[Elecraft] K3 "RF Feedback" and microHAM MK2R or microKEYER II

Wed Sep 10 17:17:21 EDT 2008

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:w3fpr at embarqmail.com] 
>> Sent: Wednesday, September 10, 2008 3:14 PM
>> To: microham at microham-usa.com
>> Cc: elecraft at mailman.qth.net; Elecraft_K3 at yahoogroups.com
>> 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. 
>>>
>>>   
>>>
>>>       
>
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