K3 RF Feedback Problem

[hb9brj]

I had an RF feedback problem when transmitting in digimodes (TX audio via LINE IN).
On some bands and at power levels above 20W, my TX audio was distorted.
BTW, my previous TRX did not show this problem in the very same setup.

Similar to MIC, also LINE IN has an RFC in its GND line (L5 on the KIO3 Audio IO board).
After removing it and placing a jumper, my problem was gone!

73, Markus

[P.B. Christensen]

In fact, several return lines are affected by the same condition on the KIO3
PCB, including:

L7 - Mic (J20);
L5 - Line In (J21);
L3- Line Out (J22);
L8 - Phones (J19); and
L15 - Speaker (J18).

On the Main DSP PCB, L4 (affecting the front panel Mic jack) is bypassed
with a jumper and removed.

IMHO, the real solution for the KIO3 is to simply disconnect the existing
ring return point on the jacks through a trace cut and bond the jack's ring
terminal to the chassis with a short jumper.  The rings should really be
placed at chassis potential as close as possible to the jacks' ring.  The
RFCs could be left in place with this option since they would removed from
the circuit anyway.

An alternative option is to remove/bypass the affected RFCs on KIO3 and
jumper the associated CN1-CN3 chip caps (e.g., jumper pins 4 and 5 on CN2
for rear panel mic shell grounding).

Whatever you decide, it's important to refer to the schematic diagrams prior
to making any K3 modification.

Paul, W9AC

----- Original Message -----
From: "hb9brj" <[hidden email]>
To: <[hidden email]>
Sent: Friday, November 07, 2008 9:46 AM
Subject: Re: [Elecraft] K3 RF Feedback Problem

>
> I had an RF feedback problem when transmitting in digimodes (TX audio via
> LINE IN).
> On some bands and at power levels above 20W, my TX audio was distorted.
> BTW, my previous TRX did not show this problem in the very same setup.
>
> Similar to MIC, also LINE IN has an RFC in its GND line (L5 on the KIO3
> Audio IO board).
> After removing it and placing a jumper, my problem was gone!
>
> 73, Markus
> --
> View this message in context:
> http://n2.nabble.com/K3-RF-Feedback-Problem-tp1442033p1469936.html
> Sent from the Elecraft mailing list archive at Nabble.com.
>
> _______________________________________________
> 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
>

_______________________________________________
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

[Joe Subich, W4TV-3]

> In fact, several return lines are affected by the same
> condition on the KIO3 PCB, including:
>
> L7 - Mic (J20);
> L5 - Line In (J21);
> L3- Line Out (J22);
> L8 - Phones (J19); and
> L15 - Speaker (J18).

The same issue effects the RS-232 return (DB9, pin 5).  In this
case, raising the return makes RxD sensitive to RF and results
in increased radiated/coupled digital "noise" from CAT polling.

> IMHO, the real solution for the KIO3 is to simply disconnect
> the existing ring return point on the jacks through a trace
> cut and bond the jack's ring terminal to the chassis with a
> short jumper.

ANY impedance in a signal circuit that returns to ground inside
the radio will result in placing any "noise" from the power
supply return (in this case unbypassed RF on the power supply)
in series with the signal input.  In particular, the chokes
in the mic circuits result in about 70 mV of RF in series
with the approximately 50 mV from the typical dynamic mic.  

73,

   ... Joe, W4TV
 

> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]] On Behalf Of Paul
> Christensen
> Sent: Friday, November 07, 2008 10:31 AM
> To: hb9brj; [hidden email]
> Subject: Re: [Elecraft] K3 RF Feedback Problem
>
>
> In fact, several return lines are affected by the same
> condition on the KIO3
> PCB, including:
>
> L7 - Mic (J20);
> L5 - Line In (J21);
> L3- Line Out (J22);
> L8 - Phones (J19); and
> L15 - Speaker (J18).
>
> On the Main DSP PCB, L4 (affecting the front panel Mic jack)
> is bypassed
> with a jumper and removed.
>
> IMHO, the real solution for the KIO3 is to simply disconnect
> the existing
> ring return point on the jacks through a trace cut and bond
> the jack's ring
> terminal to the chassis with a short jumper.  The rings
> should really be
> placed at chassis potential as close as possible to the
> jacks' ring.  The
> RFCs could be left in place with this option since they would
> removed from
> the circuit anyway.
>
> An alternative option is to remove/bypass the affected RFCs
> on KIO3 and
> jumper the associated CN1-CN3 chip caps (e.g., jumper pins 4
> and 5 on CN2
> for rear panel mic shell grounding).
>
> Whatever you decide, it's important to refer to the schematic
> diagrams prior
> to making any K3 modification.
>
> Paul, W9AC
>
> ----- Original Message -----
> From: "hb9brj" <[hidden email]>
> To: <[hidden email]>
> Sent: Friday, November 07, 2008 9:46 AM
> Subject: Re: [Elecraft] K3 RF Feedback Problem
>
>
> >
> > I had an RF feedback problem when transmitting in digimodes
> (TX audio
> > via LINE IN). On some bands and at power levels above 20W,
> my TX audio
> > was distorted. BTW, my previous TRX did not show this
> problem in the
> > very same setup.
> >
> > Similar to MIC, also LINE IN has an RFC in its GND line (L5 on the
> > KIO3 Audio IO board). After removing it and placing a jumper, my
> > problem was gone!
> >
> > 73, Markus
> > --
> > View this message in context:
> > http://n2.nabble.com/K3-RF-Feedback-Problem-tp1442033p1469936.html
> > Sent from the Elecraft mailing list archive at Nabble.com.
> >
> > _______________________________________________
> > 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
> >
>
> _______________________________________________
> 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

