[aprssig] Problems with hum on TX audio.

[Patrick Green]

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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