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Tone pot causes volume drop on Mustang PJ - need help please

Discussion in 'Pickups & Electronics [BG]' started by cattrax, Jun 13, 2018.


  1. cattrax

    cattrax

    Oct 17, 2010
    Gainesville, FL
    I have always wanted to do some modifications to a bass as a learning experience and, after much internet (Talkbass is amazing!) and youtube time, finally got brave enough to try. I have a Fender Mustang P/J to which, so far, I have done the following:

    Replaced the stock bridge with a Fender High Mass bridge
    Replaced the 3-way switch and output jack with Switchcraft brand parts
    Shielded the entire main cavity and the jazz pickup cavity with copper foil
    Adjusted the pickup heights when I reinstalled them after shielding the cavities.
    Performed a complete set up (saddle height, truss rod adjustment, intonation, etc.)
    --- I plan to swap the stock tuners for Hipshot Ultralights soon.

    Here’s where I could use some Talkbass wisdom. Once I put everything back together and did the set up, the jazz pickup has a huge problem. When I roll the tone off with the jazz pickup soloed, the volume drops completely. This does not happen with the P pickup. I get the normal sounding bright to thuddy range with the P pickup soloed as I turn the tone knob down.

    With the tone knob all the way up, the soloed jazz pickup sounds bright (although thin and at somewhat lesser volume than the P pickup, which seems to be “normal” for these Mustang basses) – roughly the same as when I first bought the bass. What’s puzzling me is the total drop in volume from the J as I turn the tone down. I removed the control plate to see if maybe something was shorting out, but even with the control plate open, I get the same result.

    I did not modify the pots at all (though I have very good soldering skills – the new switch works great). Did I somehow kill the jazz pickup or tone pot? What would my next steps be to figure this out?

    Thanks in advance for all your help!
    Kathy
     
    Last edited: Jun 13, 2018
  2. Crater

    Crater

    Oct 12, 2011
    Dallas, TX area
    If a tone control is acting like a volume control, it means that somehow the signal is "getting around" the tone capacitor and going to ground.

    Can you take a clear, close up picture of the backside of the volume and tone controls?
     
  3. cattrax

    cattrax

    Oct 17, 2010
    Gainesville, FL
    Thanks @Crater, here are some photos. If it was the capacitor, wouldn't it affect the P pickup as well? And thanks to the moderators for moving my post to the correct forum!

    IMG_5400.JPG
    IMG_5401.JPG
    IMG_5402.JPG
    IMG_5404.JPG
    IMG_5405.JPG
     
    Last edited: Jun 13, 2018
  4. Crater

    Crater

    Oct 12, 2011
    Dallas, TX area
    Nice pictures. :thumbsup: Everything looks OK as far as I can tell...

    Yes it would, so I'm a little stumped here. Is that the stock capacitor on the tone control?

    The only other thing I can think of is that somehow the Jazz pickup doesn't have a solid connection somewhere. Could be any of the solder joints, it could be the pickup switch. That's the kind of thing where you need a multimeter to hunt it down (see my signature).

    You can buy a multimeter (also called a Digital Multi Meter or DMM) at most large discount, hardware or auto parts stores, for about $20 or less.
     
  5. cattrax

    cattrax

    Oct 17, 2010
    Gainesville, FL
    Yes, stock capacitor and pots. I do have a multimeter. Would I be checking continuity or for specific resistance or other values? I have never measured pots before.
     
  6. Crater

    Crater

    Oct 12, 2011
    Dallas, TX area
    Bless you! :thumbsup:

    You can start by measuring resistance (ohms) from the hot wire at the output jack and ground.

    With the volume control all the way up ("10"), you should get about 10 k Ohms with the pickup switch set to the P pickup, and about 8.3 k Ohms when the J pickup is selected. It should drop down to around 4.5 k Ohms with both pickups ON. As you roll the volume down to zero, the resistance should drop too, until it's very close to zero with the volume all the way off. The tone control should have no effect on this DC resistance measurement, so confirm that it doesn't.

    You can check the pickup switch by measuring resistance from either switch terminal to the center terminal. Also visually inspect the switch contacts to make sure they're lining up and don't have some random debris interfering with the contacts firmly touching together.

    Based on your bass' symptoms, I expect the J pickups resistance may be higher due to a loose connection somewhere. But I'm guessing.
     
    fleabitten likes this.
  7. Bourbongangster

    Bourbongangster

    Nov 5, 2015
    Did you shield inside the hole that goes from the Jazz pup to the control cavity? If so, my guess is that the hot lead from the Jazz pup is grounding out on the shielding or connecting somehow to the ground wire going to the bridge. Just a guess though.
     
  8. cattrax

    cattrax

    Oct 17, 2010
    Gainesville, FL
    :) Thank you. Couldn't imagine getting through life without one!
    Thanks for the tutorial. I'm at work now, but will test all this tonight and report back. I will definitely check for loose connections too. I really appreciate your help!
     
  9. cattrax

    cattrax

    Oct 17, 2010
    Gainesville, FL
    I shielded the cavity beneath the jazz pickup, but not inside the small hole that the wire travels through to the main cavity (if I am understanding your question). Should I have done that? I'm not sure how to get the copper shield in there. At any rate, there certainly could be something shorting out under the jazz pup. I'll check that when I get home tonight. Thanks so much!
     
    fleabitten likes this.
  10. Bourbongangster

    Bourbongangster

    Nov 5, 2015
    Definitely sounds like you get what I'm saying. Whether or not you shield the inside of that hole is a whole different discussion that shouldn't necessarily relate directly to your problem. But the same idea applies to the shielding under the Jazz pup, so definitely worth checking out. I can't imagine how you could be having an issue with just one pickup apart from that since both pickups follow the same path after the switch. So it would have to be something between the Jazz pup and the switch. I hope that helps. Let us know what you find out.
     
