OK this is REALLY starting to frustrate me now. I'm building my first 3 stringer, discussed in a previous thread. I was just going to do a P90 and a 500 ohm volume pot. A buddy of mine talked me into adding a tone pot (partially as a learning experience) I've been screwing with this build, for the last 2 weekends, and it has had multiple mishaps. I have managed to work out fixes for all of them, BUT this one has me stumped. I got it all done, strung up, when I plugged it in, it hummed like no tomorrow. It was suggested to me, that I probably just needed to ground the strings. An Easy check, run a ground wire off of the volume pot, and tuck it under the bridge, if it stops humming, problem identified. No luck, so I went back and resoldered EVERYTHING, and shortened all the wires in the wiring harness(another possible issue, according to my researching the issue here on CBN.) I have run a wire to ground my bridge/strings that is not attached yet, but I went ahead and plugged it in to see if it hummed BEFORE I restrung it. Not only does it STILL hum, now it squelches like a Microphone too close to a speaker. I'm at wits end. I also need to figure out how to ground my bridge with the simple tools/parts that I have on hand. HELP!!!
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I used to work in a Alternator/Starter/Generator repair/rebuild shop and a customer called and asked for a price of a Starter. I asked him what car the Starter was for and he replied that it was for a 1980 Green 4 door. At that point I said, Sorry, we're all out of Green 4 door Starters, but we do have some Red 2 door Starters that might fit. My boss looked at me with a very strange look and then shook his head. Some customers can teach us - some can not.
Single coil pickups are going to have a bit of hum , but having a single coil in a box can be a big problem when it comes to shielding. Lining the inside of the box with aluminum tape, copper shielding or shielding paint is sometimes a must. Also the shielding needs to be grounded in most cases.
It's a good idea to use shielded wiring when wiring the git. You can also twist the hot and ground wires together to create your own shielded wire. All solder joints need to be good, guitar cables good, and florescent lighting, dimmer switches, bad house wiring and internal amp problems can be issues.
I once built a box around a pickup inside a hollow guitar out of scraps to get rid of noise problems.
On a side note, Is your internal wires using shielded cable? If it's just all plain wires then you might need shielded audio cable.
I agree with blind lemon, bypass and disconnect everything and run your pup to the jack. If it still hums reverse the wires.
Also agree with wayfinder. Try a different cable and a different amp and a different room. If you can, try a different pup.
I assume your 500 ohm reference really means 500k ohm?
Just for giggles, did you try another instrument cable? I had a cheap one that was not well shielded... it hummed with anything...
I assume you have a multi meter or access to one. You can use it to test your connections and the pickup, pickup could be bad. For a quick test I would by pass volume & tone pots and run pup directly into jack and see what happens.