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

[Chris Smith]

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.

Next I checked voltages on the thermistor board and input of the opamp. All good. However the input to U6A pin 3 from the LTC1451 DAC is stuck at 4.095V (top end of DAC range by the looks of it) which is forced up by the opamp. Popped U5 DIN on the scope and spun the VFO and it sends the same data word to the DAC every time regardless of VFO position. The CLK and CS lines are doing their thing correctly.

Anyone got any ideas why the control word is the same? VFO reading on display is changing fine. 

What can I do to get out of this hole?

Any help appreciated.

Best regards,

Chris M0XTE