[aprssig] Problems with hum on TX audio.

Steve Noskowicz noskosteve at yahoo.com
Mon Feb 27 15:01:33 EST 2006

  There can be, among others, ground induced hum and RF induced hum  (RF pickup somewhere, typically low lwvel audio).
  Some ideas for ground induced hum:
  Take a heavy strap, wire, cable, coax shield and "jumper "ground to ground - in parallel with what is already supposed to be there.  You may want to first measure voltage between the two before jumpering just in case.  This would most likely be between radio ald TNC, but could be caused elsewhere.
  If possible, un screw a box from whatever frame you may have them in to see if there may be parallel ground paths causing trouble.  This will rely only on the desired cable shield/ground for ground returns
  Make sure everything is plugged into the same exact power outlet.
  Try one or more of those old style three prong adapters to intentinoally isolate the groungs of power supplies and allow your cable grounds to take over.
  73, Steve, K9dci
  Message: 35
Date: Mon, 27 Feb 2006 09:48:03 -0600
From: "Patrick Green" <pagreen at gmail.com>
Subject: [aprssig] Problems with hum on TX audio.
To: "TAPR APRS Mailing List" <aprssig at lists.tapr.org>
 <54e507230602270748k7b3d8dcdj816a4f97171a956 at mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"

Hi All,

  I've changed radios out at KA9SCF-15 and in doing so, acquired this 
hum on the TXA (at any transmit power level).  There is nothing 
on the hum like a radio station, etc.  This radio is far more sensitive 
TXA than the previous and had to reduce the TXA to get it to match the
281.   When I labbed this at home 1st there was no hum at all. Here's 
run down of the change and the things that I've tried so far:

Old radio: Icom IC-281H
New radio: Icom IC-2200H

The TNC is an MFJ-1270C with a cube powersupply that came with it.
The radio is hooked up to a old Motorola repeater power supply.
The antenna system is on the same ground as the power supply.
The cable is made for an ICOM-2000H and converted to the 2200H using a 
brew jumper.

Things I've tried to isolate the problem:

Changed the cube power supply to the TNC to one that had more amps.  No
Changed the power source of the TNC to a battery pack.  No change
Changed the TNC to an MFJ-1270B and adjusted the TXA.  No change
Changed the wiring jumper to use Mic ground instead of chassis ground.  
Used the cable wired for the 2000 into the radio.  No change (I get no 
which is why I needed the jumper)

Things that are on the list to try yet:

Using toroids on the cable to the radio to the TNC (where the TXA is
Changing the radio power supply (this wouldn't be permanent).
Using the IC-2000H to test.  (the cable was made for this radio 
and had no hum but not tested at the digi site)

Can I add a resistor or something to the line to require more drive 
from the
TNC thus reducing the noise coming in on the line and placing it close 
the radio?  What value would I use?

Thanks for any help.

73 de Pat --- KA9SCF.
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