Cable for ICOM IC-720A

1

The best interfacing option is via the accessory port in the back, though finding a matching connector might be a challenge.

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Digirig’s audio cable will look like this:

ACC pin 3 → 3.5mm TRRS ring 2 (RIG_PTT)
ACC pin 4 → 3.5mm TRRS tip (RIG_AFOUT)
ACC pin 5 → 3.5mm TRRS ring 1 (RIG_AFIN)
ACC pin 8 → 3.5mm TRRS sleeve (GND)

Less desirable, but also quite acceptable connection option is via the GX16-8 mic connector:

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GX16 pin 1 → 3.5mm TRRS ring 1 (RIG_AFIN)
GX16 pin 5 → 3.5mm TRRS ring 2 (RIG_PTT)
GX16 pin 6 & 7 → 3.5mm TRRS sleeve (GND)

1/4" TRS (PHONES) tip → 3.5mm TRRS tip (RIG_AFOUT)
1/4" TRS (PHONES) sleeve → 3.5mm TRRS sleeve (GND)

Please share your project reports.

2

Audio works great but Digirig mobile does not manage to pull down RIG_PTT to GND. It ends up at ~8V. The 720 mutes the speaker but does not switch to TX.
I had some 2N7000 at home and constructed a small support circuit that helped pull the PTT line to ground. With that circuit in place everything seems to work fine.

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4

Hmm. What kind of current do you get through a closed PTT line?
Digirig’s PTT switch is over spec’d for 0.6A on PTT with up to 180V when open. Anything near that would be nuts outside of the power circuitry.

5

The current when connecting the PTT directly to ground is ~128mA.
When Digirig is connected with PTT active, the current is ~50mA. (The hardware rev. is 1.11.)

6

I did some simulations and it might be the base current of Q2 that is the limiting factor.

If I had the skills, eyes and tools(I don’t), I would like to try and exhange Q1 and Q2 with n-channel MOSFETs. Remove R19, change R15 to 100ohm and change R18 to 100k ohm.
At least from a simulation perspective it looks promising for my application.