SUT 3 position switch wiring help needed please

Two or three years ago I put together a pair of HM3 transformers, I used a pair of 3 way switches but only wired the SUT to use 2 (of the 3) positions for 1:20 and 1:40. As it stands the middle position of the switch does nothing other than cut / mute the output.

Now I’ve got my new TT it would be great to use the middle position of the 2 switches as a pass through, (so I can use MMs without having to physically unplug the SUT from the signal chain).

I need a massive favour from a kind soul to tell me how I need modify the wiring to make this happen as I’m really not sure how to do it without permanently disconnecting one of the 2 ratios.

This is what the wiring looks like currently. (pair of RCAs on the right! are the input ones)

Any ideas what I need to do please?

You can’t do it with those switches, I’m afraid. They have only two poles and you’re using one to switch SUT pin 5 and the other to switch pin 6. For a bypass option you’d also need to switch the output socket’s signal wire away from the SUT secondary (pin 9) and onto the input socket signal wire, which is now hard-wired to pin 7. You can buy four-pole three-way switches although they’re wider and I’m not sure there’s quite enough room to fit them in without touching the backs of the RCA sockets (there might be, I just can’t be sure). But even then they won’t do what you want. Using a third pole you could disconnect the output socket signal wire from the SUT’s pin 9 in the centre position, but you couldn’t connect it to the input socket, which is what you need to do.

The easiest fix would be to add a third switch which would connect the input and output signal wires either to the SUT or to one another (bypass). If you’re not currently grounding the output socket’s outer than you’ll need to do that in bypass mode too.


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So, adding a 3rd switch is the way to go. There’s probably room for that in the middle between the current switches. It’s soft aluminum so I should be able to drill through OK if I secure everything carefully first.

Thank you very much for explaining, (as you can tell I do not understand switches properly). I’ll go away and try to work out how to do this.