K3/DVD hum problem solved

[k6rb]

I've been battling what appeared to be a ground loop problem when I joined the two K3s using a DX Doubler. I would hear low-level hum on both audio outputs, and it got louder when either was keyed.

Tonight, I hooked up the radios to separate dummy loads and removed all I/O except RS-232, ACC, key in. No change. Put both radios in "test" mode and still heard low level hum but the higher level hum upon keying was not so noticeable.

In TX Normal mode, again, I noticed an interesting thing - at 10 watts out into the dummy load, the hum on keying was lower but would jump to louder as soon as I started raising the power output passed the LPA to HPA threshold (around 15 watts). But, it did not get louder when I continued to raise the power to 100 watts. So RF feedback was ruled out. It appeared to be DC current related, since that will jump as the threshold is crossed.

Both radios had been DC sourced from a common 30 amp power supply figuring that only one would be transmitting at a time so it should handle it. My next step was to separate the two radios from a common DC power source. I used two 30 amp supplies, one to each. Bingo! No more hum, no more hearing radio one in radio two when one was keyed and vice-versa.

What threw me at first was I had a similar problem with the DX Doubler using two FT-1000s, and they didn't share a common DC source. That time, the problem was a ground loop in the audio chain. So, isolation transformers worked. This time, with isolation transformers, there was no change other than loss of frequency range and volume.

Rob K6RB


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[Joe Subich, W4TV-4]

This is a classic "ground loop" (or "pin 1") problem.  When
transmitting on radio 1, some of the power supply return current
follows the shield of the PTT, mic and headphone cables to the
DXDoubler, through the other cable shields to chassis of Radio 2
and then back to the common power supply.  The loop resistance
in those shields causes the chassis of DXD (or any other accessory
device in a similar position) to "move" with respect to ground.  

Since the Mic return of the K3 is tied back to ground at the
"non-transmitting" radio, the small (as little as 10 mV)
movement in the ground reference on DXD shows up in series
with the mic line.  For a dynamic mic, 10 mV can be significant
relative to the "real" mic audio.  

This can be identified as a return current problem in that
the hum or RF feedback becomes greater at higher power (more
current draw) and is often present even when operating into
a dummy load (little or no radiated RF).  The real key is
that higher currents mean a greater voltage generated in
the ground connection.  

The classic solution is to bond the chassis of every piece
of equipment to a common ground point with a wide braid or
heavy wire.  However, that does not always eliminate the
"unit to unit" path on the shield/common of cables like PTT,
Key, headphone, etc.  Using separate, isolated power supplies
for each radio and a third power supply for any DC powered
accessories will minimize the ground loop issues.  

Kenwood recognized this problem in their DC powered radios
(although they failed to identify the cause) and all of their
manuals recommend using separate power supplies for any
"digital mode TNC."  

73,

   ... Joe, W4TV
 

> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]] On Behalf Of Rob
> Sent: Thursday, March 18, 2010 10:07 PM
> To: Elecraft Reflector
> Subject: [Elecraft] K3/DVD hum problem solved
>
>
> I've been battling what appeared to be a ground loop problem
> when I joined the two K3s using a DX Doubler. I would hear
> low-level hum on both audio outputs, and it got louder when
> either was keyed.
>
> Tonight, I hooked up the radios to separate dummy loads and
> removed all I/O except RS-232, ACC, key in. No change. Put
> both radios in "test" mode and still heard low level hum but
> the higher level hum upon keying was not so noticeable.
>
> In TX Normal mode, again, I noticed an interesting thing - at
> 10 watts out into the dummy load, the hum on keying was lower
> but would jump to louder as soon as I started raising the
> power output passed the LPA to HPA threshold (around 15
> watts). But, it did not get louder when I continued to raise
> the power to 100 watts. So RF feedback was ruled out. It
> appeared to be DC current related, since that will jump as
> the threshold is crossed.
>
> Both radios had been DC sourced from a common 30 amp power
> supply figuring that only one would be transmitting at a time
> so it should handle it. My next step was to separate the two
> radios from a common DC power source. I used two 30 amp
> supplies, one to each. Bingo! No more hum, no more hearing
> radio one in radio two when one was keyed and vice-versa.
>
> What threw me at first was I had a similar problem with the
> DX Doubler using two FT-1000s, and they didn't share a common
> DC source. That time, the problem was a ground loop in the
> audio chain. So, isolation transformers worked. This time,
> with isolation transformers, there was no change other than
> loss of frequency range and volume.
>
> Rob K6RB
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

______________________________________________________________
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Please help support this email list: http://www.qsl.net/donate.html