This is the approach that works for me and is a process outline for you to figure out the problem rather than an endless list of problems defined (although some are addressed in Common Complaints). This thread is a compliment/extension to the Bass Tone Glossary and Bass Modification Guide http://www.talkbass.com/forum/showthread.php?t=282831. Post 1 is the process, Post 2 Common complaints, Post 3 Basic Rules/Tools/Materials. The process is the same for a good bass that's gone bad as well as one that's been modified that's screwed up. The major difference is 99% of the time improper wiring/use of components is the issue in a bass that's been screwed up, a faulty component comparatively rare, and multiple problems are more likely than not. The process is also the same regardless of the complaint. In the case of wiring being screwed up, if you can't figure it out yourself, you have to either find a diagram wired like your bass (or piece a diagram together - not for the inexperienced), find a diagram with the same control layout and rewire according to it, or turn the project over to someone else. It goes without saying that the more complex the bass electronics are the more difficult it is to track a problem. How complex the bass remains primarily depends on your skills. In as much, at least in the short term, your choice may be to either let someone else do the work, tolerate a more complex bass that does not function properly, or to simplify a bass to a level you’re capable of managing. *Note: I have wired everything wrong imaginable more than once and NEVER damaged anything. There's just not enough current in a bass to damage anything I'm aware of. I've got the impression you don't want to wire a 9V battery directly to a passive pickup however. Basic Approach is a crude and simple one that works: isolate the problem by simplifying the circuit: A) Isolate the problem to the bass B) If the bass has a battery check it C) Know what the controls do D) Perform a visual inspection and simple continuity check E) Verify the bridge ground is properly connected F) Isolate the pup (pickup) from the controls (harness) G) Isolate the components in the harness H) Replace the defective component with one that works A) Isolate the problem to the bass. Sometimes the simple functioning of the bass will isolate the problem to it - a non-working pot for example. The LAST thing you want to do is tear into a bass that’s not broke however. So it’s always a good idea to either run a known good bass (preferable) or guitar through the same signal chain or run the suspect bass through a different signal chain [amp, cord, power supply (another outlet/location), and environment (location)]. Power supply and other electronics in the environment can do weird things to an instrument’s signal so NEVER fail to consider these factors in trouble shooting. If it’s not obvious that the preceding (running a known good bass through the same signal chain as the suspect bass with no such problem and/or running the suspect bass through a totally different signal chain yet getting the same problem) would implicate the suspect bass as the source of the problem then you probably should have someone else work on the bass. B) If the bass has a battery supply, verify it’s working properly (skip to C if no battery) There’s not much point to trouble-shooting an active bass if you don’t KNOW the battery(s) up to snuff. Every bass is different in terms of battery consumption, the minimal charge needed to operate, and how it behaves under those circumstances, but you're likely to have problems around 8v. They usually actually have 10v new. You can check battery level with a meter or replace with a known good battery. Battery clips are also frequently a problem source not to be overlooked as wires become disconnected at the terminals where they can’t be seen or the female clip terminals make poor contact with the male battery terminals. You can use a meter to check the clip leads for continuity by touching one meter lead to the clip terminal and the other meter lead to the end of the associated clip wire at point of contact. For poor contact just crimp the clip terminals to fit. C) Know what the controls do (Skip to D if you KNOW what the controls do) If the bass is used, there's a good chance someones jacked with it. In as much you need to KNOW what the controls do to know if something's amiss. It's actually a good idea to do with any bass. I've known guys that had a bass for years and never knew it had a function off the push/pull pot cause they never pulled up on it (not just low end gear either). To determine what the controls do simply take a small metal object (pickup screw, bridge allen wrench), turn the amp low, turn all pots off and tap on the pup mags switching all controls one at a time. Obviously a volume control off will result in no sound from anthing that's a tone pot, a tone pot that's "off" will when the volumes on. D) Perform a visual inspection and simple continuity check Often simple close visual inspection will reveal the source or a suspect and you can start there. Putting some tension on wires by moving them to the side or tugging lightly at connections can expose an issue just looking won‘t. You can take a meter and check ground continuity within the harness by going from pot back to pot back and pot back to jack ground but doing so has limitation because poor contact between the meter lead and whatever results in false readings and there may still be a ground issue elsewhere - but it's quick and easy and may indicate something of value. You can also verify the integrity of a wire by connecting the meter to each end of it. *Note: Any bass that has a copper shielded cavity is prone to shorted hot leads. Personally I place strips of electrical tape on the floor beneath each row of pot lugs and between the wall and hot pot lug of any potential contact. Paint shield is not an issue. E) Verify the bridge ground is properly connected (skip to E if hum/noise is not the complaint or a bridge ground is not utilized). This is really for a complaint of hum/noise (that increases