1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  

humming pots=fried brain see attached diag

Discussion in 'Pickups & Electronics [BG]' started by pyzoot, May 6, 2010.

  1. pyzoot


    May 6, 2010
    Ok so i'm new to all this so please be patient with me :) I just wanta get playing now and this electronics stuff is making my head hurt.

    I bought a bass off ebay really cheap with the intension of fixing it up I mean how bad could it be? New neck, machines heads, pickups and electrics later...(the bodys still original) it all feels right but! since i've rewired it i have a hum. but not all the time.

    hum=(both volumes all the way down) + (tone pot full bright[off])

    no hum = (either\both volumes > 60%) + (tone anywhere)

    no hum = (both volumes all the way down) + tone pot full dark [on])

    touch any pot\strings\bridge while hum present makes it dip slightly.

    I've made up the attached circuit from a load of different diags i found on the net, to make it do - Volume per pickup with series/parallel push pull pots, phase switch bridge/neck and single tone.

    So what i was kinda hopeing was that one of you fine peeps could santy check my diag before i go insane and start ripping out pots and resoldering it all again.

    oh makes no difference which amp i use and it's not electrical interfrance from lights etc.

    hey thanks in advance!


    Attached Files:

  2. The first thing I see is that you have the volume pots wired backwards.

    You can only wire the output to the wiper terminal if you have a single volume control, or two volumes and a pickup selector switch.
    Trying to solo either pickup will result in muting all output.

    If you have either volume all the way down, that should directly short the tip and sleeve terminals of the output jack at 0 ohms resistance. You should not be able to hear ANY sound.
  3. Have you checked all of your grounds? Is the cavity shielded?
  4. pyzoot


    May 6, 2010
    Cool thanks Line6man arh sorry mate did that part drawing yesterday, day flipped the volume pots last night, forgot to change the drawing really sorry! The volumes do now work correctly. I've updated the drawing attached

    Still humming the same though!

    I've not got any cavity sheilding but surely the hum would be constant if it was that and touching any of the metal parts would not make any difference to the hum level?

    Attached Files:

  5. Have you tried different outlets in the house? I used to live in a very old house in New Orleans, and my basses would hum/buzz when the lights in the bathroom down the hallway were turned on. Used a different outlet in the room, all quiet after that.
  6. pyzoot


    May 6, 2010
    Not sure about the external interference stuff joeyl ,but i'll be able try it in a different biulding tonight. Will post update on it tomorrow though it used to be ok before i changed the wireing and pots.

    Sept the old tone pot made no difference what so ever and the volumes used to crackle hence the rewire\upgrade.

    In my book, it's what's changed that's broken it :s, So i suspect my dodgy wiring or a faulty pot at the mo. Just trying to figure which but, i'm open to being wrong.
  7. pyzoot


    May 6, 2010
    Line6man you happy with the update or can you see any other faults in the schematic?
  8. Bassamatic

    Bassamatic keepin' the beat since the 60's Supporting Member

    A faulty pot cannot create hum. Is your bridge grounded?
    Are the pickups wired properly? Sometimes it is not clear what wire is hot and which is ground.

    Also - I am not sure of the wiring of your series/parallel switches. In the down position, the white wire is grounded and the red and green tied together. In the up position, the red and white wires are tied together. Doesn't seem right to me.
  9. pyzoot


    May 6, 2010
    Hi Gio S, Yep the bridge is grounded. I have 5 seperate saddles and a wire to the bottom bolt on each connected alll connected to the tone pot. i've tested with a meter and all the saddles have continuity.

    arh good spot on the down position will double check it later does look wrong on the drawing! Would short out the North Humbuck but would it hum with no string action?
  10. Bassamatic

    Bassamatic keepin' the beat since the 60's Supporting Member

    It will hum if you have a hot or ground that is left floating. I have just looked at some other series/parallel wiring diagrams and yours needs to be fixed, I believe.
  11. I don't know what your color code is for the pickups, but it otherwise looks fine.

    OTOH, which way is the tone pot oriented? It appears backwards.
  12. JackANSI


    Sep 12, 2006
    do all the grounds go through the cap, or did you mean soldered to the back of the tone pot? ("All earths go back to top of tone cap", which in your diagram the 'top' lead is not direct ground)

    Also I hear no mention of all the pot and switch cases being grounded, just the tone. You should have a solid ground soldered to all the pot/switch cases.

    Just throwing stuff out there to try and help..
  13. MglMatador


    May 5, 2010
    This is the first danger sign. If both pickups are off (meaning both pickup signals should be dumped to ground through the pot wipers) and it still hums then something is seriously miswired.

    You need to start at the ultimate destination of the hum (e.g. the speakers) and work backwards:

    1) Does the amp hum when turned to playing volume and there is no cable plugged in? Most amps have a shorting input jack, so this should be relatively quiet. I will assume this isn't a problem.

