to look at voltages.  All are good except Q7 which should have a
> base voltage in tune of .7 volts.  I am getting a negative .7 .8 volts
> depending on band.  Since this transistor seems to be acting as a switch
> driven by a line driver, driven by the MCU, I am wondering if it is safe to
> lift a leg of the resistor feeding it and see what shows up.  I am fearfull
> of dumping high level rf into somewhere it shouldn't be by doing this.
>  
> Has anyone else had this problem?  Looking for any help or solutions.
>  
> Thank you,
>            Richard  KK4UFY
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]
>

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