[Elecraft] K3 RF Feedback Problem

[Joe Subich, W4TV]

> 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: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Paul 
> Christensen
> Sent: Friday, November 07, 2008 10:31 AM
> To: hb9brj; elecraft at mailman.qth.net
> 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" <hb9brj at gmail.com>
> To: <elecraft at mailman.qth.net>
> 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.
> >
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