Troubleshooting a weak K2 receiver

[David Doar]

Hi guys.  I could use a little help tracking down the cause of a weak receiver on my newly built K2/10.  


All of the test and alignment procedures were satisfactory at each stage of the build. But unfortunately, when I got on the air, the receiver actually seems rather deaf.  I do hear a few signals, but rarely do they move the S-meter.  

After going through the trouble shooting section of the manual with no luck, I decided to use  the N0SS, Tom Hammond (SK) “Cheap ’N Dirty “ signal tracing method to inject a signal at the suggested  test points. I found the headphone volume drops off slightly when injecting a signal at the junction of D2 & D3, and almost totally disappears at the junction of D3 &D4. That got me checking the voltages in the T-R switch.    I found that I was only getting about 50mV at the junction of D2 and D3 (anodes). I’m also getting 0 volts at the cathode ends of D1 and D2.  Backtracking from the anodes of D2 and D3,  I get the same 50mV at RFC2.  But on the other  end of RFC2, I have the full, supplied  8 volts (receive).  


So I’m thinking that RFC2 is defective, and is the cause of the low switching voltage for the diodes.  Does it sound like I’m on the right track?






For reference, here is Tom’s “Cheap ’N Dirty Signal Tracing in the K2” article:    http://www.mmccs.com/mmarc/n0ss/k2_signal_tracing.pdf






Thanks for your help,


Dave
WB4BIN

     
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[Don Wilhelm-4]

Dave,

You are on the right track - you should have near 8 volts on the anodes
of D2 and D3 during receive.  Check RFC2 with an ohmmeter - if it is
open, request a replacement from [hidden email].  OTOH, if it shows
a low resistance, check that both leads are properly soldered.

73,
Don W3FPR

On 8/24/2014 2:08 PM, David Doar wrote:

> Hi guys.  I could use a little help tracking down the cause of a weak receiver on my newly built K2/10.
>
>
> All of the test and alignment procedures were satisfactory at each stage of the build. But unfortunately, when I got on the air, the receiver actually seems rather deaf.  I do hear a few signals, but rarely do they move the S-meter.
>
> After going through the trouble shooting section of the manual with no luck, I decided to use  the N0SS, Tom Hammond (SK) “Cheap ’N Dirty “ signal tracing method to inject a signal at the suggested  test points. I found the headphone volume drops off slightly when injecting a signal at the junction of D2 & D3, and almost totally disappears at the junction of D3 &D4. That got me checking the voltages in the T-R switch.    I found that I was only getting about 50mV at the junction of D2 and D3 (anodes). I’m also getting 0 volts at the cathode ends of D1 and D2.  Backtracking from the anodes of D2 and D3,  I get the same 50mV at RFC2.  But on the other  end of RFC2, I have the full, supplied  8 volts (receive).
>
>
> So I’m thinking that RFC2 is defective, and is the cause of the low switching voltage for the diodes.  Does it sound like I’m on the right track?
>
>
>
>
>
>
> For reference, here is Tom’s “Cheap ’N Dirty Signal Tracing in the K2” article:    http://www.mmccs.com/mmarc/n0ss/k2_signal_tracing.pdf
>
>
>
>
>
>
> Thanks for your help,
>
>
> Dave
> WB4BIN
>
>    
> ______________________________________________________________
> Elecraft mailing list
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> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]
>

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[David Doar]

Don, thanks so much for the help!  I've learned a lot from reading your responses.

I just checked  RFC2, and it is indeed open.  I'll get in touch with the parts department.
On a side note, the signal tracing method that Tom recommended worked like a champ.  I used the N-generator for a signal source, and that made it very easy to find out where  the problem was originating. A great resource troubleshooting problems!
I'm still not quite sure  how to properly use the mailing list, though.  I guess I'll figure it out eventually. :)

73,
DaveWB4BIN


     
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[Don Wilhelm-4]

David,

It appears that you are using the email list just fine.  If you are
subscribed to receive individual emails, make a folder under your inbox
named 'Elecraft' or something else you choose.  Then create a filter to
automatically put all email having [Elecraft] in the subject line into
that folder.

As far as posting, "reply" goes only to the originator of the post,
'Reply All' goes to the list and the originator.  If your email client
has a "Reply to List", that will go only to the list.  If you do not
have that feature and wish to reply only to the list, use "Reply All"
and delete the original poster's email from the TO: list in your reply.  
If I suspect that the poster receives only the digest format, I will
always do a "Reply All" if the matter is such that a quick answer is needed.

Yes, the N0SS signal tracing method is a good tool to have at your
disposal.  I use that technique often even though I have several other
means of chasing down a problem, including an HP8640B signal generator
that is capable of "forcing a strong signal 'down the throat'" of the
most obstinate problem situation so I can view the progress through the
various stages with my 'scope.

One adapts his techniques to the tools available - better tools only
mean that one can isolate the problem more quickly, lesser tools can
also get to the root of the problem, but it takes more care and time.

73,
Don W3FPR

On 8/24/2014 3:09 PM, David Doar wrote:
> Don, thanks so much for the help!  I've learned a lot from reading your responses.
>
> I just checked  RFC2, and it is indeed open.  I'll get in touch with the parts department.
> On a side note, the signal tracing method that Tom recommended worked like a champ.  I used the N-generator for a signal source, and that made it very easy to find out where  the problem was originating. A great resource troubleshooting problems!
> I'm still not quite sure  how to properly use the mailing list, though.  I guess I'll figure it out eventually. :)
>
>

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[David Doar]

Happy days!
The replacement choke came today, and I promptly installed it.  I'm now getting 7.85 volts at the D2 and D3 anodes, and much stronger signals across all bands. Picked up several S-9 signals, and even a couple that that were S-9 +10.
I also completed the installation of the KAT2 antenna tuner, and it seems to be working as advertised.  I even managed a quickie QSO with W1AW/1 in Maine (about 1700 miles) with 10 watts and my attic dipole.
Now it's time to start in on the KAF2, KNB2, and KB2.
I'm have a ton of fun building this little radio!


DaveWB4BIN    
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