[K2] VCO Test fail

[Stephen vk2rh]

I'm enjoying building K2 #7076 and have made it to the Alignment and
Test, Part II section on page 63 of the revision H manual.

I've completed the 4MHz Oscillator Calibration check and the PLL
Reference Oscillator Range Test and the K2 is within expected spec.

For the 4 MHz Oscillator Calibration the counter reading was 12098.69
with regular fluctuation 10-20 Hz or so. Is that okay or a sign of
trouble ahead? Wasn't sure whether this was what the manual meant by
'changing rapidly' and whether the readout needs to be rock steady or
not. I've used both an external frequency counter and a K3. The signal
sounds solid to my old ears!

Assuming it was okay I proceeded on to the PLL Ref. Oscillator Range
Test. This yielded a range of 12.46 kHz - within acceptable range from
9.8 - 15 kHz.

However the VCO Test on page 63 results in a frequency of 7470.10
instead of one within the acceptable 8 - 10 MHz range. It's not clear
which parts of the troubleshooting advice apply at such an early
alignment stage. I've checked and re-checked components and solder
joints in the PLL Reference Oscillator and the PLL Synthesizer parts
of the board.

So I'd like any advice about what I need to check or do to get the VCO
Test to land within the correct 8 - 10 MHz frequency range. Most
advice I could find via searching previous email discussions was about
the VCO Alignment which is the next stage, but I figure I need to get
over this hurdle before that next one.

73
Stephen VK2RH
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

[Don Wilhelm-4]

  Stephen,

The 4 MHz Oscillator Calibration will only be as correct as your
external frequency counter or your ham-band receiver.  You can re-do
that calibration with precision using the N6KR method and receiving WWV
once your K2 is complete.  I highly recommend that - see the K2 Dial
Calibration article on my website www.w3fpr.com for all the details.

The "dithering" of the low order digit on the K2 CAL FCTR display is
normal, and is not what is referred to as "rapidly changing".

While the target frequency when you are tuned to 4000 kHz may be off,
there can be a number of reasons for that, including the VCO voltage
alignment.  I suggest that you first do the next step which is to check
the DC voltage at the left end of R30 for all band edges.  After you
have successfully done that, the other  intervening tests. and then run
the VFO Linearization step on page 66 - re-visit the frequency of the
VCO when tuned to 4000 kHz - if it is not correct, then send another
query to the reflector.  Any problems here revolve around the VCO which
involves T5 and the capacitors in the VCO circuit.  You may want to
check the varactor diodes to be certain they are all of the right type
and are installed correctly.  The smaller 1SV149 varactors do not have a
fully rounded back, but the two tapered edges denote the "rounded"
side.  Look at them from the top and the rounded side should be obvious.

73,
Don W3FPR


On 1/6/2011 11:32 PM, Stephen vk2rh wrote:

> I'm enjoying building K2 #7076 and have made it to the Alignment and
> Test, Part II section on page 63 of the revision H manual.
>
> I've completed the 4MHz Oscillator Calibration check and the PLL
> Reference Oscillator Range Test and the K2 is within expected spec.
>
> For the 4 MHz Oscillator Calibration the counter reading was 12098.69
> with regular fluctuation 10-20 Hz or so. Is that okay or a sign of
> trouble ahead? Wasn't sure whether this was what the manual meant by
> 'changing rapidly' and whether the readout needs to be rock steady or
> not. I've used both an external frequency counter and a K3. The signal
> sounds solid to my old ears!
>
> Assuming it was okay I proceeded on to the PLL Ref. Oscillator Range
> Test. This yielded a range of 12.46 kHz - within acceptable range from
> 9.8 - 15 kHz.
>
> However the VCO Test on page 63 results in a frequency of 7470.10
> instead of one within the acceptable 8 - 10 MHz range. It's not clear
> which parts of the troubleshooting advice apply at such an early
> alignment stage. I've checked and re-checked components and solder
> joints in the PLL Reference Oscillator and the PLL Synthesizer parts
> of the board.
>
> So I'd like any advice about what I need to check or do to get the VCO
> Test to land within the correct 8 - 10 MHz frequency range. Most
> advice I could find via searching previous email discussions was about
> the VCO Alignment which is the next stage, but I figure I need to get
> over this hurdle before that next one.
>
> 73
> Stephen VK2RH
>

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html