[Elecraft] K2 PLL range test failure

[Don Wilhelm]

Michael,

First check RP2, D16, D17 and R19 for correct values and good soldering 
- in the case of the varactors, be certain they are oriented correctly.

If there is still a problem, try to narrow it down to the source.  With 
the K2 menu set for CAL FCTR, measure the DC voltage at RF Board U6 pin 
3 - when you tap the BAND+ button, the voltage should rise to about 4 
volts, and when you tap the BAND- button it should reduce to almost zero 
volts.  If so, the DAC (U5) is functioning correctly and you can move on 
to the next point which is U6 pin 1.

U6 pin 1 should have almost 2 times the voltage seen at U6 pin 3 - so 
tap the BAND+ button and see if it goes up to about 8 volts, and tap 
BAND- to see if it goes to zero (or close to zero).  If that is 
happening, the thermistor board is likely OK and the problem lies with 
R19,  RP2, D16 or D17, L31 or perhaps even the crystal X1.

A separate check on the 'health' of the thermistor board is to check the 
voltage at U6 pin 5.  It should be very near 4 volts - if it is not, you 
likely have a solder bridge on the thermistor board.  Look carefully 
where RE and RC have their ends near each other.  There should be no 
connection between these two resistors at that point, but the solder 
pads are quite close and easy to bridge with solder.  You can often gain 
adequate visibility and access to the back of the thermistor board 
without removing it by removing the front panel and Control Board.

73,
Don W3FPR

Michael Wells G7VJR wrote:
> Hi -
>
> I'm part way through building a K2 (previously, I have built a KX1,
> which was a success). On the K2, I am on the Alignment and Test, Part
> II section where the PLL reference oscillator range test is performed
> using the probe connected to TP3. This is on page 63 of the guide.
>
> Unfortunately, my numbers don't look good. The Ref. High Freq is
> 12109.52 and the Ref. Low Freq is 12107.88 (the difference is just
> 1.64kHz, far below the acceptable range of 9.8 and 15kHz). In
> particular, I was expecting the low freq. to be 12080-12090.
>