OK, I bit the bullet and did it; I ripped out the harness from my beloved 2016 '64 ES-345 VOS (Mono) and modified it.
So I've always loved the Varitone, but the treble roll off when turning down the volumes always got to me as I'm used to the 'Vintage' wiring approach on my Les Pauls and use the volumes a lot. Also the guitar was really dark sounding even with everything open.
So, job 1, carefully remove the harness. Easier said than done and it took hours.
Job 2, carefully trace out the circuit so I understood exactly what's here. What I found is in the first attachment. This circuit is almost exactly like the (difficult to read) Gibson stereo diagram except the outputs of each pick up are connected together at the mono jack. So, two Varitone's ganged together, in parallel with the tone pots, 'before' (connected to the top lug of) the volume.
Yes the tone pots are wired opposite to each other but in passive electronics terms this makes no difference (a capacitor and resistor in series works the same which ever way 'round they are).
I drew this schematic as I found Gibson's really hard to interpret, particularly how the Varitone is wired. This was important to understand as I was going to rip it apart and put it back together again.
The second picture is the Varitone and its' two networks. These were mounted as two blue Surface Mount 'ICs' on the switch.
The way the Varitone switch works is as follows:
Although in a true passive electronics sense, it’s not really correct to refer to ‘inputs’ and ‘outputs’ on a simple filter circuit like this one, it does help illustrate how the circuit is wired.
So, for the IC (NOT the switch), consider Pin 1 to be the input (from the pickups), and Pin 2 to be the output (towards your amplifier). All the other switch connections are there simply to bring different components into and out of the signal path.
In switch position 1 (Bypass), IC pins 1 & 2 are shorted so the signal ‘sees’ the IC as a 2MΩ load (10MΩ/5 parallel resistors) which is so much bigger than the alternative paths via the volume control and amplifier input impedance that it has no effect in theory (bypass mode).
In position 2, pins 2 & 3 are shorted, so the signal ‘sees’ a notch filter consisting of a 100kΩ resistor, a 0.001µF capacitor and a 15H inductor (not shown here, connected to IC pin 5);
in position 3, IC pins 2 and 4 are shorted, so the signal ‘sees’ a notch filter consisting of a 100kΩ resistor, a 0.033µF capacitor and a 15H inductor (not shown here, connected to IC pin 5);
in position 4, IC pins 2 and 6 are shorted, etc;
in position 5, IC pins 2 and 7 are shorted, etc;
in position 6, IC pins 2 and 8 are shorted, etc.
Hope that helps.
I heard from a guitar tech that the IC's are of variable quality so I built my own single network out of discrete MIL spec components.
Next step was to decide on a new wiring solution. There are a number of options here; I could have moved the volumes to before the tone's and kept the rest in tact, but Gibson's own Mono circuit uses 'Modern' wiring with a single Varitone after the pickup selector switch. I chose to wire it as a 'Vintage' circuit, and then add a single Varitone after the pickup selector switch. The last diagram shows the circuit I used.
The results are simply astounding. The guitar rings like a bell, it's resonant with clear highs and a huge bottom end. The volumes now work like my Les Pauls but best of all the Varitone still sounds the same.