K3 "RF Feedback" and microHAM MK2R or microKEYER II

[Joe Subich, W4TV-3]

There have been persistent reports of "RF feedback" in the
microHAM microKEYER II and MK2R/MK2R+ when used with the K3.
The problem seems to be related to the design of the K3 front
panel mic jack and a work around has been found for microHAM
interfaces.  

The easiest solution for the user is to modify the microHAM
DB37-EL-K3 cable by cutting the wire currently connected to
pin 8 of the Foster (microphone) plug.  There is no problem
when the DB37-EL-K3 cable is used with the Elecraft K2 or
when the DB37-El-K3 is used with a "breakout" cable (In-line
Foster jack, RCA and 3.2mm plugs) for the rear panel mic and
PTT inputs of the K3.  

This same issue can appear with any external interface that
connects via the microphone jack.  For those interfaces, the
microphone return (pin 7) must be separate from any shield on
the microphone cable.  The cable shield and "PTT ground" must
be connected to the shell of the Foster mic jack (chassis);
they can not be connected to the "mic ground" (pin 7).

The root of the problem appears to be in the wiring of the
K3 mic jack.  Pin 7 (mic ground) and pin 8 (PTT ground) are
connected in parallel, through a common RF choke (L4 on
the front panel board), then to the circuit common (ground).
Note in the K2, both PTT (pin 8) and mic (pin 7) returns
are connected directly to the circuit ground through P1
and the K2 does not have the "RFI" problem.

73,

   ... Joe Subich, W4TV
       microHAM America
       http://www.microHAM-USA.com 
       http://groups.yahoo.com/group/microHAM 
       [hidden email]
 

PS. Thanks to N6TV, K5WA, N6XI, AB7R and others for their
    help in identifying the problem and testing various
    solutions in what often seemed to be a losing battle.
 

_______________________________________________
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

[Don Wilhelm-4]

Joe,

I believe the 'root of the problem' is that your cable ties the shield
to pin 8 rather than the mic plug shell.  That is poor engineering practice.

Yes, the K3 makes the situation worse by tying pins 7 and 8 together and
running them to ground through an inductor, but I believe the real
source of the problem is the connection of the cable shield to a line
that is also used as a signal return.  Normal practice would connect the
cable shield to the connector shell.

The K2 does not have the same problem because there is no inductor in
the path for either pin 7 or pin 8.

This is a classic example of 'the pin 1 problem' that Jim Brown
continually refers to.

73,
Don W3FPR

Joe Subich, W4TV wrote:

> There have been persistent reports of "RF feedback" in the
> microHAM microKEYER II and MK2R/MK2R+ when used with the K3.
> The problem seems to be related to the design of the K3 front
> panel mic jack and a work around has been found for microHAM
> interfaces.  
>
> The easiest solution for the user is to modify the microHAM
> DB37-EL-K3 cable by cutting the wire currently connected to
> pin 8 of the Foster (microphone) plug.  There is no problem
> when the DB37-EL-K3 cable is used with the Elecraft K2 or
> when the DB37-El-K3 is used with a "breakout" cable (In-line
> Foster jack, RCA and 3.2mm plugs) for the rear panel mic and
> PTT inputs of the K3.  
>
> This same issue can appear with any external interface that
> connects via the microphone jack.  For those interfaces, the
> microphone return (pin 7) must be separate from any shield on
> the microphone cable.  The cable shield and "PTT ground" must
> be connected to the shell of the Foster mic jack (chassis);
> they can not be connected to the "mic ground" (pin 7).
>
> The root of the problem appears to be in the wiring of the
> K3 mic jack.  Pin 7 (mic ground) and pin 8 (PTT ground) are
> connected in parallel, through a common RF choke (L4 on
> the front panel board), then to the circuit common (ground).
> Note in the K2, both PTT (pin 8) and mic (pin 7) returns
> are connected directly to the circuit ground through P1
> and the K2 does not have the "RFI" problem.
>
>  
>

_______________________________________________
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]

Don,

> I believe the 'root of the problem' is that your cable ties
> the shield to pin 8 rather than the mic plug shell.  That is
> poor engineering practice.

You are incorrect.  The shield in the microHAM cables is
connected to the shell of the Foster connector and the shell
of the DB37 connector following proper engineering practice.
As in the K3, PTT return is also circuit ground/power supply
ground but in the microHAM products it is connected directly
to the chassis which is also proper engineering practice.  

> Yes, the K3 makes the situation worse by tying pins 7 and 8
> together and running them to ground through an inductor, but
> I believe the real source of the problem is the connection of
> the cable shield to a line that is also used as a signal
> return.  

The K3 causes the situation by tying pins 7 and 8 together and
lifting the PTT return.  Again, the cable shield is properly
connected to the shell/chassis at both ends.  Lifting the cable
shield on the K3 end does not eliminate the problem - the only
fix for the user is separating the PTT and mic grounds by using
the shield as the PTT return (isolating pin 8) or using the RP
jacks (with their independent returns).  

