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I wired the guitar just like this.

But I have a bad hum coming from the input jack. It goes away when I touch it, so it is obviously a grounding issue, but I just can't figure it out.

I know you said you wired it just like the diagram, but..... the most common cause I've seen for this problem is that the wires at the jack are wrong. Usually, reversing the wires at the jack fixes it. It's just two wires. Switch them and see.

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Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

I had already tried that. I have no idea what's going on with it. I can directly tie it into a cable and it works just fine, as soon as I wire it into the input jack, it's hum city.

Check the ground wire is making good contact with the bridge and that the solder joints shine (dull joints are dry joints) and are secure on the back of the pots. Most common reason for a hum is a bad earth. Another common problem is to over heat the pot when trying to establish a solder point on the back..this will destroy the pot but more often, its just a bad earth.

Cheers

Kim

Check to see if when your chord is plugged in that the tabs aren't hitting the cavity walls. Sometimes you have to spin the jack and find the spot where the 1/4 inch won't make the tab hit the wood. I also bend the ground and hot tabs in just a bit to make sure my signal is as it should be, not touching anything they shouldn't be touching.

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Todd Lunneborg
http://www.tlguitars.com/
http://www.fretboardjournal.com/

How well shielded is your cavity? You say when you touch the jack the hum goes away. Is your bridge grounded? If it is, check for continuity with the jack ground by touching the bridge and the jack ground with the probes on your multimeter. If there's no continuity, scrape the paint off the bottom of the bridge where it touches the ground wire.
If the hum goes away when you tought the jack, it should go away when you toiuch the strings, that's the purpose of the string ground, the hum might just be the normal RF interference being picked up by the pots and wires in the control cavity. Shielding the cavity with some good quality shielding and GROUNDING THE SHIELD TO THE BACK OF A POT will reduce the noise. Also shield the control cavity cover and make sure it is grounded to the cavity shield, I fold a shield section over where one of the screws goes. You might also consider using a shielded wire from the volume output to the jack.

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Old growth, shmold growth!

Also.... have you tried a different cable? One never knows.

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"Act your age, not your shoe size" - Prince

I have gold monster cables I have been trying. I didn't shield the cavity at all, Just didn't want to mess with it for some reason, but today I resoldered a few points, and it seemed to help some, I think if I shield it, it will take a lot of the hum out.