I'm trying to set up an APRS station using an old icom HT. Receiving is fine, but I'm having some trouble with the transmission side. When I connect all my parts together, the transmit light immediately goes on.

I think I might just not understand the instructions, but the thing I've built works with my testing.

I've got two parts basically. The first is the standard serial PTT circuit for soundmodem:

Serial PTT Circuit

I've verified this is working with my DMM. There's no continuity when normal, but there is continuity when xastir is transmitting.

For the other part, I took wired two 3.5mm jacks onto my breadboard such that the signal lines are directly connected and one (the input) goes directly to ground and the other (the output) goes to ground through the PTT circuit as shown in the diagram from the manual:

icom jack

Again, I've verified I can hear the output with headphones and that ground is not connected end-to-end except when the PTT circuit says it should be.

Can anyone suggest what I might be doing wrong here? I've verified repeatedly everything works the way I think it should, so I can only assume there's a flaw in my understanding of how things should actually work.

  • \$\begingroup\$ What is the problem? That it transmits when it's not supposed to? If so, try removing parts until you make it stop, and figure out what is happening that isn't supposed to. Maybe you mixed up connections? Do you have the series cap on the audio? Does it still transmit if you leave the audio unconnected? \$\endgroup\$ Commented Nov 20, 2011 at 0:13
  • \$\begingroup\$ It transmits OK, but it's always got PTT down. The cap must be the trick. I didn't read that as a cap. I'll shove something small there and see if it helps. Thanks for the pointer. \$\endgroup\$
    – Dustin
    Commented Nov 20, 2011 at 0:50
  • \$\begingroup\$ I think the documentation is wrong. I moved stuff around until it worked. I'll draw up what worked successfully later. \$\endgroup\$
    – Dustin
    Commented Nov 20, 2011 at 1:24
  • \$\begingroup\$ That's most definitely a series cap, and required unless there's one in what is sourcing the audio. \$\endgroup\$ Commented Nov 20, 2011 at 1:40

1 Answer 1


I don't know that it's exactly correct, but this actually seems to work. It's a bit confusing to me as it seems quite a bit different from what I was reading above, but when I put it all together, it seems to actually function.

I tried lots of combinations basically poking around with wires until I got both audio signal and the PTT signal onto the radio before coming up with this. It seems to be the answer, but I'd like a couple other opinions on it.

xmit circuit


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