Troubleshooting hum with parallel effects paths

[kitschead]

Hi everyone. This is my first post here. Any help would be appreciated with this.
My pedal rig involves a Y selector switch (I recently built) which toggles between two paths. One is a dry signal and the other one goes through an MXR carbon copy. They then go into a mixer and then to the amp. I've got an issue with hum though. It only seems to be present when using the delay pedal. I've tried other pedals and there doesn't seem to be a problem. I'm using a good power supply and there is no hum when using the carbon copy by itself or with other pedals in series. The hum is there when the signal from the guitar is bypassing the delay and going through the dry path. It seems like it is some sort of ground loop causing the interference because it is significantly quieter when I make a connection between the guitar and the pedal with my body or if I connect the chassis of the delay and the switch pedals (it disappeared when I lay a spanner across the two pedals). I'm confused though because I would have thought there would already be a ground connection between the two pedals made by the sleeve of the patch cables.
If anyone has any ideas as to how to fix this problem it would be greatly appreciated!

Thanks a lot

[kitschead]

Also thought I should mention;
I've just used the chassis of the switch unit as grounding, I haven't got any grounding wires or anything.
Could this be problematic?

[R.G.]

I believe you have an open or high impedance ground path. Using the spanner for a shorting bar indicates that a good ground between the two units would fix it, so there's an open or high impedance in the grounding somewhere.

Now the problem is *finding* it.

[kitschead]

Each pedal works fine in isolation from each other. its only a problem when they're hooked up together.

[PRR]

> disappeared when I lay a spanner across the two pedals

Sometimes it don't pay to ask "why?". If the spanner works, leave it there. (Or run a bit of wire between screws.)

> ...there would already be a ground connection between the two pedals made by the sleeve of the patch cables.

True. But bad cables love to nest in critical patches. It may be worth checking all cables for under-1-Ohm from shell to shell. Bad cables MUST be hunted-down and captured. If fix-able, tie knots in the end so you won't use it until fixed. If not fixable, CUT the end off and toss it.

If you have common signal ground all through, and common power ground from a shared power supply, there are issues. At pedal-load size, often not-a-problem, but sometimes it is.