Question Regarding Direct Conversion Receiver

[Bernie KF0QS]

Dear Folks:

I have now built a K1 (#1024) and a K2 (#6922) and have deeply enjoyed doing both.  I am now currently building the SSB option for the K2.  They are both great rigs, and building them and getting them to work has been some of the best stuff I've done in my ham radio career.  I've worked hundreds of stations with them both, and a fair amount of DX.

Certainly, my confidence in doing my own work has grown by leaps and bounds because of the experience I've gained.  I've even built one thing from scratch, just using a schematic (a relatively simple circuit, but it worked perfectly the first time).

I now have a question about something I did years ago.  When I got the K1, I decided to build something smaller first in order to practice soldering.  I built a Vectronics (actually, from MFJ) direct conversion receiver for 20 meters (I also built the companion transmitter, and it works fine).  The problem is that merely touching the receiver makes its change frequency, and tuning it is an uncertain process.  As a result, it is unstable, and not much fun to operate (though it is reasonably sensitive despite its simplicity).  There must be some capacitance that it is getting from my touch, that is adversely affecting its performance.

Did I do something wrong in building this receiver, or is it just the price of a really bare bones design?

Thanks for any advice anyone can give me.

73 de Bernie KF0QS

[Don Wilhelm-4]

The stability of a Direct Conversion receiver is determined solely by
the Local Oscillator (VFO).
It must be stable both mechanically and electrically.  It sounds like
yours is suffering from hand capacity effects.
If it uses an air variable capacitor, it should be mounted rigidly and
behind a metallic panel to reduce the hand capacity effect.
If OTOH the oscillator is tuned with a varactor, the power source to the
oscillator must be regulated for stability, and that is typically done
with a zener diode.
Is the inductor a toroid that is mounted rigidly to the board? - the
toroid turns should be tight - this is not a place for loose windings.

Since you did not give the model of that kit, so I could not look up the
schematic, so I can only give generalities.

73,
Don W3FPR

On 1/23/2013 12:25 AM, Bernie KF0QS wrote:

> I now have a question about something I did years ago.  When I got the K1, I
> decided to build something smaller first in order to practice soldering.  I
> built a Vectronics (actually, from MFJ) direct conversion receiver for 20
> meters (I also built the companion transmitter, and it works fine).  The
> problem is that merely touching the receiver makes its change frequency, and
> tuning it is an uncertain process.  As a result, it is unstable, and not
> much fun to operate (though it is reasonably sensitive despite its
> simplicity).  There must be some capacitance that it is getting from my
> touch, that is adversely affecting its performance.
>
> Did I do something wrong in building this receiver, or is it just the price
> of a really bare bones design?
>
>

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[Mike Furrey]

Hi Bernie,
I have built two DC receivers (one being the Neophyte that was in QST years
ago) and several regens (including one in Sept 2000 QST) over the years. The
tuning has been stable in all and I securely attach the tuning capacitor's
body that hold the rotor plates to the chassis.

Hope this helps.

73, Mike WA5POK

--------------------------------------------------
From: "Bernie KF0QS" <[hidden email]>
Sent: Wednesday, January 23, 2013 12:25 AM
To: <[hidden email]>
Subject: [Elecraft] Question Regarding Direct Conversion Receiver

> Dear Folks:
>
> I have now built a K1 (#1024) and a K2 (#6922) and have deeply enjoyed
> doing
> both.  I am now currently building the SSB option for the K2.  They are
> both
> great rigs, and building them and getting them to work has been some of
> the
> best stuff I've done in my ham radio career.  I've worked hundreds of
> stations with them both, and a fair amount of DX.
>
> Certainly, my confidence in doing my own work has grown by leaps and
> bounds
> because of the experience I've gained.  I've even built one thing from
> scratch, just using a schematic (a relatively simple circuit, but it
> worked
> perfectly the first time).
>
> I now have a question about something I did years ago.  When I got the K1,
> I
> decided to build something smaller first in order to practice soldering.
> I
> built a Vectronics (actually, from MFJ) direct conversion receiver for 20
> meters (I also built the companion transmitter, and it works fine).  The
> problem is that merely touching the receiver makes its change frequency,
> and
> tuning it is an uncertain process.  As a result, it is unstable, and not
> much fun to operate (though it is reasonably sensitive despite its
> simplicity).  There must be some capacitance that it is getting from my
> touch, that is adversely affecting its performance.
>
> Did I do something wrong in building this receiver, or is it just the
> price
> of a really bare bones design?
>
> Thanks for any advice anyone can give me.
>
> 73 de Bernie KF0QS
>
>
>
> --
> View this message in context:
> http://elecraft.365791.n2.nabble.com/Question-Regarding-Direct-Conversion-Receiver-tp7569046.html
> Sent from the Elecraft mailing list archive at Nabble.com.
> ______________________________________________________________
> 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 

______________________________________________________________
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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