instead of decreases when you touch the strings/hardware). Some basses (EMG actives in particular) don‘t utilize a bridge ground. You check the bridge ground by running a continuity check with a meter using one lead to the jack ground and the other simply touched to the bridge itself. Alternatively you can remove the bay cover and simply solder a separate wire from the jack ground ran externally directly to the bridge using one of the bridge screws to attach the bridge end. Fire the bass up and check it. If there’s no change in performance then either your temporary jumper is not making good contact or the bridge ground is good and the problem is elsewhere. My brother had a digital meter the other day he bought new for $3 (hint hint). If hum increases instead of decreasing when the string’s/hardware are touched that’s an indication of a faulty ground SOMEWHERE in the circuit. If touching significantly reduces hum, grounding is probably about as good as it gets and perhaps added foil or paint shielding ((see FAQ for shielding) will help, but no gaurantee. By design, single coil pickups are inherently more noisy than humbucking but there are exceptions to both. Active pickups have invariably been dead quiet in my experience and passive pickups used with a preamp have been significantly quieter. F) Isolate the pup from the controls (harness) Wiring one pup at a time straight to the jack eliminates everything but the pickup, jack, and YOUR WIRING as a problem source. If you fire the bass up and the problem is resolved then in all probability the problem is in the harness or the other pickup. If the problem remains then the pup, jack or YOUR WIRING is the issue. Replace the jack with a good one and recheck. If the problem persists it’s the pup or YOUR WIRING. Alternatively you can address the pickup itself by putting the leads of a meter to the DISCONNECTED leads of the pickup to check DCR. You can also check function of the jack in the bass by sticking a phone plug (guitar plug) with pigtails (open end wire) into the jack and checking the pigtails for continuity with a meter. You could also just plug the cord in and check continuity touching one meter lead to the tip and the other meter lead to the shaft of the unplugged end. Comparing the difference in resistance of these two would give you some idea of the integrity of your cord. G) Isolate the components in the harness Usually a given component in a harness will be suspect so you don‘t have to check them all. Standard pots can be tested with a meter for continuity by DISCONNECTING the pot from the harness and checking individually. In the case of pots, by connecting one meter lead to the center lug and the other to the outside lugs one at a time, rotating the stem, and observing the meter reading. For a switch it's just flipping the lever and checking contacts. Scratchie pots are sometimes corrected with spray cleaner. Without a meter the process is basically one of simplifying the harness (which requires a knowledge of how to rewire to accomplish as much) OR replacing the suspect component with a known good unit and testing through an amp. In the case of a modular preamp like a typical Bartolini or Aguilar it’s the same process. Some components like the pre units themselves and capacitors cannot be checked with a meter (however an oscilloscope can be used to check a preamp). Lacking an oscilloscope, it's replacement with known good units or doing the best you can to rule everything else out leaving the units as the only source of the problem. In the case of a printed circuit board unit like Music Man I have no clue and most techs probably won‘t either. *Note: In order to have better access, sometimes you may be able to get by with simply removing the nut and pulling the component in question from it's mounting while still wired to the harness. In the case of a more complex harness or a cluttered control cavity, it’s often easier (if not required) to simply remove the electronics from the bass and work on them outside of it. This can be accomplished by simply mounting the entire harness onto a piece of cardboard, connecting the pups to the harness, plugging into the jack, turning on the amp, and tapping on the pickups with a small steel object (like pickup screw or allen wrench) to test for function. Not a reliable means to resolve some issues (like poor ground) but will test basic functioning, everything is readily accessible, is much easier to see, is a lot faster for the more troublesome issues, and much easier to produce quality work with much less probability of unintentionally melting leads while soldering. If you’ve got two wires to connect that both have bare ends you can simply use small wire-nuts for temporary connections instead of soldering unsoldering them. H) Replace the defective component with one that works You can use something temporary that will work for testing - for example a 250K audio for pretty much any pot except a 25K audio. Probably best to temporarily use a 25k for a 50K pot. An audio pot can be substituted for linear (tone) pot but not visa versa. A stacked pot is basically just two standard pots so you can subsitute a standard pot for half a stack (which can be all that's needed for testing purposes). A blend can be temporarily replaced with two volumes for testing. A stereo jack can be used in place of a mono jack but not visa versa (unless you want a constant drain on the battery). But ideally you want the same part from the same source for a permanent fix, otherwise an equivalent is more likely than not different in some way. Pots vary in resistance, diameter, thread, shaft (that come to mind). Other parts can be almost as bad. You wire the replacement component the same way it was before (if it's an equivalent replacement). Ideally make only one change at a time and test otherwise you don't specifically KNOW what may have resolved the problem or the source of any newly created problems. In addition, the more things you change at one time, the higher the probability of creating more problems and lower probability of resolving the presenting complaint.