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I have this cheap copy of a 50s Precision Bass, which came with an ugly pickguard. So I made a new pickguard, and in the process of fitting it to the bass, I scraped off the protection tape on the side of the pickup, and the wires became visible.

When I was done, I noticed that the bass is now picking up hum when I'm not touching the strings, which I'm fairly sure it didn't do before. The hum goes away when I touch the strings, and I can turn it off by turning off the volume pot on the bass. It doesn't seem to be the cable, or the amp - plugging it into my guitar doesn't result in hum.

So I have a couple of related questions:

  1. What is wrong with the bass? Did I accidentally damage the pickup, or tear off a ground wire? How do I diagnose this?
  2. What can I do to fix it? A brand replacement pickup would cost roughly as much as the bass...

Edit: As requested, here are some images, and the results of some tests with the multimeter.

The pickup The cavity The rear side of the pickup As far as I can tell, all points that should be grounded have a good connection (1 Ohm or less) to the outside of the output jack. What worries me is that I cannot measure the resistance of the pickup - from what I understand, there should be some 5-20 kiloohm between the two poles of the pickup, depending on the construction, but my multimeter shows infinite resistance.

  • Find a short length of wire and hold it on the bridge and pup. If the hum disappears, it's an earth wire that needs re-soldering. – Tim Feb 27 at 8:51
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    Did the old pickguard have any foil shielding on the back, and/or any wire connecting it to the earth in the wiring circuit? "picking up hum when I'm not touching the strings" can be fairly standard behaviour for instruments that are wired ok, but poorly shielded. And yep, some pictures would be good. – topo Reinstate Monica Feb 27 at 9:12
  • topo morto: the old pickguard was pure plastic, and we used the bass without any pickguard for a long time. Poor shielding could be an issue, since the cavity is not shielded from the back and sides. – Richard Metzler Feb 27 at 17:54
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I disagree that this could be an earthing fault. You need to go right back to the basic design of this bass. The P bass from 1951 to 1957 was built with a single coil pickup, which inherently picks up electrostatic interference. Conversely the human body is a good antenna for attracting AC electromagnetic interference. Your bass gets quieter when you touch the strings, or other grounded metal hardware, as this is all connected to ground, forming a pathway for this interference to go to ground. Your bass doesn't have any shielding, so will have always been likely to have picked up background interference from any number of sources.

From the picture you posted, the control cavity and pickup cavity in your bass would benefit from shielding (either copper tape or shielding paint) to cut down on electromagnetic interference. This will minimise the difference in noise levels from your bass, regardless of whether you are touching any grounded part of the bass or not.

However the pickup in your bass will always pick up electrostatic interference, and there isn't much you can do about that. The damage to the pickup coil might be superficial or serious. The best test is to see if you still get a DC resistance reading from the pickup coil. If it is between ~ 5 and 7 k ohms then the pickup is ok. If it reads as an open circuit then you cut into the coil and broke it. A pickup with a broken coil still produces output (Geddy Lee's famous black Jazz bass, which he purchased in a pawn shop, has an open coil in the bridge pickup. Likewise Roy Buchanan's famously bright Telecaster tone came, in part, from a broken pickup coil).

A broken pickup coil still passes high frequencies, but the tone control acts like a second volume control when this happens. In the case of your bass I think I can see a broken copper wire within the pickup coil, but I can't be sure.

If the pickup is broken, there are cheap Wilkinson and other unbranded single coil P bass pickups on Ebay for a lot less than you paid for the bass (assuming that it is 1) the Harley Henton '51 P bass copy and 2) you bought it new).

  • I have a jazz bass, similar pickups and it became almost silent by improving the earthing with grounded foil . However the OP stated that it was quiet before therefore its unlikely to be a construction and design issue. I have one of these myself and it is quiet. – bigbadmouse Mar 1 at 9:47
  • You improved the shielding, rather than the earthing, in your Jazz bass. Likely OP has damaged the pickup in their bass, but it is hard to prove this definitively without a DC resistance reading from the pickup coil, as there are multiple ways their bass could be picking up interference. A good example is my old Japanese Fender Mustang bass. It had no shielding at all, and was making a horrible hum when I rewired it. The hum was simply the temperature control unit in my soldering iron, which was next to the bass on my bench. – ABragg Mar 1 at 12:36
  • it had none, but a modern bass (which this was not), would likely comes with some – bigbadmouse Mar 1 at 14:04
  • Factory shielding is usually very limited in quality and application. OP's bass in new; it is just a budget model. Fenders (US and Mexican-built) come with shielding paint in the control cavities but it is incomplete and doesn't really provide any shielding properties. I've measured a few painted cavities in Fenders and there is poor-to-no conductivity through the paint. In fairness, a $2000 Rickenbacker 4003 bass leaves the factory with no shielding installed in the control and pickup cavities. – ABragg Mar 1 at 15:55
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    ABragg, thank you for your detailed and well-informed answer. Your pointer regarding the tone knob was right on target - the pickup is indeed damaged. Unfortunately, I didn't have much success with cheap replacements on German Ebay, but I suppose the Seymour Duncan I just ordered will do a fine job. And yes, it's a Harley Benton - not a bad instrument at all, considering the price. – Richard Metzler Mar 2 at 9:13
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Please add some pictures. You have an earthing fault. The most likely explanation is that you accidentally tugged on the wires at the jack socket, or pulled an earth from one of the knobs. Unscrew the control plate (where the knobs are) and inspect carefully for loose wires (probably black).

If you have a multimeter you can test for earth continuity to the jack socket's outer ring where the black wire connects.

  • Thanks, will do that when I get home from work tonight. – Richard Metzler Feb 27 at 8:50
  • Get the probe on the jack's outer ring and touch each black wire's bare spot with the other probe, testing for continuity. Its unlikely you damaged the pickup, and this is all pretty easy to fix with a soldering iron. Its all pretty low tech. Look out for dull solder indicating a poor but working connection. Good solder should be bright and shiny. If you're confident with a multi-meter, set it to Ohms and do the same tests for poor connections, resistance between two given earth points should be milli-ohms at most. – bigbadmouse Feb 27 at 8:58
  • Perhaps my downvoter could explain why? The OP found my suggestion helpful as a route to try and that is surely the point of his asking the question ? – bigbadmouse Mar 1 at 9:48
  • you should also check that the bridge connects to the jack-socket outer ring using the multi-meter. One bass I bought once had a broken earth wire, – bigbadmouse Mar 1 at 9:50
  • As the 'downvoter', OP's issues didn't sound like an earthing fault. Any poorly shielded instrument will buzz if you remove your hands from the strings (perhaps there is a case to file 'no shielding' under 'earthing fault'). From OP's description it sounded like they had damaged the coil of the pickup, which was comfirmed by the OP. Any earthing fault would be a more correct diagnosis if, say, the volume pot or tone pot didn't work, or turning down the volume introducted a steady hum into the signal path. – ABragg Mar 4 at 9:28

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