Strange behaviour in K2

[la2moa-2]

Hi group!

I have just finished the alignments and tests (part II) in my (second) K2-project. Sofar all resistance checks are ok, and the results of the alignment and tests have also been ok. Except for one thing: I think that the AGC in the radio behaves rather strangely. It's pretty nervous in the "Fast" position and switching to slow makes it a little bit better, but not perfect. I have also noticed a "chuffing" or "clicking" sound in my heaphones while tuning the radio aound on 40 metres. This sound (which indeed is very difficult to describe) corresponds with the change of digits on the display (regardless of tuning rate) and is very significant with the appearance of a strong signal. Tuning the BC-band above 7100 KHz is very annoying. The behaviour disappears when the antenna is disconnected, but occours again when connecting a signal generator or other RF-source to the antenna jack.

Measuring the AGC-treashold voltage on pin 5 of U2 (control board) gives a range from 2.65-3.74 volts when adjusting R1 from counter-clockwise to clockwise. The recommended voltage is 3.80 volts, which I never reach. I've checked the control board very carefully, looking for solder bridges, cold solderings, incorrect components etc, but everything seems to be fine. I've even re-heated all solder joints on the control board without success.

Any ideas, anyone?

Regards,

Hans, LA2MOA
K2 #6341 - unfinished

_________________________________________________________________
Trött på krångliga mejladresser? Skaffa en live.se-adress här!
 http://get.live.com/mail/options_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Geoffrey Mackenzie-Kennedy]

Hello Hans,

The noises which you are hearing on 40m as you tune appear to be very much
like the noises that my K2 #3255 produces, especially above 7100 kHz when
the BC stations are strong, or with a signal generator connected to the
antenna jack. I had started to look into ways to get rid of the problem but
have not had the time yet to complete the job.

I am reasonably certain that the root cause of the problem is the lack of
shielding around the VCO and the 12.xxx MHz Reference oscillator, and / or
the lack of RF filtering of the DC and control lines which enter the area.
It can be demonstrated that a strong received signal will get into the VCO /
Reference area, and at signal frequencies in the 40m band as the result of a
process of mixing a product can attempt to take over the job of the
reference oscillator or certainly disturb the VCO loop causing 'clicks' as
you tune. Sometimes my K2 will lock onto a strong BC signal and only lets go
with a 'whoop' after I have tuned away from the signal a kHz or so - how far
depends on the strength of the BC or generator signal and the frequency.

Although I am unable at this time to suggest a simple cure for the problem,
and a cure might not be simple, I hope that the above will help in some way.
In Northern France my K2 does not 'click' on 40m because the level of BC
signals received there are considerably lower than those received in this
part of Scotland, here the individual carriers can reach -10dbm to -5dbm.

73,
Geoff
GM4ESD


Hans Ostnell  <[hidden email]> wrote:


Hi group!

I have just finished the alignments and tests (part II) in my (second)
K2-project. Sofar all resistance checks are ok, and the results of the
alignment and tests have also been ok. Except for one thing: I think that
the AGC in the radio behaves rather strangely. It's pretty nervous in the
"Fast" position and switching to slow makes it a little bit better, but not
perfect. I have also noticed a "chuffing" or "clicking" sound in my
heaphones while tuning the radio aound on 40 metres. This sound (which
indeed is very difficult to describe) corresponds with the change of digits
on the display (regardless of tuning rate) and is very significant with the
appearance of a strong signal. Tuning the BC-band above 7100 KHz is very
annoying. The behaviour disappears when the antenna is disconnected, but
occours again when connecting a signal generator or other RF-source to the
antenna jack.

Measuring the AGC-treashold voltage on pin 5 of U2 (control board) gives a
range from 2.65-3.74 volts when adjusting R1 from counter-clockwise to
clockwise. The recommended voltage is 3.80 volts, which I never reach. I've
checked the control board very carefully, looking for solder bridges, cold
solderings, incorrect components etc, but everything seems to be fine. I've
even re-heated all solder joints on the control board without success.

Any ideas, anyone?

Regards,

Hans, LA2MOA
K2 #6341 - unfinished



_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Geoffrey Mackenzie-Kennedy]

Hi Hans,

One possible reason why the problem does not show up on all K2s, assuming that they are all exposed to the same strong signals, is that all K2s are not exactly the same in terms of actual component values due to allowed tolerances, which in turn can result in a different behaviour of  unwanted 'sneak paths'. There are several other possible reasons which include mechanical construction, but to keep this short I'll not go deeper now :-)  A related subject is 'Birdies'.

73,
Geoff
GM4ESD


Hans LA2MOA wrote:

>Thanks a lot for your response! Yes, our problems seems to be similar, indeed! Yesterday evening, there was a lot of kicking, popping and "whoooing" in my K2, due to all very strong contest-CW >signals on 40. Very strange, indeed. This is my second K2, and I didn't experience something even similar to this in my previous one.

>I did send a copy of the description of my problem to the support guys on Elecraft, so let's see what they come up with. However, your explanation of the problem seems fair enough - but it's still strange >that this doesn't occour on all K2:s...

>Well, think that the best I can do for now is to complete the rig, by assembling part III, do all the finishing work and see if this also occours on other bands.

>Thanks and 73!

>Hans / LA2MOA
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[VR2BrettGraham]

LA2MOA noted undesirable behavior in his K2's receiver,
the reason for which was suggested by GM4ESD.  This
behavior is due to an environment that apparently the K2
was not really meant to be used in - one with plenty of
strong signals about.

Yet this is one of the things that many folks rave about
the rig & is one of the main reasons why I got one myself -
its strong signal-handling ability.

Sadly, the K2 wilts here - with what LA2MOA observed, but
even more so from its AGC image response.  Also
unfortunate is that it appears that mods developed in EU to
address these two issues are not sufficient for the
environment here.

I sincerely hope that the K3 will not be like American or
some European cars that can't deal with extreme cold/heat/
humidity/whatever & is only found out by unwitting customers
after it is too late - after all, the K3 is meant to be a "real radio",
not something for backpacking.

Hans: I suspect you will find the aggregate energy at the
input of your new K2 to be higher now than your when using
your previous K2.  Just like the SteppIR antenna here, there
are days when things are pedestrian enough that these
problems do not manifest themselves in my K2.  If you have a
spectrum analyzer, it is very clear what is going on.

Oh, how I hope the K3 is suitable for other than just those
in ITU Region 2.  ;^)

73, VR2BrettGraham.

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Geoffrey Mackenzie-Kennedy]

VR2BrettGraham  <[hidden email]> wote:

> This
> behavior is due to an environment that apparently the K2
> was not really meant to be used in - one with plenty of
> strong signals about.

Brett,

To be fair to the K2, the design of the variable IF filter and the use of a
MC1350G as IF amplifier and NE602 as product detector does suggest that the
K2 was not designed to cope with very strong unwanted signals close to the
operating frequency, which is the 40m environment above 7100 kHz in *some*
parts of Europe. With 6 or more BC stations spaced 5 kHz and/or 10 kHz
rolling in with carrier levels up in the -10dbm region, > S9 +60db, it is no
wonder that the K2 using these 'weak' devices cannot cope properly if asked
to listen amongst these monsters.

The tuning 'click' problem appears to be caused by layout, lack of shielding
and lack of lead filtering - a separate issue which could have been avoided
IMHO.

> Also
> unfortunate is that it appears that mods developed in EU to
> address these two issues are not sufficient for the
> environment here.

Probably not for Hans LA2MOA in Norway either, nor here in Northern UK.

73,
Geoff
GM4ESD






_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com