Grayhill 71 Series Switch StopsThe demo mixer uses a lot of PCB mounted 71 series Grayhill switches. These are all basically 12 position switches but they can be adjusted for fewer positions if required. This is done by inserting one or more pins into small holes in the front of the switch to act as end stops. To stop them falling out, Grayhill provides a self-adhesive cover. With no pins fitted the switch just goes round and round which can be useful in some circumstances.
Fortunately, every time I have used the Grayhill 71 series switches, I always laid out the PCBs starting from position 1 so that if any positions were left over they always started at 12 and worked downwards. This means there is one standard way of adding the stops to the Grayhills that only varies with the number of positions required.
Even if all 12 positions are used, you still need to add one pin if you want to stop the switch continuing from position 12 back to position 1. This is exactly the case with the mic pre gain control where the maximum gain position is 12 and the minimum gain position is 1. The last thing you want is to accidentally switch from minimum to maximum gain. So for 12 positions you need a single pin and for fewer positions you need two of them. The diagram below shows how to locate the right holes into which to fit the pins and lists the various switches used in the mixer and their corresponding pin positions.