[Elecraft] [K2] No Transmit Power

[Don Wilhelm]

  Brian,

VPWR is high because there is not enough output being sensed by the RF 
detector, and the K2 firmware is telling the control circuit to "add 
more power".
With that fact in mind, there does not seem to be sufficient RF voltage 
at the Xmit Mixer Output, and the other points beyond it as well - but 
we will come back to those at a later time if necessary.

You are correct that the Pre-driver output is the first point 
significantly lower than the expected value (and all measurements beyond 
that add no further value at this time), so the Q5 circuit is the one to 
investigate.  I would suggest temporarily removing T3 to remove the 12 
volts from Q7 and Q8 and gives you the ability to safely make 
measurements without the heat sink installed (Do NOT transmit with the 
heat sink removed and 12 volts connected to the collectors of Q7 and Q8 
- the PA transistors will fry quickly).

The first thing would be to make DC measurements at Q5 and compare with 
the chart in the manual.  If nothing looks awry with those measurements, 
then look at the schematic and find all components in the Q5 stage - 
examine each one for proper values, good soldering - even to the point 
of arbitrarily re-flowing the solder with a hot soldering iron (750 
deg/F).  Add a wee bit of solder during the re-flow process to add some 
additional flux.  If there is a blob of solder on any connections 
already, wick the excess away and then resolder with a smaller amount.

Most new build problems turn out to be soldering problems, followed by 
toroid lead tinning and by component mis-placement.  Those components 
jump out of their proper holes and get soldered into the wrong places :-) .

73,
Don W3FPR


On 9/7/2010 10:16 PM, Brian Denley wrote:
> Don:
> I brought home a DSO5054A (very nice 500 MHz Agilent scope!) and I was able
> to take the better measurements.  My measurements were in P-to-P but I have
> divided them by 2.8 for RMS comparison:\
> (all measurments Keydown)
>
> 1.)    VPWR line (pin 2 of U8 on control board):    5.0 volts        this is
> HIGH
>
> 2.)    Xmit Mixer Output (pin 4 of U10):    28 mV
>
> 3.)    Buffer Output (pin 6 of U9):    121 mV
>
> 4.)    Band-Pass Filter Output (W6):    42 mV
>
> 5.)    T-R Switch #1 Output (anode of D1):    41 mV
>
> 6.)    Pre-Driver Output (collector of Q5):    34 mV           this seems
> LOW
>