Telecaster 4-way Switch Conversion -> Background Hum

[bob p]

I'm having a minor hum problem after adding the Fender 4-way Tele switch to my new G&L ASAT Classic (basic Telecaster type guitar). The 4-way switch adds a series connection to the standard Tele wiring. I followed the official Fender instructions that came with the switch, which I've attached below.



The new selector switch works fine, but I'm having a minor noise problem. If I don't have my hands on the guitar, and I touch the metal control plate or the stalk of the selector switch, I get hum. (No noise if I touch the metal knobs on the pots, which are floating on plastic shafts.) If I keep my hand on the metal control plate and touch the strings then the buzz stops. The buzz appears to happen in all switch positions; in other words it's not coming from an un-shielded neck pickup, whose cover's ground connection hast to be cut and re-established with a new wire with this mod. I can touch the neck pickup cover without any buzzing.

The wiring that adds the 4th switch setting requires that the neck pickup cover be disconnected from the pickup's ground wire, so that a new dedicated ground wire can be added that connects only the neck pickup cover and not to the pickup itself. This provides a dedicated path from the pickup cover to pot case star ground. The original ground wire for the neck pickup then goes to the switch as shown in the wiring diagram. In this model the neck and bridge pickups' hot leads are switched for normal parallel pickup selection, and the neck pickup's ground lead is switched to provide series wiring.

The guitar as it came from the factory does not have a ground wire soldered directly to the metal control cover plate and it does not hum like this. (I think most Tele's don't have a solder point on the control plate.) Before the mod my guitar didn't have a hum/buzz problem when I touched the cover plate, presumably because the original Cortec pickup switch that I removed had a metal box body with a ground post on it that connected directly to the pot star ground point.

Interestingly, the new Fender 4-way switch does not have a ground terminal on it's frame, so the grounding of the new switch has to be made through the screws, to the cover plate, and then via the bodies of the pots to the pot start ground. The screws and nuts are all nice and tight.

Is this a common problem with the Fender 4-way wiring? It doesn't have a dedicated ground connection from the frame of the switch to the pot star ground. I think that if this were a common problem I would have heard about it, but I'm not seeing the problem being reported online.

As to why I'm getting buzz when touch the control cover plate -- it would seem that the plate doesn't seem to be all that well grounded with the new wiring paradigm -- apparently my body is acting like an antenna when I touch only the cover plate. If I keep my right hand in contact with the cover plate I'll get buzz, but I get grounded out and the buzz goes away as soon as I touch the strings with my other hand because the bridge is well grounded with it's own ground wire.

Right now I'm liking the series connection that the mod offers, but I'm definitely not liking the buzz that happens if I touch the cover plate without keeping my other hand on the strings. Has anyone else experienced this problem? I have to admit, I'm not at all happy about what seems to be the only solution -- grinding down on the cover plate to rough it up enough to add a ground wire going to the pot star ground. I would have thought the connection of the cover plate through the pot bodies would have been good enough.

Am I missing something?

[Steve A.]

I've put 4-way tele switches on several guitars without any problems like that. Thoughts...
#1. If you temporarily reconnect the neck pickup case to the terminal you disconnected it from does that change anything? Not that it should... (Obviously you would avoid the series linkage when you were trying this.)
#2. On Fender and Squire teles ground wires are soldered to the backs of the two pots which then connect to the metal plate through the metal pot bushings and the frame of the selector switch is grounded to the metal plate through the 2 screws.
#3. On the original non-commercial GuitarNuts site back in ~1997 there was an article about keeping the signal returns from the pickups isolated from the grounds until they were joined together at a single point. Until I read that I had been treating the signal returns and grounds interchangeably on my custom wiring harnesses figuring that the less wire used the quieter the guitar would be. Wrong.
After reading the article and implementing some of the ideas my guitars usually got quieter. My theory was that the various grounds pick up crap from what I've always called "the ether" and by joining them all together some of that crap gets cancelled out. By connecting the signal returns to the grounds in a single point you might end up with less crap from the "ether" on the output signal from your guitar. (This is not exactly the same as star grounds on amps which have to with current flow on the chassis.)
#4. Many of my Asian guitars come with conductive paint inside the control cavity and quite often the pickup cavities as well which my US guitars usually don't. I'll go through my guitars and do everything on my OCD list to minimize the noise like shielding the cavities and sometimes using shielded cable on Fender-style single coil pickups (one trick is to wrap an insulated wire around the individual leads and connecting that wire to ground. IMO tedious but effective.)

Good luck exorcizing those darned gremlins!

Steve A.

P.S. I usually wire in a phase switch on a push-pull tone pot since I love the OOP series linkage. On guitars with only a master volume control the parallel OOP linkage can be too thin. On a guitar with individual volume controls you can reduce that thinness by turning down one of the controls a little bit. I finally figured out that you can get similar results on a Tele by adding in a resistor or trim pot to ground from the normally grounded pickup lead on the DPDT switch. (The resistor or trim pot is out of the circuit when the pickups are in phase.)

This idea can also be used on guitars with two volume controls but you will need to decide which pickup you want to attenuate and wire that one to the phase switch. By doing this you can instantly switch to a decent sounding OOP linkage with a single "click."

P.P.S. You can get a better ground on the control plate by wrapping a short length of small gauge stranded wire around one of the screws securing the switch to the plate and soldering it to the back of the pot. (With small gauge stranded wire squished between the switch body and the control plate the switch won't be cockeyed as it would be if you used, say, 18GA solid wire.)

