PTTs sorted? Maybe…

I’ve been thinking about the X’air PTT connections and I think I’ve worked out what’s going on. I found some information somewhere that the way the PTT system works on these old Icom radios is a bit weird. As well as completing the PTT circuit, the button also has to earth another circuit at the same time and that’s why there are 4 wires in the X’air switches. The yellow and red pair are (I think) the PTT wires and the black and white are the pair that need to be earthed.

But there’s an added complication. Whoever wired in the X’air PTTs appears to have used the wrong switches for the job! Instead of using simple on/off momentary switches that separately ‘make’ two 2-wire pairs and then ‘break’ them again when released and that would need 4 connections to work, they have used another weird type of switch that has 6 connections, 2 of which aren’t really used, and 3 positions.

So in the picture I posted previously (that I’ve now modified by adding labels to the terminals), the red PTT wire and the black ‘earthing’ wire are connected conventionally to two terminals in the centre position of the switch – labelled terminals 0/1 and 0/2 with the PTT wire on terminal 0/1 and the earthing wire on 0/2. The white wire is on one of the 2 terminals that are in the ‘unused’ switch position – labelled terminals 1/1 and 1/2 with the white connected to 1/2. Terminals 2/1 and 2/2 are on the other side of the switch. The yellow PTT wire is on terminal 2/1 and when the switch is operated, the PTT circuit is completed when connections are ‘made’ between terminals 0/1 and 2/1. The earthing circuit is completed when connections are ‘made’ between terminals 0/2 and 2/2. The black earthing wire is on terminal 0/2 and to complete the circuit between the black and white wires, there is a bridge between terminals 1/2 to which the white lead is connected and 2/2.

So when the switch is fully back, it makes contact with the white wire that connects to nothing. When the switch is central, it makes separate contacts with the red and black wires that also connect to nothing else in that position. And finally, when the switch is fully forward, it makes separate connections between the red and yellow wires and the black and white. Simples!

Now all we need to do is to find two double pole, single throw push-button switches. Hmmmmmm…… 😕