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Silencing the Noise on a Marcus Miller Signature Bass -
March 2007
- Phil D’Eon
- pdeon@rogers.com
I have an older Marcus Miller Signature bass, and it used to be highly susceptible to noise
picked up through the vintage single-coil pups (unless they were exactly balanced in
volume, and also light buzzing (that would go away when I touched the strings). That’s
all gone now. It’s absolutely silent until I thunder a string - because I re-wired it to
remove the ground loops, and I shielded the pups. It was great before – it’s unbelievable
now.
I opened up the control cavity and what I found in there shocked me, considering the
presumed build quality of this bass. It was a textbook case of how NOT to wire and
shield a guitar – or any audio device for that matter! So, if your control panel is wired
like the picture below, and you are getting noise, I now know you can get rid of it simply
by eliminating the ground loops and shielding the pups. (What I can’t understand is –
why was it built that way?)
Fig 1:
Key things to notice are (1) the short black wires joining the shells of the pots, (2) some
of the pot terminals are bent back and soldered to the shell, and (3) the conductive paint
coating does not go high enough to make contact with the foil-backed pick-guard.
What I did was this:
(1) added a foil liner to bridge the gap from the conductive paint in the cavity, up
over the edge to make contact with the foil on the back of the pick guard,
(2) gave all the signal grounds their own wires and ran them all to one point, removing the ground loops, and
(3) shielded the pups with copper foil, and grounded the shields.
These steps are described below in some detail. Here’s the end result.
Fig 2:
What a difference! Dead silence now regardless of volume and pickup
balance.
[edit] INSTRUCTIONS:
FIRST THING – Thanks to John Atchley and his Guitar Nuts website that inspired me to
do this project. Please visit the Guitar Nuts website at http://www.guitarnuts.com/wiring/index.php. You will notice that John strongly recommends a capacitor to prevent electrocution in the event of amplifier malfunction. My bass didn’t have that capacitor, and I haven’t added it to mine
yet, but I intend to look into it further – I have already bought the capacitor.
SECOND THING – These instructions are not step-by-step, but rather they assume you
know enough about soldering and simple switch-circuit wiring to provide self guidance
and not do damage. Think ahead and apply common sense liberally. If you are unsure, I
recommend you ask for help from someone who is more comfortable with these things.
There is no visible difference to the guitar from the outside after this work, but it sounds
clean.
[edit] PART ONE – Shielding the control cavity.
Remove the battery.
This bass already came with aluminum foil on the back of the pick guard, a very nice coat
of conductive paint inside the cavity. If you need to add these things to your bass, the
instructions are on the Guitar Nuts website above.
But unfortunately on this bass, the conductive paint did not come up and over the edge,
and therefore the pick guard foil couldn’t make contact. All I needed to do here was to
complete the shielding all around the edge by adding a foil strip around the lip of the
cavity such it makes electrical connection with the pick guard. On the inside, the new
foil touches the conductive paint; on the top side, the pick guard sits on the foil when
installed.
I chose to use aluminum foil tape for this - the shiny thin metal aluminum tape that you
get at a hardware store for dryer duct taping (NOT fabric-based duct tape!). This tape is
about 2 inches wide, so I cut it in half length-wise into 1-inch strips. A box cutter or
exacto knife is all you need to work with this.
You need to make good contact with the conductive paint, and the adhesive side of the
tape is not conductive. So, to get the shiny conductive side up against the paint, I folded
over about ¼” onto itself (Fig 3 - start) (Fig 4 – ready ), and stuck the strip on the side of
the cavity such that the folded shiny foil was in contact with the conductive paint (Fig 5).
Apply it in strips of convenient length to work with, but apply pieces to go all around the
cavity lip. Make sure the edges butt together or overlap.
Fig 3:
Fig 4:
Fig 5:
I have three tips when it comes to bending the foil over on to the top surface of the guitar:
(1) Slit the upstanding foil vertically at the bends – once at each side of the bend, and one
or more times in the bend, as you see fit. Folding back the straight sections is easy now,
because you have slit the foil before the bend. And slitting the bends will let you
approximate several straight sections.
(2) Focus on getting the foil flat, because wrinkles will end up raising the pick guard and
reducing its overall contact. Don’t worry about gaps on the top side – it is the inside that
matters most. And don’t worry about overlapping the foil sections – you will easily cut
out the overlaps in the next step.
(3) Where possible, cover over a screw hole, because that will provide the tightest
electrical contact when the pick guard is installed.
I then made secure electrical contact between the folded foil and the conductive paint on
the side of the cavity by winding tiny short screws into it. Tips on the screws:
(1) Put the screw through the folded part of the foil. That’s where the shiny foil is on the
other side up against the paint – which is the whole point of this.
(2) Use the tiniest screws you can find. I used #4 x ½”.
(3) Avoid twisting the foil under the screw head by putting a washer under the screw, or
at least don’t over-tighten – all you need is contact.
(4) Make a starter hole with a needle-shaped scratch awl or something.
(5) Don’t put screws where the heads will interfere with the pot shell when the pick guard
is installed.
(6) You don’t need many screws – maybe even none at all - but I used one per foil strip.
Next, remove the overlaps in the foil. Simply slit down one side of the overlap and peel
off the top layer that you just freed with the slit.
