Need help understanding wiring diagram

I've built probably something like 10 kits but never taken the time to try and understand the wiring diagram. By that stage, I'm usually just keen to get it hooked up and making noise. But this time I think I need to understand this diagram so I can understand the implications of my upgrade choices and what more I might need to do.

I'm building an AGM-3F which has 3 humbuckers, 3 volume pots, 1 tone pot and a 3 way switch. Wiring diagram is attached. I originally ordered the AG-1F which is a 2 humbucker model and upgraded the pups to toneriders. However, there was an issue with stock and I was offered the AGM-3F as a free upgrade. I didn't bother with upgrading the third pickup - figured I'd just not use the middle for now and get around to an upgrade later on if I felt like it was needed and when I had a better idea of what I might want.

However, if I'm reading this wiring diagram correctly (which there's a really good chance I'm not), the master tone and jack hot wire come off the middle pickup which leads me to believe that the middle pickup is in play in all three switch positions - 1) Neck/Middle, 2) Middle Only, 3) Middle/Bridge. So, if that's the case, with that wiring the only way I can take the middle (ie muddy standard kit model) pickup out of play is to turn the middle volume down to zero which means I can then only have neck or bridge and not both.

If that is the case, I see a few options but I'm not really sure how I'd achieve some of these:
1) Upgrade the middle pickup as I have the other two (presumably another bridge pickup but preferably something different).
2) Replace the centre volume pot with another tone pot and wire it up as a 2 humbucker, 2 volume, 2 tone, 3 way switch and just use the middle pickup as filler but not actually wire it up. This is not as stupid as it sounds since I was only after a dual humbucker in the first place.
3) Same as option 2 but use the middle cavity as a battery holder and drop in some active pickups.
4) Same as option 2 but put some stupid bright led light in the middle cavity that shines in everyone's eyes when I play.

Anyway, I've digressed a bit. My main query is the bit about what pickups are used in each switch position in the attached diagram. If any of the plethora of experts here can help me out, would be much appreciated.

Attachment 40985

The middle pickup pot is always connected, but if you turn its volume control down, then you can use the switch to select B, B+N and N just like on a 2-pickup SG. Using the middle pickup’s volume control (as you now have three volumes and one master tone knob with this arrangement) you can also ‘blend in’ as much or as little of the middle pickup as you want.

If you look closely, you’ll find that the pot wiring is different to normal, and the input from the pickup now goes to the central tab and the output is connected to the normal input tab. This always keeps the full pot resistance between the output signal and ground when the pot is turned down, so you don’t get the output being cut off when one pickup is turned fully down like you would with a standard 2-pickup Gibson wiring scheme. This affects the way the volume control reacts a bit, but it is a small price to pay for being able to turn down a pickup fully and still have output.

What I’d do if this was mine would be to get 3 x 1meg audio taper pots for the volume controls and a 500k log pot for the tone control (check, but I’m sure you’ll get 4 x 500k pots with the kit, and probably two are log and two linear). Having that middle pickup always connected to the output means that you’ve got a 500k resistor from signal to ground, effectively halving the pot resistance and knocking off a reasonable amount of treble and taking the sparkle away, even when you have selected just the neck or bridge pickup.

So using three 1 meg pots will give you an effective 500k resistance with a single pickup selected. As per normal, you will get a lower overall resistance when in the middle switch position, but as normal on a LP or SG, this is compensated for by the combined signal being a lot brighter to start with.

The kit knobs will be for 18-spline shafts, so if you get new pots, I’d get Alpha pots (or Bourns or CTS pots that definitely have 18-spline shafts as they're normally 24-spline as standard). Or go for four CTS pots (tone pot stays 500k) and get some 24-spline knobs of a different style (though sets with writing on rarely come in 3 volume and 1 tone arrangements).

You do have the option of wiring the middle pickup as reverse polarity to the neck and bridge, as this is what Gibson did for a more interesting range of sounds on their 3-pickup LPs and SGs. In fact, there’s a 50/50 chance this will happen anyway if you mix the kit pickup with the Toneriders, as despite manufacturers showing north and south start and end bobbin wires on their diagrams, it seems they can’t agree on what north and south are, and the output signal polarity can end up reversed when you use pickups from different makers. This is fine if you have 4-wire pickups as you just swap the signal ground and hot signal wires over (but the shield always goes to ground). But if you have single wire plus screen pickups from different sources that are opposite polarity, you are either stuck with that, or you have to take one pickup apart and flip its magnet.

If you’ve gone for Alnico Toneriders, then the ceramic magnet kit pickup will almost certainly be louder, so an upgrade to a matching Tonerider will give a better balance to the sound. I’d go with another neck pickup, as another bridge pickup will still result in the middle being the loudest. Make sure you fit a neck position kit pickup in the middle for this reason).

If the above doesn't appeal, you may just want to fit the middle pickup but not wire it in, and have a standard 2 x vol 2 x tone arrangement. If so, I’d set the middle pickup low to minimise its magnetic drag on the strings for better sustain.

Thanks Simon. I'm glad it's that simple. :-p

The Toneriders I got are the AC4. I could grab another AC4 or maybe even an AC2 or AC5 for something a bit different. I've really got no idea, but I kinda like the idea of experimenting a bit rather than playing it safe. I don't play well enough to be too worried about it sounding bad - I think the weakest link is my playing.

As for the pots, I hadn't even looked at the kit pots until now. Surprisingly, the pots, switch and jack are all pre-wired which I've never had outside of strat kit. Wasn't expecting that. However, it's wired for a two pickup, two volume and two tone as far as I can tell. Although the A pots have the caps, so it looks like they've got them arse about. I wasn't intending on using the kit pots anyway as Adam sent me a different set of pots that they were looking at using in kits and needed trialling. However, looking at those pots, I'm not sure if they're all the same or not. All the numbers and markings appear to be the same on all four. Might need to test them with a multimeter. At this stage, might just invest in all new decent pots, switch and jack to be safe and go with the first option, and get a replacement middle pickup. I like the sound of the reverse polarity wiring. Never thought of that.

Thanks for all your help and advice, Simon. Can always rely on you to provide comprehensive information. At least I'll now be going into it with my eyes partially open.