    2) Now plug a cable in, then take a piece of wire and short the tip to the sleeve on the end that you would plug into the bass. There should be no hum. If there is hum, you have a broken cable shield. I will assume this is fine as well.

    3) Now plug in the bass with the pickups fully off. You said there is hum in this case. Solder a wire to the closest ground connection point (which in this case is the solder lug connected to the sleeve) , and strip the other end about 1/2 half an inch. This will be your test point.

    What you want to do is to short out (using the bare end of this new wire) different points starting at the output jack and leading back to the pickups. Start with the signal (tip connection) of the output jack. The hum should instantly disappear when you short out this point). If it doesn't, then you have a faulty connection at the output jack: either the basses ground is not connecting to the cable sleeve, or the tip lug needs to be resoldered.

    Continue this process, going to the tone pot, and backwards in the signal chain towards the pickups. Eventually you'll get to a signal connection where the hum doesn't disappear when shorted, which means that point in the path has no ground reference (which in this case is your wire).
    You can also check the ground points in a similar fashion: go from ground point to ground point, touching your new wire to each one, and if the hum disappears then you know which connection is missing.

    I would also suggest greatly simplifying your wiring and making sure that is correct as a starting point. Completely disconnect the pickups and output jack, leaving in your new ground test wire. Solder each humbucker in a fixed series config, and then run your new ground wire attached to the output jack to the pickup ground, then run the new signal signal wire directly to the output jack, and see if this configuration hums. Humbucking pickups in this configuration should be relatively quiet (that's the whole point), so if you have a loud hum here then you've likely found the problem. Do the same for the other pickup.

    Then assemble the series/parallel portion and do the same test (leaving the volume pots and tone control out of the circuit). You'll get to a point where you find the problematic area and then the fix will likely become obvious (missing connection, bad solder joint, etc).
  14. mech

    mech Supporting Member

    Jun 20, 2008
    Meridian, MS, USA
    I didn't trace all the parallel/series switching to check it, but if it's wired correctly and the hum with both vol controls turned off is relatively low level......

    With both PUs turned off, the PUs are shorted to ground and the input of the amp only sees the parallel resistance of the two 500K ohm pots, or 250K ohms to ground. This is similar to, but not as bad as, having an open cable plugged into the input of the amp. There will be some hum due to this type of independent volume control circuit. Jazz Bass and Peavey T-40 is the same and have some hum when turned all the way down. They use 250K ohm pots that provide more load to the input of the amp (125K ohms).

    With either or both of the vol pots turned up to some degree, the hum should be reduced since the input of the amp is starting to "see" the PU, which has a lower resistance/impedance value than the pot.

    As an experiment you can connect the 500K pots across your cable (the two outside terminals) and see if it has about the same amount of hum you have with both vol controls turned down when wired in the guitar.

    Also make sure the bodies of the pots are grounded.

  15. pyzoot


    May 6, 2010
    Wow so many replys.

    lyeoj - Still hums in different socket and different building and different amp

    Gio S - You were right about the diagram being wrong I put it down from memory and transposed the White and Red wires. The bass is correct though i double checked it last night. (i've attached the updated diagram for completeness)

    JackANSI - I've soldered all grounds to the back of the tone pot, including pot\switch cases. The DPDT is all plastic so not case grounded. I've added some clarity to the top pot in the diagram.

    Mech - will check this out later when i get home from work and get back to you thanks!

    Thanks everyone all for your help!

    MlgMatador - 1. No the amp doesn't hum without the jack plugged in at playing volume. Will try the your trouble shoot process tonight for the rest of the path (i'm at work right now)

    Attached Files:

  16. BTW, you need to fix the capacitor value in all your diagrams.

    0.47uF is ridiculously dark! I'm fairly sure that you mean 0.047uF, right?
  17. wow dyslexic typing, I have not seen that before :p hopefully you have it sorted out soon.
  18. JackANSI


    Sep 12, 2006
    In those cases I try to solder a lead onto the lock washer or something else that touches the metal parts.

    best diagram on here in a long time, btw.
  19. pyzoot


    May 6, 2010
    well the prize goes to MglMatador. Was my lead :( if you only new how many times I've check and rechecked the wireing, Re-soldered, re-tested i've been going round in circles for weeks. Big thanks to all!

    Line6man. Yep it's not a typo, the bass player in our band described it more like an off switch than a tone pot :) I've got a stack of caps to try this weekend should find one that's more suited.

    Out of interest anyone... If i put a small cap in-line with the ground to the pot tab would that have any beneficial decoupling type effects from the other pickup? or would it have adverse effects on the tone. see attached...


    Attached Files:

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.