Finally, trim off the excess foil. I laid the pick guard in place and traced around it with a
sharp pencil. Then I lifted off the pick guard and used the knife lightly to trim off the foil
to within about ¼” of that line so it’s not visible when the pick guard is installed.
See Fig 2 for the end result.
[edit] PART TWO - Fixing the ground loops
The basic goal here is really simple: Separate the “signal ground” wires from the
equipment grounds, join them together at a single point, and wire that point to the low
side of the output jack. We are NOT going to touch any of the other (signal “hot”) wires.
“Signal grounds” are the (typically) black wires coming from the pickups and anything
wired to the arms of a pot that presently goes directly to ground or the shell of a pot –
including those pot terminals bent back and soldered to the shell. “Equipment grounds”
are the bridge ground, shield grounds, and the minus side circuit of the battery.
FYI: The minus side of the battery runs to the ring terminal of the output jack, which acts
as an on-off switch when you plug in a cable. This works because the shaft of the mono
jack on the cable shorts the ring terminal to the body of the jack, thereby connecting the
battery terminal to ground, and to the black (minus) power input wire for the pre-amp.
On the output jack, find the solder lug connected to the body of the jack (the part with the
nut on it). It will likely have wires that you can trace to the battery (-) terminal and to the
pre-amp (-) power input, usually a black wire. Leave those wires where they are.
Isolate the signal grounds:
(1) I took the black (grounded) wires from the pickups and cut them free at the far
end, leaving the longest length of wire possible. (To identify the black wire from
the low side of the pickup, versus a the black wire coming through the same hole
that is the bridge ground wire, you can lift the ends of the wires and measure
continuity. The pickup signal ground wire should measure thousands of ohms
between the end of that wire and end of the other wire (typically white) that is the
hot side of the pickup.)
(2) Wherever the terminal of a pot was soldered to the shell, I de-soldered it, moved
it back away from the shell, and added a length of black wire. I also removed the screw and lug that secures two black wires to the floor of the cavity (visible in Fig 5). One of these is a signal ground, which must be removed from that grounding point, and the other is an equipment ground, which you could leave there if you wanted - but I didn't.
(3) At the output jack, the signal grounds and equipment grounds come together. We
want this to be the ONLY place they come together. Figure out which is the
signal ground – that is the wire you are going to replace. On my bass, I followed
the signal ground wire that was currently connected to the jack and followed it
back to its source at the pot shell. Remove it and replace it with a wire. In Fig 2,
that is the white wire with a black stripe.
(4) Bring all these wires together, trim them to a neat length, and solder them
together in a bundle. You should have six wires in that bundle: Two coming pickups, two coming from pots, one from the preamp, and one from the cable jack. These wires should not touch equipment ground anywhere except via the jack body. Insulate this connection – you don’t want it touching ground or you could get the ground loop buzzing back.
Then, connect the bridge grounding wire to the foil. You will find it coming through a
passage hole near the bridge, possibly running through the bridge pickup cavity, and most
likely soldered at the other end to a pot shell. To identify it, remove it at the pot shell
end, and you should measure continuity (0 ohms, roughly) between the end of the wire
and the bridge body. Solder that wire to a ring terminal, and put the ring terminal under
one of the screws that you used to anchor the aluminum foil in Part One.
Finally, remove all those short black wires soldered on the the shells of the pots. When you are done, nothing should be soldered to the shell of any pot.
[edit] PART THREE – Shielding the pickups
This is simply a matter of getting access to the pickups, wrapping them in copper foil,
and grounding the foil inside the control cavity.
Loosen the strings, lift out the pickups, and lift off the plastic covers.
I wrapped a little strip of electrical tape around the terminals where the leads connect to
the coil. This is a precaution against the copper foil touching the terminals after being
wrapped and the cover crammed back on. But be very careful not to disturb the coil
winding wire because it can break off very easily.
Cut a length of adhesive copper foil that will wrap around the coil in the same direction
as the windings (obviously) with about an inch overlap. (I used a strip of 1” copper foil -
available at many guitar parts supply stores.) Peel off the backing and wrap it carefully
around the coil with a little bit bent over the top of the coil form, as seen in Fig 6.
Press ONLY on the plastic coil spool form, and NOT on the coil windings. In fact try to
avoid the copper touching the coil winding wire anywhere, even though it is insulated
and should not be a big deal if it does touch.
Fig 6:
Tack-solder the foil ends using not too much heat. For each pickup, cut a wire long
enough to reach easily into the control cavity from its pickup cavity, and solder one end
to the foil. This is shown in Fig 7.
Fig 7:
Carefully put the plastic covers back on the pickups. Then put the new wire through the
hole where the pickup leads run, and fish them into the control cavity. Reposition the
pickups in their cavities, screw them in place, and re-tension the strings.
There should now be only the two new shield wires that are loose. Solder them together
on a ring terminal, and secure the ring terminal under one of the screws that you used to
anchor the aluminum foil in Part One.
Clean up all loose debris, check it over, and re-attach the pick guard over the control
cavity being careful to avoid pinching wires or any unnecessary binding contact between
the pots. If you have trouble fitting the pick guard back in, be sure to check that you are
not pressing up against one of those little screws in the foil. Remove it if you are.
Re-attach the battery and the cover. Tune up - the usual stuff.
You’re done. Now you can get out there and play the spaces between the notes with REAL silence!
[edit] Comments
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