[aprssig] Problems with hum on TX audio.

Tim Cunningham tim_cunningham at mindspring.com
Mon Feb 27 15:09:38 EST 2006

If the problem is not a PL tone you could try or verify:

1. Make sure the GND signal between the TNC audio and the radio microphone jack is disconnected in the connector on the microphone side so it does not make a connection from the TNC GND to the microphone GND on the radio. This is important for several reasons.

2. Make sure the GND from the power connector from the TNC and the GND from the power connector on the radio are connected at the same point at the power supply. This will provide your single point ground source at the power supply.

3. This step is re-iterated from step #1. Make sure the TNC GND from the 5-pin DIN connector and the GND on the radio microphone connector are not connected.

Now if you have other equipment connected to the radio, power supply, or TNC all bets are off without a full description of what is connected and how it is connected.

73's de Tim - N8DEU
Huntsville, Alabama

  ----- Original Message ----- 
  From: Patrick Green 
  To: TAPR APRS Mailing List 
  Sent: Monday, February 27, 2006 9:48 AM
  Subject: [aprssig] Problems with hum on TX audio.

  Hi All,

    I've changed radios out at KA9SCF-15 and in doing so, acquired this nasty hum on the TXA (at any transmit power level).  There is nothing superimposed on the hum like a radio station, etc.  This radio is far more sensitive on 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 the 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 home 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 change
  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.  No change.
  Used the cable wired for the 2000 into the radio.  No change (I get no RXA 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 carried).
  Changing the radio power supply (this wouldn't be permanent).
  Using the IC-2000H to test.  (the cable was made for this radio initially 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 to the radio?  What value would I use?

  Thanks for any help.

  73 de Pat --- KA9SCF.


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