  11. cattrax

    cattrax

    Oct 17, 2010
    Gainesville, FL
    Calling @Crater and @Bourbongangster !!
    Okay, so I unmounted the pups and checked all connections. Everything looks solid. No stray dirt/debris or anything that might short something out. I will cover the pickup terminals with electrical tape when reassembling - just in case. I measured the resistance as suggested by Crater and got the following readings (all were measured from the hot wire on the output jack to the ground lug that is screwed into the cavity):

    Neck “P” pup soloed:
    Volume at 10 = 11.31 k ohms
    Volume at 0 = 247.7 k ohms

    Both pups on (switch in middle position):
    Volume at 10 = 11.31 k ohms
    Volume at 0 = 247.7 k ohms

    Bridge “J” pup soloed:
    Volume at 10 = 247.7 k ohms
    Volume at 0 = 247.7 k ohms

    Some of these readings certainly don't follow Crater's example above. There was no change in resistance when rotating the tone knob in any of the above positions, with volume at 10 or at 0.

    I then measured the resistance at the switch. See the drawing below for the values.
    What could be the problem? Where do I go from here?
    Thanks!
    IMG_5412.jpg IMG_5409.jpg
     
    Last edited: Jun 13, 2018
  12. honeyiscool

    honeyiscool

    Jan 28, 2011
    San Diego, CA
    Yuck 50s wiring. Yuck reverse volume knob.

    The wire from the switch should be moved from middle lug to outside lug, and the outside lug wire to output jack should move to middle lug. Tone pot can stay where it is.
     
    sissy kathy likes this.
  13. cattrax

    cattrax

    Oct 17, 2010
    Gainesville, FL
    The wire from the switch should be moved from middle lug to outside lug, and the outside lug wire to output jack should move to middle lug. Tone pot can stay where it is.
    Are you referring to the lugs on the volume pot? This is my first experience with pickup wiring and pots. What would that change?
     
    Last edited: Jun 14, 2018
  14. honeyiscool

    honeyiscool

    Jan 28, 2011
    San Diego, CA
    That’s just the nature of the reverse volume knob. Notice that the input is in the middle lug and the output is in the outer lug and the ground is on the other outer lug. There is always 250k resistance between the two outer lugs. That means when you take the volume down, the pickup output goes to ground but the output jack becomes 250k away from ground. Nothing is wrong. That is how that wiring works.

    Furthermore, the 50s wiring puts the tone pot parallel to the output jack. This means that as you roll down the volume, the tone is not rolled off as much. But it does mean that as you roll down the tone, you lose lots of volume.

    My two suggestions in my above post solves both problems. When you face the volume knob, there are three lugs. Let’s call them 1, 2, and 3 where 3 is grounded. Currently, pickup switch output goes to 2 and jack output and tone pot are connected to 1. Instead, connect pickup switch output and tone pot to 1 and jack output to 2. This will solve all your problems.
     
    sissy kathy likes this.
  15. cattrax

    cattrax

    Oct 17, 2010
    Gainesville, FL
    That's making more sense to me now. Thank you for explaining. What would cause a reading of 250k ohms on the jazz pup with the volume at 0 and also at 10?
     
  16. honeyiscool

    honeyiscool

    Jan 28, 2011
    San Diego, CA
    Unless you’ve connected the shielded cavity to the rest of the shielding, you haven’t shielded the jazz pickup cavity. You just put copper in there. And judging from how many wires are coming out of the cavity (1 white wire and 2 black wires) you probably haven’t actually connected the shielding to the rest of the shielding. Furthermore it does sound like there is something wrong with that cavity. The pickup shouldn’t measure 250k to ground with the volume on 10 unless something is really wrong. How is the jazz pickup in there? Springs? Foam? There’s a good chance that in your attempt to shield the cavity, you actually connected the hot lead on the pickup to the copper.
     
    BlueTalon and Bourbongangster like this.
  17. cattrax

    cattrax

    Oct 17, 2010
    Gainesville, FL
    Good point on the shielding in the jazz cavity. @Bourbongangster had a similar point and I will correct that. The pup has foam underneath. I did check that the hot lead was not touching the copper, but will check it again. Thanks so much for your help!
     
    Bourbongangster likes this.
  18. LUpton

    LUpton Supporting Member

    Oct 22, 2012
    Tampa, FL
    Probably too old for this sh--
    Disconnect the jazz pickup from the switch and read it w the meter. If it reads much higher than 10k, the pickup has a problem...

    +1 on connecting the output of the switch to lug 1 of the volume pot.
     
  19. cattrax

    cattrax

    Oct 17, 2010
    Gainesville, FL
    Thanks @LUpton, I'll check that out. My project for the evening...
     
  20. Axstar

    Axstar Inactive

    Jul 8, 2016
    Scotland.
    Two options spring to mind:

    1. The coil in the Jazz pickup is broken internally. Tone controls act as volume controls on pickups with broken coils.

    2. The tone control appears to roll off the volume, but isn't. The capacitor in those PJ Mustangs is 0.1 uF, which is quite a high capacitance value for the job required. I've changed out the caps in both my Mustangs to 0.022 uF, and I think it is an improvement. I also rewired both of mine using Telecaster wiring schemes. From memory this reverses the order of the wires in the volume pot, where and how the tone control is wired and configured in the circuit.

    The stock wiring in the PJ Mustangs isn't all that good. One of mine came from the factory with the ground wire unsoldered at the 3-way switch. It was simply wrapped through the eyelet on the switch and allowed to flop around.

    This was my revised wiring scheme for one of my Mustangs:

    O31KI1B.jpg
     
    ELynx and sissy kathy like this.
  21. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
    Apr 12, 2021

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