K2 IF Amp Alignment and AGC Threshold

[Chuck Gehring]

I have a couple of questions about readings I have collected during my present build process.

First:  The procedure outlined for setting the AGC Threshold on page 47 recommends setting the voltage from U2 pin 5 to 3.8 volts by adjusting R-1.  In the troubleshooting section Appendix E Page 6, the procedure recommends setting the voltage to 3.9 volts.  As I understand it this is a initial adjustment that I will fine tune later and that I am just adjusting for a peak voltage.  However the maximum I can adjust my U2 Pin 5 voltage is 3.78.  Comments?

Second:  The IF Amplifier Alignment, outlines a procedure to tune the VFO to 7.000 MHz then with the L34 slug turned down 1 1/2 turns from the top of the can, adjust the L34 for signal peak.  I hear a tone at 6,998 MHz and the audible tone is pretty solid for a full one and half turns in either direction on the L34.  I hooked up a DMM to the headphone output and the voltage appears to peak at the top of the L34 can.  Comments?

Third:  I remember seeing a posting that discussed a rather elaborate procedure to Calibrate the K2 S-meter using the Elecraft XG1.  Or was I dreaming?  Can someone point me in the right direction to speed up my search.

73, KI4DGH
Chuck G.

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[Masleid, Michael A.-2]

Hello Chuck,

I also never saw much change on the DMM, but the L34 peak was close to the
top.  Later, I used the Spectrogram program.  The peak still didn't change
much, but the background noise spectrum does change.  Much of the high
frequency noise goes away as the slug is turned down, so I know that
the setting should not be too near the top.  I haven't figured out a
good way to center the band pass around the signal though, because the
audio from the low side of the IF filter bandwidth gets lost in the
low frequency audio cut off frequency.

>Second:  The IF Amplifier Alignment, outlines a procedure to tune
>the VFO to 7.000 MHz then with the L34 slug turned down 1 1/2 turns
>from the top of the can, adjust the L34 for signal peak.  I hear a
>tone at 6,998 MHz and the audible tone is pretty solid for a full one
>and half turns in either direction on the L34.  I hooked up a DMM to
>the headphone output and the voltage appears to peak at the top of
>the L34 can.  Comments?

73 de AB9GV
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[Rich Lentz]

Rather than using a DMM, while you have the rig hooked up to the PC (Speaker
jack NOT Phone jack) and using Spectrogram, follow the steps for the IF
Amplifier alignment - BUT look for the least amount of noise and a peak in
the signal.  If you don't have Spectrogram, use any PSK program with a
waterfall display and minimize the background clutter (like snow on a TV)
and maximize the brightness of the tone.  Works great and much easier.  Made
a tremendous difference on my rig.  I think if people that complained of
"Low output volume" there problems would go away.

Rich,
KE0X

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[Don Wilhelm-3]

Chuck,

The AGC Threshold setting of 3.8 volts is the normal recommended starting
value.  If you are measuring 3.78 volts, thatt is likely OK, but just as a
quick sanity check, do check the output of the 8 volt regulator U4 on the
control board.  If it is less than 8 volts, that may be the reason you
cannot get the AGC Threshold up to the 3.8 volt level - check the soldering
on the 8A line (refer to the schematic to see the points connected).

In any case, you can modify the AGC Threshold after you have the K2 up and
running to achieve maximum receiver sensitivity - simply switch between AGC
on and AGC off on a quiet band - the AGC Threshold should be set so there is
no reduction in receiver gain with AGC on.  I usually find that the final
AGC Threshold voltage is somewhere between 3.6 and 3.8 volts.

You have received several other responses about the IF alignment question -
I can add nothing more.

In answer to your 3rd question about the S-Meter calibration - Yes, one can
use the Elecraft XG1 (or any other 50 uV signal) to set the S-Meter at S-9.
The easy way to do that is just what is mentioned in the manual - change S
HI so the S-meter reads the way you want it to, but if you also insist on
full S-meter deflection with a VERY strong signal, things get a bit more
complex.  The settings of S-Lo, S-Hi, and the AGC Threshold all interact and
can be set to provide whatever S-meter action is pleasing to you - so if you
are adventure minded, I encourage you to experiment with it until you have
the settings you desire - If you are not so adventuresome and just 'want it
to work', do what the manual tells you to do - that will result in a
perfectly adequate S-meter response.

73,
Don W3FPR

----- Original Message -----

>I have a couple of questions about readings I have collected during my
>present build process.
>
> First:  The procedure outlined for setting the AGC Threshold on page 47
> recommends setting the voltage from U2 pin 5 to 3.8 volts by adjusting
> R-1.  In the troubleshooting section Appendix E Page 6, the procedure
> recommends setting the voltage to 3.9 volts.  As I understand it this is a
> initial adjustment that I will fine tune later and that I am just
> adjusting for a peak voltage.  However the maximum I can adjust my U2 Pin
> 5 voltage is 3.78.  Comments?
>
> Second:  The IF Amplifier Alignment, outlines a procedure to tune the VFO
> to 7.000 MHz then with the L34 slug turned down 1 1/2 turns from the top
> of the can, adjust the L34 for signal peak.  I hear a tone at 6,998 MHz
> and the audible tone is pretty solid for a full one and half turns in
> either direction on the L34.  I hooked up a DMM to the headphone output
> and the voltage appears to peak at the top of the L34 can.  Comments?
>
> Third:  I remember seeing a posting that discussed a rather elaborate
> procedure to Calibrate the K2 S-meter using the Elecraft XG1.  Or was I
> dreaming?  Can someone point me in the right direction to speed up my
> search.
>
> 73, KI4DGH
> Chuck G.

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