[Elecraft] PX3 noise generated on KX3-2M

[David Anderson]

Alan,

Thanks for taking the time to investigate this. 


> On 27 Feb 2016, at 05:35, Alan <n1al at sonic.net> wrote:
> 
> I finally got around to checking this.
> 
>> 
> 
> It is not a "pin 1" problem. The metal shell of the chassis-mount power supply connector is mounted directly to the metal side panel of the PX3 and the paint is masked around the hole to make sure it makes a good connection.

Agree that is the case on mine also.

> 
> On 02/23/2016 10:26 AM, David Anderson wrote:
> 
> > Since I purchased my PX3 I have suffered from radiated noise at 144
> > MHz.
> ...
> > I had already checked that the DC Jack was grounded properly to the
> > side panel.

First thing I checked.



> However what I hadn't appreciated is that the common of
> > the PCB is connected to a switch in the Jack and is not directly
> > connected to the chassis of the Jack.

We agree


> What was happening was the
> > barrel of the plug was connecting to the pole of this switch and the
> > PCB common, but only loosely to the chassis of the Jack.

The switch doesn’t connect to anything, BUT the contact connection to the barrel of the plug makes much better contact to switch contact spring than the regular connection.



> Movement of
> > the plug sideways with enough pressure did connect the plug better to
> > chassis and resulted in the noise going away.

This is the problem, not the connection from the socket to the chassis which is perfectly fine. It is the relatively poor connection between the plug and socket. Exerting sideways pressure decrease the contact resistance from DC return to the chassis. Note nothing to do with the tightness of the nut on the socket, that wasn’t moving and was not slack. 



> 
> I'm trying to understand that.  The power jack has three pins, the connector shell (tied to the printed circuit board ground plane in the PX3), the center pin (connected to the +12V input) and a switch.  The switch shorts to the connector shell when nothing is plugged in and opens when a power plug is inserted.

OK
> 
> But the thing is, the switch is not used and that pin is not connected to anything inside the PX3.  (The only reason I used a jack with a switch was that it was the only suitable chassis-mount type I could find.)  Besides, that pin is internally disconnected whenever the plug is inserted anyway.

However using the unused switch contact switch is a much better connection from plug DC return and connecting that to the chassis reduced the intermittent poor contact bonding between the DC plug and the chassis.

> 
> I just confirmed all that by double-checking the PCB artwork and by measuring an actual PX3 with an ohmmeter.  If you'd like to check it yourself, the three connector contacts are arranged like this:  The switch pin is the one on the side of the connector.  The +12V pin is the one farthest from the panel.  The shield/ground pin is the one that is almost hidden underneath the connector.
> 
> > I opened up the back of
> > the PX3 and connected the chassis part of the Jack to the PCB common
> > side of the jack's switch by soldering a small piece of copper EMI
> > tape across the gap. This ensures that the PCB common is stoutly
> > connected directly to chassis at the DC Jack.
> >
> > This has had the desired effect, no more intermittent  increases in
> > spurs heard in the receiver as the leads are touched or moved,
> 
> I'm not sure from that description exactly what was connected to what so it's hard to figure out how that could have affected the noise.  It sounds like you probably connected the switch contact to the connector shell, but I don't see how that could have affected anything since that contact is disconnected whenever a plug is inserted and it's a no-connect on the PC board.

By connecting that very good connection between the switch contact and the barrel of the DC plug to the chassis of the PX3 with a very short wide piece of copper tape the resistance from barrel to chassis was consistently low and the radiated noise was eliminated.


> 
> You might check to make sure the mounting nut on the power connector is good and tight and the connector body is making a solid contact to the inside of the side panel.  I'm guessing that's the most likely cause of the noise radiation.


No, that was not the case, it was the very first thing I checked. It is purely the contact resistance from plug  outer to the socket not being good enough, and the solution was to reduce that by using the unused contact going to the switch and bonding that contact to the chassis as well. It completely eliminated the problem I had.


73

David GM4JJJ