P.P.P.S. I bet that there is not full continuity between the backs of the pots and the bushings because normally that extra wire would not be needed (I only add them if I need to ground a terminal on the switch.)

[bob p]

Thanks for your help, Steve.

It does seem that the noise is the worst when the ground from the neck pickup is lifted -- that is to say, when the bridge pickup is selected, or when the pickups are on in parallel.

I think the idea of adding a dedicated ground wire from the switchplate and/or switch to the grounding point on the back of the pot is a good idea. The original Cortec switch had a dedicated solder tab for ground on the switch case, but the Fender switch doesn't. I thought about that when I was wiring things up, but I ASSumed that everything would be fine because the pots made good contact with the cover plate through their bushings. At the time I didn't consider taking resistance measurements from their bushings to the shells. If there isn't continuity there then that could be helping the buzz. I'm going to try adding dedicated ground wire from the cover plate to the common ground point on the pot case. I like your idea of sandwiching a wire between the cover plate/switch at the screw. that should kill two birds with one stone.

[Mick Bailey]

I've wired quite a few of these switches and never had a problem with hum as a result. When checking hum problems that occur or disappear when touching something I meter everything out for continuity and then use a clip lead from the plug body to various points to see what needs grounding.

[rjb]

Maybe not very scientific, but this ROT works for me:
Don't depend on mere physical contact at the metal control plate to provide electrical continuity. Run a wire.

In this situation, that would mean:

> Add a ground wire between the volume & tone pot cases.
(If you're concerned with star grounding, replace the wire from jack-ring to tone pot case with one from jack-ring to volume pot case.)

Assumptions: Control and bridge cavities are shielded. Bridge plate, bridge pickup base plate, and bridge cavity shield are wired to control cavity shield.

Then:
> Add a wire from the control cavity shield to the volume pot case.
Note: The stock wiring in my Squier mini-Strat (which I converted into a 5-string mando) includes a ground wire between the shield-painted cavity and the volume pot, which is mounted on an aluminum foil-backed pickguard. I don't remember the details, but they did it somehow.

Bonus:
Here's something I've done with 4-conductor shielded pickups. It allows you to terminate the cable shields near the leads:
Mount a solder tab under the "left" end of the pickup selector switch frame; insert a washer under the "right" end, so the frame sits level.
Run a ground wire from the volume pot case to the tab, then terminate pickup shields at that point.

P.S. I usually wire in a phase switch on a push-pull tone pot since I love the OOP series linkage. On guitars with only a master volume control the parallel OOP linkage can be too thin. On a guitar with individual volume controls you can reduce that thinness by turning down one of the controls a little bit. I finally figured out that you can get similar results on a Tele by adding in a resistor or trim pot to ground from the normally grounded pickup lead on the DPDT switch. (The resistor or trim pot is out of the circuit when the pickups are in phase.)

Another way is to use a variation of Bill Lawrence's "Half Out of Phase" circuit. Replace one of the "criss-cross" wires with a ~10nF capacitor. The cap blocks bass content in one pickup, so it doesn't cancel bass in the other. I did get occasional "pops" with this circuit, but silenced them with a ~10M resistor. [Unfortunately, I've misplaced the schematic, don't remember if it was a bleed (shunt) or pullup (series) configuration, and am too burned out from home construction work to think about it right now.]

-rb

[bob p]

I've wired quite a few of these switches and never had a problem with hum as a result. When checking hum problems that occur or disappear when touching something I meter everything out for continuity and then use a clip lead from the plug body to various points to see what needs grounding.

I was foolhardy enough to assume that the bolted connections would have good continuity. They didn't. When I metered the cover plate to the back of the volume pot the reading was OL. Same for switch body to volume pot. I soldered a jumper wire from the plate/switch/screw to the back of the volume pot. I'm wondering if the cover plate might have some sort of coating that's poorly conductive.

[rjb]

Don't depend on mere physical contact at the metal control plate to provide electrical continuity. Run a wire.

I was foolhardy enough to assume that the bolted connections would have good continuity. They didn't. When I metered the cover plate to the back of the volume pot the reading was OL

Woo hoo, I win!

[Steve A.]

I'm wondering if the cover plate might have some sort of coating that's poorly conductive.

Kinda late to try this now but scratching up the surface on the control plate would have helped with that. I'm thinking more that the cheap pots used might not have continuity between the back of the pot and the threaded bushing.

Those terminals that rjb mentioned are really cool - you mount them under a screw and can solder a wire to them. I forget what they are called but I used to pick them up in small boxes of 10 before the local electronics parts house went out of business 15 or 20 years ago.

Steve A.

[Steve A.]

Another way is to use a variation of Bill Lawrence's "Half Out of Phase" circuit. Replace one of the "criss-cross" wires with a ~10nF capacitor. The cap blocks bass content in one pickup, so it doesn't cancel bass in the other. I did get occasional "pops" with this circuit, but silenced them with a ~10M resistor. [Unfortunately, I've misplaced the schematic, don't remember if it was a bleed (shunt) or pullup (series) configuration, and am too burned out from home construction work to think about it right now.]

-rb

Here is the Mod Garage article on that mod...

https://www.premierguitar.com/articl...caster-circuit

The Jerry Donahue custom Tele has a 5 way super switch — one position is half out of phase...

Jerry Donahue Telecaster Wiring - Red Herring Tone Bones

BTW they now make 4P5T strat style super switches with the terminals facing inward ... I had to rout out the control cavity on one of my teles 15 or 20 years ago to accommodate the original super switch with the terminals facing out to facilitate easy wiring on the very roomy strat pickguard (but a royal pain in the butt for teles!)

Steve A.