Passive bass, bridge properly grounded, no hum... but a click

[anzoid]

I've been working on a self-build P-bass type thing (this one: https://www.basschat.co.uk/topic/493469-guitarbuildcouk-bass-3).

 

The setup is a little out of the ordinary - the pickup is a Tonerider TRP1 and it's wired directly to the output jack - no controls of any kind, and it sounds pretty awesome.

 

The cavity is shielded with copper foil and the bridge is properly earthed. I checked this with a multimeter from strings to output jack. Although parts of the coated aluminium Hipshot B style bridge don't seem to be grounded, the the saddles and screws are. When plugged in and not touching the strings the bass is silent - not even a hint of hum. However, when I touch the strings I get a click, and if I touch alternating poles of the pickup (starting on the outside of the E string), I get a horrendous buzz/hum. The jack is a bog standard Switchcraft mono 1/5" jack and it doesn't touch any of the copper foil anywhere because I've wrapped electrical tape round it so that it can't touch the body at all.

 

So, I'm kinda stumped as to what could be causing the problem and would be grateful for any wisdom from the the wonderful folk in this part of the forum. Thank you!

[tauzero]

Have you tried swapping the connections to the jack socket?

[Hellzero]

Did you earth ALL shielded cavities including the pickup one?

 

If not, you have the culprit, because if you shield something without grounding it's totally useless (and way too common).

 

And did you shield and ground the back of your pickguard as it plays a role in protecting the whole electronics from electromagnetic fields?

[anzoid]

9 hours ago, tauzero said:

Have you tried swapping the connections to the jack socket?

Nope, hadn't even thought of that! Thank you!

Just given it a try and it makes no difference. Cranking my amp - there is a buzz/hum with hands off the strings, but it's not that loud. 😕
I've tried disconnecting the earth wire from shielding - instant horrendous buzz. Also tried not earthing the bridge - instant horrendous buzz.
With everything wired up I have continuity from each the string and the bridge through to the sleeve of the jack at the far end of my guitar lead.

 

2 minutes ago, Hellzero said:

Did you earth ALL shielded cavities including the pickup one?

 

If not, you have the culprit, because if you shield something without grounding it's totally useless (and way too common).

 

And did you shield and ground the back of your pickguard as it plays a role in protecting the whole electronics from electromagnetic fields?

Ah, no, the pickup cavity isn't grounded... I was just about to ask that question - should I ground the pickup cavity...

Looks like that might be today's job, the bass doesn't have a scratchplate, control cavity is rear routed and the front is just wood:

Control cavity wiring is pretty basic 😆

[anzoid]

Thank you both for your help! Off to do some wiring!

[Chienmortbb]

Yes any copper shielding must be connected to the ground/sleeve terminal of the jack. It is usually easy to solder to?

Lovely bass by the way.

Edited September 7, 2024 by Chienmortbb

[anzoid]

Looks like I'm going to have to put copper tape in the pickup cavity as the shielding paint I used appears to be useless - vary variable continuity using the multimeter probes in different parts of the cavity. With copper tape does it have to go all the way to the top edge of the cavity, or is something like half way up the sides sufficient? Going to be really difficult to keep it looking neat otherwise

 

[Hellzero]

I see that your shielding is not grounded...

 

Ground it as well as the pickup cavity and shield the back of the lid of the electronics compartment that will be grounded by contact with the cavity shielding.

[anzoid]

8 minutes ago, Chienmortbb said:

Lovely bass by the way.

It's shaping up pretty well. The neck is gorgeous to play, and body has come out looking pretty good, not 100% what I was imagining, but close enough

[Hellzero]

Yep, shielding paint is pretty much useless as conductivity is often really poor.

[anzoid]

Just now, Hellzero said:

I see that your shielding is not grounded...

 

Ground it as well as the pickup cavity and shield the back of the lid of the electronics compartment that will be grounded by contact with the cavity shielding.

Lid has copper tape already done. I think the shielding is grounded OK - the screw going through the shielding with the wire wrapped round of it seems to be making good contact and electrical continuity throughout the cavity is OK, though I shall check some more points to be sure!

 

[ezbass]

1 minute ago, Hellzero said:

I see that your shielding is not grounded...

Pretty sure I can see a star washer or some such screwed into the shielding on the right of the photo, where the it is attached to the bridge ground.

 

[anzoid]

Just now, Hellzero said:

Yep, shielding paint is pretty much useless as conductivity is often really poor.