The proper fix would be to connect pins 7 and 8 directly to
the circuit common (ground) or short L4.

> Normal practice would connect the cable shield to the connector
> shell.

Again, the cable shield IS correctly connected to the connector
shell at both ends.  

> The K2 does not have the same problem because there is no inductor
> in the path for either pin 7 or pin 8.
>
> This is a classic example of 'the pin 1 problem' that Jim Brown
> continually refers to.

100% correct, the K3 has a pin 1 problem.  Comparing figures 1 and
2 of K9YC's "Understanding and Solving RF Interference Problems"  
will show that impedances in the signal reference line (circuit
common) and between the signal reference and supply common (PSU
ground or -DC) are the very definition of "pin 1 problems."  The
problem with the mic input can be fixed as indicated above -
by connecting pins 7 and 8 to the chassis or replacing L4 with
a jumper.  

In the meantime, users of microHAM interfaces (and those with
similarly impacted products) will need to work around the issue
by connecting the PTT return to the connector shell instead of
pin 8 or use the rear panel jacks (which, although they also have
a pin 1 problem, do not exhibit the serious symptoms since none
of the signal returns - except the ACC connector - share common
impedances).

73,

   ... Joe, W4TV
 

> -----Original Message-----
> From: Don Wilhelm [mailto:[hidden email]]
> Sent: Wednesday, September 10, 2008 3:14 PM
> To: [hidden email]
> Cc: [hidden email]; [hidden email]
> Subject: Re: [Elecraft] K3 "RF Feedback" and microHAM MK2R or
> microKEYER II
>
>
> Joe,
>
> I believe the 'root of the problem' is that your cable ties
> the shield
> to pin 8 rather than the mic plug shell.  That is poor
> engineering practice.
>
> Yes, the K3 makes the situation worse by tying pins 7 and 8
> together and
> running them to ground through an inductor, but I believe the real
> source of the problem is the connection of the cable shield to a line
> that is also used as a signal return.  Normal practice would
> connect the
> cable shield to the connector shell.
>
> The K2 does not have the same problem because there is no inductor in
> the path for either pin 7 or pin 8.
>
> This is a classic example of 'the pin 1 problem' that Jim Brown
> continually refers to.
>
> 73,
> Don W3FPR
>
> Joe Subich, W4TV wrote:
> > There have been persistent reports of "RF feedback" in the
> > microHAM microKEYER II and MK2R/MK2R+ when used with the K3.
> > The problem seems to be related to the design of the K3 front
> > panel mic jack and a work around has been found for microHAM
> > interfaces.  
> >
> > The easiest solution for the user is to modify the microHAM
> > DB37-EL-K3 cable by cutting the wire currently connected to
> > pin 8 of the Foster (microphone) plug.  There is no problem
> > when the DB37-EL-K3 cable is used with the Elecraft K2 or
> > when the DB37-El-K3 is used with a "breakout" cable (In-line
> > Foster jack, RCA and 3.2mm plugs) for the rear panel mic and
> > PTT inputs of the K3.  
> >
> > This same issue can appear with any external interface that
> > connects via the microphone jack.  For those interfaces, the
> > microphone return (pin 7) must be separate from any shield on
> > the microphone cable.  The cable shield and "PTT ground" must
> > be connected to the shell of the Foster mic jack (chassis);
> > they can not be connected to the "mic ground" (pin 7).
> >
> > The root of the problem appears to be in the wiring of the
> > K3 mic jack.  Pin 7 (mic ground) and pin 8 (PTT ground) are
> > connected in parallel, through a common RF choke (L4 on
> > the front panel board), then to the circuit common (ground).
> > Note in the K2, both PTT (pin 8) and mic (pin 7) returns
> > are connected directly to the circuit ground through P1
> > and the K2 does not have the "RFI" problem.
> >
> >  
> >

_______________________________________________
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

[Don Wilhelm-4]

Joe,

If indeed your cable has the shield bonded to the shell and not other
conductors at both ends, I agree that it is well constructed.
I am sorry - I jumped to conclusions, I have not physically/electrically
examined any MicroHam cables.
I initially read your post as indicating that your PTT ground and your
cable shield were common and both connected to pin 8.  Upon re-reading,
I find that is not what you wrote.

