[aprssig] Re: Problems with hum on TX audio.

Patrick Green pagreen at gmail.com
Mon Feb 27 23:26:45 CST 2006

Hi All,

  I found the problem in the PC to TNC link.  When I disconnect the PC, the
hum goes away.  I need the PC tho as it's the brains of the operation.
Keying with a mic was tested as well as torroids.  I put a couple of
torroids on the serial cable and it improved slightly.  I need to get the PC
grounded better.

73 de Pat --- KA9SCF.

Tnx for all the info.  There is .4 mv of AC on the DC PS and it's putting
out 11.4 volts DC.  A little low but it doesn't drop more than .2 volts with
a load.

On 2/27/06, Patrick Green <pagreen at gmail.com> wrote:
> 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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