K2 PLL range test failure

[g7vjr]

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.

Everything else is ok up to this point, eg C22 trim to 4MHz was fine,
and the control board, front panel etc are all working fine, as far
as I am aware.

I am not sure what to try in terms of diagnosis on this. So far I've
retraced all the steps building on the RF board (and actually found a
mis-placed capacitor, so that's fixed). I've also cleaned the board
in case it's that, although I must stress I am not using water based
solder, just a quality lead/tin solder. However, after all this (and
with high hopes!) I powered up again and got the same numbers.

I have been wondering if the thermistor board could be involved. It
is a tight fit and I suppose I might have damaged (ESD or heat)
something. Any suggestions gratefully received, I'm stuck!

73
Michael G7VJR

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[Don Wilhelm-4]

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.
>

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