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:[EMAIL PROTECTED]
Sent: Wednesday, September 10, 2008 3:14 PM
To: [EMAIL PROTECTED]
Cc: elecraft@mailman.qth.net; [EMAIL PROTECTED]
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: Elecraft@mailman.qth.net
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: Elecraft@mailman.qth.net
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
