A Simple PTT Breakout (Interface) Box

The current rating of the PTT relay on my IC-746 and many other modern rigs is very low. Even though I used a small interface relay between the transceiver and a linear amplifier I still had problems with the PTT relay sticking closed. It is only too easy to weld the contacts together. To overcome the problem I built a simple solid state interface which also works as a breakout box and allows multiple devices, for example more than one linear amplifier, to be switched from the radio.

The interface has been in use here for about two years and as well as day to day use in the shack it has been used in several contests. It is used to key my 144Mhz amplifier and a preamplifier as well as an old homebrew HF amplifier. I also use this breakout box with my home made soundcard interface. Since building it I have not had any further problems with the PTT relay in the IC746.

The interface is built into a diecast metal box for screening and in addition to the decoupling capacitors shown in the schematic, ferrite beads have also been placed on the input and output lines.


The circuit is very simple and designed to be used with any rig that has a closing PTT output. On receive Q1, a BC548C (but almost any small NPN will do) conducts and holds the base of Q2, the PTT keying transistor low. On transmit the rig's PTT relay sinks less than 1mA through R1 and cuts off Q1. As a result Q2 conducts and keys the output PTT lines.

The PTT output lines are isolated using diodes which allows multiple devices to be connected to the breakout box. Q2, the keying transistor is a TIP31C rated at up to 3A at 100V. This should be sufficient to key almost anything that uses a positive PTT line taken to ground for transmit. Some older amplifiers that use negative bias keying on the PTT will still need the addition of an isolation relay.

Construction is "ugly" style with the components soldered directly onto the back of the input and output phono sockets.

I'm about to replace this breakout box with a proper sequencer kit sold by Jim W6PQL. Although I have had no problems on HF or 2m using this buffer box, on 70cms with a FT736R and K2RIW I blew up the Gaasfet preamp in the last contest!

73 Paul G4DCV

Email address OK at qrz.com