_______________________________________________
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

[KK7P]

> L5 - Line In (J21);
> L3- Line Out (J22);

These are transformer isolated and not returned to frame or chassis
ground specifically to avoid ground loop problems.  The inductors are
there to help keep RF out.  These "commons" are notational - they are
not common with anything.

73,

Lyle KK7P
_______________________________________________
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

[Igor Sokolov-2]

I have just received my K3 factory built and been playing with it for the
last two days. I immediately discovered RF feedback problem which was cured
by connecting pin 7 of the mike plug to the shell of the  connector. I do
plan to use rear mike and audio connectors  as well but I am getting a bit
confused with what exactly needs to be modified to avoid RF feedback. I would
really like to get inside  only once. May be Elecraft can officially issue
recommendations for the mod regarding this RF feedback issue. Hopefully the
one that would require minimum surgery on the radio.

73, Igor UA9CDC

>> L5 - Line In (J21);
>> L3- Line Out (J22);
>
> These are transformer isolated and not returned to frame or chassis ground
> specifically to avoid ground loop problems.  The inductors are there to
> help keep RF out.  These "commons" are notational - they are not common
> with anything.
>
> 73,
>
> Lyle KK7P
> _______________________________________________
> 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 

_______________________________________________
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

[P.B. Christensen]

>> L5 - Line In (J21);
>> L3- Line Out (J22);
>
> These are transformer isolated and not returned to frame or chassis ground
> specifically to avoid ground loop problems.

Lyle,

That's right.  I should have looked "off page" on the preceding schematic to
see that -- although the issue still stands on the other stated jacks.
What's odd is that his RFI problem went away with the removal of L5.  I
would not have expected that result on the transformer output although one
line is shared commonly between two transformers.

Paul, W9AC
 

_______________________________________________
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

[Dean Straw]

Lyle:

Is remove of L4 on the front panel board and L7 on the KIO3 audio board "official" Elecraft modifications?

I'd hate to void my warranty by messing with something that isn't official.

Thanks, Dean Straw, N6BV

Lyle Johnson wrote

> I bit the bullet and removed L4 and L7 this morning.  It is not
> difficult; they are both on the back of the front panel board.

L4 on the front panel board is related to the issues some folks are
having when using the front panel mic connector with various audio
routers.  It can be shorted with a jumper from pin 8 of the mic jack to
ground.

L7 on the KIO3 audio I/O board can also be bypassed for the same reasons
when using the rear panel mic jack in similar installations.

L7 on the front panel board is for reduction of noise from the RS232
serial port lines and should definitely not be bypassed!

73,

Lyle KK7P


_______________________________________________
Elecraft mailing list
Post to: [e-mail]
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

[KK7P]

> Is remove of L4 on the front panel board and L7 on the KIO3 audio board
> "official" Elecraft modifications?

Bypassing these two inductors will not impact your warranty.

73,

Lyle KK7P
_______________________________________________
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

[Dean Straw]

Thanks, Lyle.

 

73, Dean, N6BV

-----Original Message-----
From: Lyle Johnson (via Nabble) [mailto:[hidden email]]
Sent: Monday, November 10, 2008 11:52 AM
To: Dean Straw
Subject: Re: [Elecraft] K3 RF Feedback Problem

> Is remove of L4 on the front panel board and L7 on the KIO3 audio board
> "official" Elecraft modifications?

Bypassing these two inductors will not impact your warranty.

73,

Lyle KK7P
_______________________________________________
Elecraft mailing list
Post to: Elecraft@...
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

---

This email is a reply to your post @ http://n2.nabble.com/K3-RF-Feedback-Problem-tp1442033p1481946.html
You can reply by email or by visting the link above.