What's going on here then? (Jazz bass wiring wierdness)

[Phil_T]

This is a photo of the control plate from 91 MIJ squier jazz that I bought this week. I'm puzzled about why there is no ground wire connecting to the jack. The copper plate that I pulled out sits in the bass of the control cavity and has three black wires soldered to it, 1 from the front pickup cavity and 2 from the back pickup cavity (I assume these are from ground shields in the pickup cavities/bridge but haven't looked yet). Two further black wires, one from each pickup cavity are connected to the back of the first pot and a wire then connects the pot to the copper shield. None of this, however, is connected to the jack ground unless it's somehow going through the control plate itself. Weird, anyone know what's going on?

The bass isn't particularly noisy so it appears to be working somehow. There's very little hum and what there is reduces when the strings are being touched. Touching the poles of the bridge pickup introduces a bit of noise but not much wheres the front poles are really bad so they are obviously not grounded.

[Islander]

It looks like the metal plate is being used as an earth conductor which is fine as long as all of the fittings are tight and making a good connection.

[Phil_T]

1 minute ago, Islander said:

It looks like the metal plate is being used as an earth conductor which is fine as long as all of the fittings are tight and making a good connection.

by metal plate you mean the chrome control plate presumably, not the copper plate?

[Islander]

Yes, the control plate

[Unknown_User]

If the hum reduces when you touch the strings then presumably there is a ground connection to the bridge?  I think there ought to be a ground connection to the back of each of the pots and also to the jack socket.

 

Edited January 21, 2019 by Unknown_User

[Islander]

I'll just add that that's the way I'd expect it to be wired - it removes the dependency on good contact with the control plate

 

[Unknown_User]

4 minutes ago, Islander said:

I'll just add that that's the way I'd expect it to be wired - it removes the dependency on good contact with the control plate

 

If I understand, that control plate is supposed to press up against the backs of the pots?

[Islander]

Yes but there's a difference between a good mechanical contact and a good electrical one.  The first is just a matter of doing everything up tight enough, the second means that you have to ensure that contact areas are both clean and tight.  It's possible and acceptable to use the control plate as an earth path rather than having the additional wiring, but it does mean you have to ensure that good electrical contact is made between the pots and jack and the control plate.  Using a loop of wire soldered to the back of each pot and to the earth tag of the jack is generally more reliable in the long term as long as good solder joints are made.

[Phil_T]

Yes, I expected to see a ground wire connecting all the pots to the jack which is why I was a bit confused. It only occurred to me that the control plate might be in the circuit while I was typing up the the original post

[Marc S]

I can't tell from your photo - but is there a copper strip earthing the pickup(s) to the bridge?
I'd still expect to see the jack socket earthed though..... Perhaps you ought to add one?
Maybe then the slight humming would stop

I've got a MIJ Jazz, and from memory, I'm sure the jack socket is earthed....

[Geek99]

No, earths though the bell plate and pot bodies. 

Surprised that’s strings not earthed though

i 

[Phil_T]

1 hour ago, Marc S said:

I can't tell from your photo - but is there a copper strip earthing the pickup(s) to the bridge?
I'd still expect to see the jack socket earthed though..... Perhaps you ought to add one?
Maybe then the slight humming would stop

I've got a MIJ Jazz, and from memory, I'm sure the jack socket is earthed....

yes, there is copper strip bridge earth. I haven't taken the pickup out yet to check thats it's connected properly but the eveidence suggests that it is

[Geek99]

Multimeter between bridge and bell plate set to ohms should tell you that. If 0 ohms or near it then you have connectivity with no resistance 

Edited January 21, 2019 by Geek99