My first build: left-handed JM-1L

Best to post a photo of the pot wiring before doing anything. Looking down on the back of the pot, with the three tabs facing you, the grounded tab should be on the right, the output wire on the middle tab and the input wire from the pickup selector switch on the left-handed tab. 10 should give zero resistance between the input (left) and output (centre) tabs.

10 on the tone pot should give maximum resistance. When the knob is at 0, there is no resistance so the signal is directly connected to the tone cap for maximum treble roll-off. At 10, there's 500k resistance between the signal and the tone cap, so almost none of the signal passes to the tone cap, so you get maximum brightness.

This is how it would be oriented for a leftie, looking down on it, this is on zero with it at the top
https://i.imgur.com/xBunxE5.jpg
at zero like this the input to output shows zero resistance.
https://i.imgur.com/mjTYioJ.jpg
I suspect the tone pots are backwards too from your description.

Unless the pots are all anti-log pots, then they have wired them the wrong way round. If you can undo one of the pot nuts and have a look at the underside, it should say something like 500A or 500B printed on the small pit of board the tabs are connected to (A is for an audio/log pot, B is linear taper). To be an anti-log pot they should say C at the end e.g. 500C or 500k(C). But with anti-log pots, which work in the reverse direction, you'd also need knobs with the numbers running the other way, and those ones are standard. Reverse numbered knobs are rare beasts, so almost all lefties use standard pots and knobs.

So something is wrong. Using the normal pots, you don't swap the connections to them over just because the guitar is a leftie!

I should have said that linear(B) pots can happily be reverse wired to work in the opposite direction, but you are still left with the knobs numbered in the right-handed way, so 0 is full up etc.

Though you could always swap to non-numbered knobs (like the knurled Telecaster ones) if you got an anti-log pots for the volume control. Tone pots can be log or linear types, I prefer the way a log pot behaves on tone, though TBH I rarely touch the tone pot.

That makes sense. I'm pretty comfortable with soldering so I've swapped the wires no problems. The pot says B500k on the top.

There is a little bridge on the wiring diagram between two of the pickup selector soldering points that looks wrong to me. It looks like you would always be selecting the humbucker and middle pickup together. Which pickups should get the tone pot? Can someone check that the diagram is right for me?

To avoid any confusion, which diagram are you using?

The 5-way switch is really two sets of contacts in one package. On more expensive /traditional 5-way switches, you have the contacts laid out on either side of a central wafer. With these 'import' switches, all the contacts are laid out in a line with the two 'common' terminals in the middle. On some switches, as the commons are always connected for standard Strat-style applications, you just get 7 connections with the two commons pre-linked.

So one 'side' of the switch takes the inputs from the pickups and takes them to the common output depending on switch position. The other half of the switch takes the common output and passes them to the connections running off to the tone pots. Obviously only two tone pots (not quite sure what Leo was thinking when he did the Strat), and on a Strat you'd traditionally have the mid and neck pickups with a tone control, the bridge without. It's only a question of swapping the connected tabs over if you want a different arrangement, and you can have two pickups sharing the same tone control.

If it were mine, I'd wire one tone for the humbucker and one for the mid and neck pickups. What would you like?

It's the one labelled One Humbucker, 2 Single Coils to a Control Panel. It says suitable for JM-1 under it.
Your suggestion seems reasonable. How do i wire two pickups to a tone pot without shorting them together across the 5 way switch?

It’s easy. Remember that the other half of the switch is sending the output of the first half of the switch to the tone pots, so you’ve already got the pickup selection sorted with no shorting.

Will mark up a diagram when I’m back at my PC.

The standard JM-1 wiring diagram shows the switch linked for a common bridge and middle pickup tone control, with the neck on its own tone pot. This is a carry-over from a standard Strat modification. A standard Strat has no tone control for the bridge pickup, so using the mid tone control for both middle and bridge pickup allows you to knock some treble off the often quite strident bridge pickup on a Strat.

Attachment 41217

But with a bridge humbucker, you don't really want a common tone control for it and the middle pickup. By moving that connecting link as shown below, the humbucker has its own tone control whilst the two single coils share one. This is far more useful as with a 500k volume pot (normally 250k with all single coils), the two single coils can sound quite bright, so you may well want to have their tone control rolled back a bit.

Attachment 41218

Note that as drawn, the tone pot nearest the volume is the bridge tone, the one furthest away is the mid/neck. If you'd prefer it the other way round, then swap the connecting wires over either at the switch end, or at the pot end, (but not both :) ).

Oh I think I'm misunderstanding the switch. Do I wire the pickups to the three spare posts? It is a bit confusing that the existing wiring is not shown. The end result being that all 7 posts have something attached to them?