Modifying Ibanez HSH/VT/5-way's tone control

[Prostheta]

Objective: Modify a tone control so that it only affects the neck pickup rather than the entire circuit.

Since I'm currently stuck in the position of not being able to advance any major projects, I figured that I'd look at simpler ones I've had on my mind for a while. Anybody that's chatted with me for any length of time will know how much I absolutely gush over my 2001 Ibanez S1540FM.

I could take or leave that single coil and positions 2-4 of the 5-way, however ripping it out and putting a 3-way switch in is something I'm not prepared to consider right now. I'd rather wait until I can build my ideal HH Saber with a wraparound sometime in the future.

The sound I get from dialling down the tone on the neck SD '59 pickup is sublime. Tubular, smooth and really carries woodiness rather than a dulled empty tone. Leaving the tone control dialled in to what I like for the neck so I can switch in from (say) the bridge straight to my neck tone would be sweet.

 

Circuit: Under the hood we have a pretty standard Ibanez circuit for an HSH/VT/5-way circuit.

 

Proposal: The alteration as I see it is a simple one; moving the wire connecting the signal tap of the tone control on the volume pot and attaching it directly to the "hot" (north coil start) red wire of the neck pickup. Since my neck pickup isn't a DiMarzio any more, the corresponding wire colour for an SD is black rather than red. I'll check this out in the instrument itself for the second post.

 

Potential Problem(s): On a simple level this should work nicely, however it may affect position 4 unless the tone is brought "out of circuit" from the north coil since only the middle and single coils are switched in. I think it'll be quicker to just try it than to think it through. My brain is crap anyway.

 

So, let's see what we actually have under the hood. Quite likely, @curtisa will call me an overthinker or tell me exactly where I'm going wrong. Unless what I'm proposing is correct, which I would expect is unlikely. 

[Prostheta]

So let's see what we have here.

Firstly, let's check out the tone control. It gets pretty busy with all the grounding wires, the capacitor tucked between the case and the tags plus the shielded conductor cables.

 

This is what we're interested in though. The shielded single conductor wire that ties the tone pot with the volume pot. As per the diagram above, the central (white) wire is soldered to the centre tag and the shield to the case. All good here, and if my idea holds weight then we don't need to do anything here (other than maybe consider a less cheesy cap?).

[Prostheta]

This one is far harder to discern, but corresponds to the diagram also. The shield connects to the right-hand tag of the volume pot and the white inner wire to the opposite tag. These will need desoldering for re-placement.

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Huh. A JEM VLX91 switch rather than the 8-contact imports you normally see, and as illustrated in the diagram above. Okay. Not the end of the world.

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Now this is interesting. It seems that when the previous owner (or their tech) installed Seymour Duncans in place of the original DiMarzios, the wiring schema was altered. Here's the neck pickup cable coming in:

 

....the white and red wires are soldered together, likely making the neck pickup a full-time humbucker. I'm not yet familiar with the VLX91 switch and it could still be "shorting" out the neck pickup's north coil to achieve position 4's neck south/single coil combo. This might complicate my proposed simple solution to the tone control placement. Hmm. Do I really want to be bashing my brain in trying to figure out how the VLX91 switch works on a Sunday.....hmmmmmmmm

[Prostheta]

Huh. Doesn't look too oppressive.

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Okay, so it seems that what is being done here is that one of the four sections of the switch is doing what I suspected it to and that's the "cheating" way of turning the bridge and neck humbuckers into single coils in positions 2 and 4. That is, grounding the pole and bonding the red/white series link of the humbucker (black and white for DiMarzio coding) to positions 2 and 4. Whilst effective in its current form, I'm not sure how this might affect my proposed way of attaching the tone to the neck pickup.

[Prostheta]

This reminds me of how deeply I dislike HSH setups. I'm half-tempted to rip out the single as an act of revenge.

[Prostheta]

Okay, so I de-soldered the white wire where it met the volume pot and brought this over to the switch wafer where the black (north start) wire is soldered. I didn't like how this wire was soldered in, because it looked lazy to me. Positions 4 and 5 were bridged with a solder blob with the wire jumping the two. This can be better.

Each of the four pole and ways on the VLX91 switch have two soldering points on each pad, so I de-soldered the black wire with some fluxed copper braid and cleaned the pads of excess solder, crafted a small solid wire "staple" jumper and soldered this over positions 4 and 5 in the outer points. The black and white wires were then soldered into the remaining two holes. Far neater.

In test this works what I *think* is correctly. It's difficult to tell, so I'm going to draw out the switching of the circuit in positions 1 and 5. I believe that it is electronically-valid without any additional load or other weirdnesses having been introduced.