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
