[Elecraft] K3 Ground Loop Issues using Ameritron ARB-704

[Joe Subich, W4TV]

How are the amplifiers wired?  Is the neutral connected to the 
chassis (does not meet UL standards)?  

If the neutral is connected to the chassis (ground) you will 
place any unbalance in the input power on the chassis (ground) 
and a portion of that current will inevitably find its way IN 
SERIES with the microphone due to the choke in the mic return 
(mic ground). 

First, to be code compliant, any amplifier that has a neutral 
REQUIRES a four wire power cord (hot-Neutral-hot plus ground). 
The neutral must be isolated from the chassis (safety ground) 
and must be returned to the source (breaker panel) separately. 
The neutral may not be tied to the amplifier chassis. 

In the K3, the mic ground jumper MUST be installed on the Audio 
I/O board as shown in the Elecraft RF Immunity modification: 
<www.elecraft.com/K3/mods/ELECRAFT_Application_Note_Improving_the_Immunity_of_
the_Rear-Panel_KIO3_RS232_and_Audio_Connectors_to_RF.pdf>  (see the 
second item on page 3).  If the jumper is not installed and 
every piece of equipment is not grounded separately to a common 
station ground, any common mode hum (e.g., hum on the chassis of 
an amplifier often due to neutral currents) flows from box to 
box through the shields of interconnect cables.  When it reaches 
the K3 it appears across the RF choke between the shield of the  
rear panel microphone cable and the chassis (in series with the 
microphone signal).  

73, 

   ... Joe, W4TV 
 


 

> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Clark 
> L. Stewart
> Sent: Thursday, June 04, 2009 12:51 PM
> To: Elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3 Ground Loop Issues using Ameritron ARB-704
> 
> 
> At 09:55 PM 6/2/2009 -0400, Alfred Frugoli wrote:
> >I've just gotten my K3 built and integrated into the shack 
> about a week 
> >ago.  Operated WPX CW this weekend with GREAT success.  Was 
> very happy.
> >
> >Tonight in setting up FSK, I was getting a lot of "AGN?" 
> after my cq's. 
> >Listening to my signal on a 2nd receiver, I realized that my 
> signal had 
> >a hum on it - basically sounded terrible. Tested in CW mode as well, 
> >and the hum was also there (haven't dug out the mic yet for 
> the dreaded 
> >SSB mode to know if the hum was there as well, but I would 
> suspect it 
> >would have been). Tried changing a bunch of menu settings to 
> no avail.  
> >Then I disconnected everything from the back of the K3 
> except for the 
> >antenna, power and FSK input.  Beautiful clean signal.  The hum 
> >reappeared when I connected the amp keying line.
> >
> ><<<< SNIP >>>>>
> >
> >Thanks.
> >
> >73 de Al, KE1FO
> 
> 
> Elecrafters,
> 
>       My friend, KC8UHE, just received his brand-new K-3 and 
> in helping him 
> set it up, we encountered a similar problem.  But, we are not 
> using the 
> ARB-704.  This K-3 is connected to a microHAM microKEYER II using the 
> microHAM DB37-EL-K3R cable set.
> 
>       When the headset microphone is connected to the 
> microKEYER and the 
> Amplifier is connected to the microKEYER, a serious amount of hum is 
> present on the mike audio.  This is evident both in TX TEST 
> mode and in 
> on-the-air reports.
> 
>       If the microphone is connected directly to the rear 
> MIKE input on the 
> K-3, there is no hum even with the amplifier connected.  If 
> the microphone 
> is run through the microKEYER and NO amplifier is connected to the 
> microKEYER relay output, there is no hum.
> 
>       This hum is present when connecting the microKEYER relay output 
> directly to either the Ameritron AL-82 or the Heathkit SB-200 
> which has 
> been converted to 6-M.  (Only one amp is connected at a 
> time.)  Both amps 
> are connected to 220 VAC.
> 
>       The microKEYER has been modified with the RFI "fix" on the USB 
> cable.  And, we swapped out KC8UHE's microKEYER with my own 
> microKEYER and 
> the hum is present with either microKEYER in line.
> 
>       Mike level on the K-3 was 20, CMP was Ø, and the 
> internal pots of the 
> microKEYER as well as the Router software were adjusted as per the 
> manual.  The headset is a Heil Pro Set Plus using the HC-4 element.
> 
>       Does anyone have a suggestion to cure this as it is not 
> possible for 
> KC8UHE to operate his K-3 with either amp on SSB while this hum is 
> present.  Thanks.
> 
> 
> Clark, W8TN
> 
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