I was surprised how rubbish it is - a few feeble beeps from the multimeter in a couple of places  The other two basses I've built have EMG pickups and don't even have shielding and are silent (except the noise I make :D)

[ezbass]

1 minute ago, anzoid said:

 I think the shielding is grounded OK - the screw going through the shielding with the wire wrapped round of it seems to be making good contact and electrical continuity throughout the cavity is OK,

 

Ah, beaten to it.

[anzoid]

Just now, ezbass said:

Pretty sure I can see a star washer or some such screwed into the shielding on the right of the photo, where the it is attached to the bridge ground.

 

No washer at the moment, just the wire wrapped round the screw and tightened down... Off to fit a star washer now, I like that idea!

[anzoid]

Job done! Thank you to everyone for the help and advice.
Pickup cavity now shielded with copper tape and I checked every piece for connectivity as it was applied - it's about as shielded as I can get it. Everything is grounded to the shielding in the control cavity and I somehow ended up with a spare piece of wire afterwards ¯\_(ツ)_/¯
Final step was putting the lid back on the control cavity and I now have a bass with no hum. Only oddity is I get a slight click touching the G-string (ooh er)...

Edited September 7, 2024 by anzoid

[Hellzero]

8 hours ago, anzoid said:

Only oddity is I get a slight click touching the G-string (ooh er)...

Try lightly sanding the top of the G saddle, it could be a very slight connectivity issue.

 

Or maybe the silk of that string is on the saddle preventing good connectivity (grounding).

[anzoid]

11 hours ago, Hellzero said:

Try lightly sanding the top of the G saddle, it could be a very slight connectivity issue.

 

Or maybe the silk of that string is on the saddle preventing good connectivity (grounding).

That'll be today's job  Thank you!

[anzoid]

Saddles sanded slightly, connectivity checked at every conceivable point, everything put back together and... the bass is silent when I don't touch the strings. Can get a bit of buzz from touching the pickup poles though I did put copper tape on the bottom of the pickup and then round the pickup foams so as to be touching the shielding - but something wasn't quite right there. I'm currently not sure whether I'm going to be keeping the TRP1 or trying something like an SD Quarter Pound... we shall see.

 

Thank you @tauzero, @Hellzero, @Chienmortbb and @ezbass for the help! Very much appreciated because I don't think I would have got this sorted without your advice.

[tauzero]

The pole pieces issue has me puzzled. That's why I suggested swapping the pickup wires round. Try checking resistance between the pole pieces and each of the pickup wires, and if one is no resistance and the other is a few k, the one with no resistance should be the ground.

[anzoid]

Have taken the pickup out this evening and, as far as I can tell the pole pieces are completely electrically isolated. Resistance between the black and white wires is 10.6 kΩ, but from either wire to any pole piece... nothing. My multimeter is a bit of a naff auto-ranging/function thing and it doesn't find anything to measure, continuity works fine (which is how I used it for shielding). 

I actually caved last week and bought a Delano PMVC 4 FE/M2 (the one with the chunky pole pieces) because I've been wanting to try one for ages, and should have probably bought it in the first place. Installed and it's both silent with respect to any hum/buzz and silent if I touch any of the pole pieces.

Took it to band practice on Thursday with the Tonerider in it and the sound was very good... made me wonder if the Delano will really be much better. 

Edited September 14, 2024 by anzoid

[Hellzero]

According to its guts, you'll lose a bit of mids making this Delano sound clearer (more precision in the ... sound rendering) than the Tonerider, but it's no big deal as long as you like the tone.

[anzoid]

Yeah, it sems a touch mellower than the Tonerider with sweet highs. Will be taking it to practice on Thursday to put it through its paces

[rmorris]

On 07/09/2024 at 11:13, anzoid said:

Looks like I'm going to have to put copper tape in the pickup cavity as the shielding paint I used appears to be useless - vary variable continuity using the multimeter probes in different parts of the cavity. With copper tape does it have to go all the way to the top edge of the cavity, or is something like half way up the sides sufficient? Going to be really difficult to keep it looking neat otherwise

 

 

The more complete the shielding the more effective it is. But how good is good enough is subjective.

[anzoid]

16 hours ago, rmorris said:

 

The more complete the shielding the more effective it is. But how good is good enough is subjective.

I think I just about got there. I ended up buying some narrower copper tape than used on the first attempt in the pickup cavity and that made it a whole lot easier to do as I wasn't trying to fold the (and rip up...) the tape round odd corners.