[Elecraft] K2 PLL reference oscillator problems (DAC side!)

[Don Wilhelm]

Chris,

If your kit was shipped between mid-August 2018 and mid-January 2019, 
then the most likely problem is that Q19 should be replaced with a new 
J310 from Elecraft.

As for the 7.07 volts from U6A pin 1, that is not necessarily the cause 
of the problem (Q19 should oscillate even with almost 8 volts from U6 
pin 1) - if it does not, then the problem is Q19.

While waiting for a replacement J310 from Elecraft, try putting a small 
value capacitor from the source of Q19 to ground - use 10 to 15pF.  Then 
measure the PLL Reference oscillator range using the counter probe in 
TP3 and entering CAL FCTR in the menu.  You should have oscillation at 
both the low (BAND-) and high (BAND+) and the frequency of the high end 
should be near 12100 kHz.

Another potential problem if the voltage on U5 pin 7 (and as a result U6 
pin 1) can be that the /DAC2CS input to U5 pin 3 is always at or near 
ground - check for that condition, and if present, find the solder 
bridge.  That signal line should go to ground as you rotate the VFO, but 
otherwise should be at 5 volts.  You will need an oscilloscope to see 
the negative transitions.

73,
Don W3FPR

On 5/21/2019 12:27 PM, Chris Smith wrote:
> Hi,
> 
> I've just completed my K2 up to the PLL reference oscillator.
> 
> Initially when I assembled it the PLL reference oscillator would not oscillate at all. No reading on internal counter. I popped it on my external Racal counter and still dead. After tracing the problem out I found that the voltage on input to the varactors was pegged at 7.07V which was too high for the oscillation to carry on. I lifted one end of R19 on the output of U6A and the thing oscillates fine isolating this to a control voltage problem.
>