73,
Don W3FPR

Joe Subich, W4TV wrote:

> Don,
>
>  
>> I believe the 'root of the problem' is that your cable ties
>> the shield to pin 8 rather than the mic plug shell.  That is
>> poor engineering practice.
>>    
>
> You are incorrect.  The shield in the microHAM cables is
> connected to the shell of the Foster connector and the shell
> of the DB37 connector following proper engineering practice.
> As in the K3, PTT return is also circuit ground/power supply
> ground but in the microHAM products it is connected directly
> to the chassis which is also proper engineering practice.  
>
>  
>> Yes, the K3 makes the situation worse by tying pins 7 and 8
>> together and running them to ground through an inductor, but
>> I believe the real source of the problem is the connection of
>> the cable shield to a line that is also used as a signal
>> return.  
>>    
>
> The K3 causes the situation by tying pins 7 and 8 together and
> lifting the PTT return.  Again, the cable shield is properly
> connected to the shell/chassis at both ends.  Lifting the cable
> shield on the K3 end does not eliminate the problem - the only
> fix for the user is separating the PTT and mic grounds by using
> the shield as the PTT return (isolating pin 8) or using the RP
> jacks (with their independent returns).  
>
> The proper fix would be to connect pins 7 and 8 directly to
> the circuit common (ground) or short L4.
>
>  
>> Normal practice would connect the cable shield to the connector
>> shell.
>>    
>
> Again, the cable shield IS correctly connected to the connector
> shell at both ends.  
>
>  
>> The K2 does not have the same problem because there is no inductor
>> in the path for either pin 7 or pin 8.
>>
>> This is a classic example of 'the pin 1 problem' that Jim Brown
>> continually refers to.
>>    
>
> 100% correct, the K3 has a pin 1 problem.  Comparing figures 1 and
> 2 of K9YC's "Understanding and Solving RF Interference Problems"  
> will show that impedances in the signal reference line (circuit
> common) and between the signal reference and supply common (PSU
> ground or -DC) are the very definition of "pin 1 problems."  The
> problem with the mic input can be fixed as indicated above -
> by connecting pins 7 and 8 to the chassis or replacing L4 with
> a jumper.  
>
> In the meantime, users of microHAM interfaces (and those with
> similarly impacted products) will need to work around the issue
> by connecting the PTT return to the connector shell instead of
> pin 8 or use the rear panel jacks (which, although they also have
> a pin 1 problem, do not exhibit the serious symptoms since none
> of the signal returns - except the ACC connector - share common
> impedances).
>
> 73,
>
>    ... Joe, W4TV
>  
>
>
>  
>> -----Original Message-----
>> From: Don Wilhelm [mailto:[hidden email]]
>> Sent: Wednesday, September 10, 2008 3:14 PM
>> To: [hidden email]
>> Cc: [hidden email]; [hidden email]
>> Subject: Re: [Elecraft] K3 "RF Feedback" and microHAM MK2R or
>> microKEYER II
>>
>>
>> Joe,
>>
>> I believe the 'root of the problem' is that your cable ties
>> the shield
>> to pin 8 rather than the mic plug shell.  That is poor
>> engineering practice.
>>
>> Yes, the K3 makes the situation worse by tying pins 7 and 8
>> together and
>> running them to ground through an inductor, but I believe the real
>> source of the problem is the connection of the cable shield to a line
>> that is also used as a signal return.  Normal practice would
>> connect the
>> cable shield to the connector shell.
>>
>> The K2 does not have the same problem because there is no inductor in
>> the path for either pin 7 or pin 8.
>>
>> This is a classic example of 'the pin 1 problem' that Jim Brown
>> continually refers to.
>>
>> 73,
>> Don W3FPR
>>
>> Joe Subich, W4TV wrote:
>>    
>>> There have been persistent reports of "RF feedback" in the
>>> microHAM microKEYER II and MK2R/MK2R+ when used with the K3.
>>> The problem seems to be related to the design of the K3 front
>>> panel mic jack and a work around has been found for microHAM
>>> interfaces.  
>>>
>>> The easiest solution for the user is to modify the microHAM
>>> DB37-EL-K3 cable by cutting the wire currently connected to
>>> pin 8 of the Foster (microphone) plug.  There is no problem
>>> when the DB37-EL-K3 cable is used with the Elecraft K2 or
>>> when the DB37-El-K3 is used with a "breakout" cable (In-line
>>> Foster jack, RCA and 3.2mm plugs) for the rear panel mic and
>>> PTT inputs of the K3.  
>>>
>>> This same issue can appear with any external interface that
>>> connects via the microphone jack.  For those interfaces, the
>>> microphone return (pin 7) must be separate from any shield on
>>> the microphone cable.  The cable shield and "PTT ground" must
>>> be connected to the shell of the Foster mic jack (chassis);
>>> they can not be connected to the "mic ground" (pin 7).
>>>
>>> The root of the problem appears to be in the wiring of the
>>> K3 mic jack.  Pin 7 (mic ground) and pin 8 (PTT ground) are
>>> connected in parallel, through a common RF choke (L4 on
>>> the front panel board), then to the circuit common (ground).
>>> Note in the K2, both PTT (pin 8) and mic (pin 7) returns
>>> are connected directly to the circuit ground through P1
>>> and the K2 does not have the "RFI" problem.
>>>
>>>  
>>>
>>>      
>
> _______________________________________________
> 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
> ------------------------------------------------------------------------
>
>
> No virus found in this incoming message.
> Checked by AVG - http://www.avg.com 
> Version: 8.0.169 / Virus Database: 270.6.19/1664 - Release Date: 9/10/2008 6:00 AM
>
>  

_______________________________________________
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