[Elecraft] K2 question regarding Pin 24 U6 control board operation

[Don Wilhelm]

Will,

Yes, the voltage on U6 24 is a 'red herring'.  That is a serial data 
line, and while you might observe the +5 volts active signal at times 
with a 'scope, much of the time it is inactive and at zero volts.

You can check the operation the data into the DAC which controls the PLL 
reference oscillator most easily by observing the DAC output - set the 
K2 to CAL FCTR and observe the voltage at RF Board U5 pin 7 as you tap 
the BAND+ and BAND- buttons alternately.  You should see the voltage 
change between close to zero volts to about 4 volts.  If you do see a 
change, then you can conclude that the SDO signal is being properly 
received by the DAC (U5).

It would appear that your PLL reference oscillator is not oscillating  - 
you should see a signal at approximately the PLL crystal frequency of 
12096 mHz.  Check the voltages at Q19, check to be certain the leads of 
RFC14 are well stripped, be certain D18 is oriented correctly.  If that 
does not solve the problem, look at the schematic for RF Board sheet 1 
and find the PLL Reference Oscillator in the upper left corner. 
Identify all the components associated with that part of the circuit and 
check them for correct values and proper soldering.

73,
Don W3FPR

Will wrote:
> 
> Hi all,
>     This question is more to fulfill my curiosity, but maybe it has to 
> do with the problem I'm experiencing with the PPL reference oscillator 
> in the synthesiser section.
> 
>     While trying to debug why I was getting a reading of 190.00 -/+ 2KHz 
> in the TP3 PLL reference oscillator test point, I decided to do some 
> voltage checks.  I discovered that pin 2 of U5 on the RF board wasn't 
> getting the 5V I was expecting according to the voltage tables.  I 
> traced this all the way back to the control board, U6 pin 24.  the 
> voltage reading was 0V, so thinking this is a digital output, I 
> hooked-up a logic probe and confirmed the voltage reading.  I conirmed 
> this same reading at pin 5 (SD0) of U4 on the RF board.
>