[Elecraft] KX1 build problem

[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
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