ikay

Useful tip, but won't a hairdryer stir up a lot of dust and swap one problem for another?

This (old) TB thread mentions that British Audio Services (Nashville) sell a CD which includes the AH600SMX schematic for $15. May be worth contacting them if you can't find it elsewhere. https://www.talkbass.com/threads/need-schematic-for-trace-elliot-ah600-smx.389111/ Or try a UK Trace repair centre such as https://www.proaudioservice.co.uk/bass-amplifier-repair/

If the bass is a P with an added J then generally the P is in the exact same place. If the bass is a J with a P neck pickup then it's generally a little bit north of where it should be. The other common positional variation with P/Js is having the J pickup in the 60s or 70s position. I agree with Misdee about Yamaha BBs being among the best sounding P/Js. Particularly models with a hot blade J bridge pickup like the BB1024 etc. A regular J pickup just sounds weak when matched with a P and isn't man enough for the job IME.

It depends how the P and J pups are wired. If it has a single Vol and Tone and a 3-way pickup selector switch (P, P+J. J) then, when in the P position it will be exactly the same as a P bass. On the other hand, if the pickups are wired Vol/Vol/Tone (or Vol/Blend/Tone) then the resistance of the additional pot changes the pickup loading and the solo'd P will sound slightly darker. It's not hugely noticeable, but there is a subtle tonal difference. If you want to retain the original tonal range of the P pickup then wire it with a selector switch.

Fender vintage saddle screws are generally UNC 6-32 x 1/2" (or length to suit)

I'm glad I wasn't there lol!

If no one here can help then there's a long and active thread on TB about Ampeg scroll basses - https://www.talkbass.com/threads/love-of-ampeg-and-other-scroll-basses-part-2.1044767/page-68. Lots of AEB-1 owners on there who may be able to help. Visual comparison with P-bass below might be useful as a guide.

Have you tried swapping the hot/cold wires on the Jazz pup? That's an easy way of checking for a phasing issue. Or try reversing the phase of one pup channel in Logic as in vid below

Ah, how annoying, wonder why they did that!

Excellent!

The Mustang, Bronco and Jaguar SS are all 30" scale so it is possible the JSS neck will fit. The pocket and mounting hole positions look very similar.

The headstock reshape was a brave attempt lol! The SD single coil with the bigger pole pieces is probably quite similar to my Sentell pup. There's something about the sound of a bass with one single-coil that is so open and natural it just feels 'right' to me.

I've spent a lot over the years in my quest for 'the special one' and have owned exotic and beautiful instruments from Zon, Rick Turner and Rob Allen as well as many other expensive-ish basses. More recently I've been playing around with budget basses and have been very pleasantly surprised! My latest one is this Squier Bronco which was professionally modded by the previous owner with a Hipshot bridge, Hipshot tuners, new pots/harness and a Sentell 'Big Sis' handwound pickup (http://www.sentellpickups.net/bass.html). An ebay special for the princely sum of £150. A bit on the steep side for a used Bronco maybe, but it's a little gem! It feels and plays well above it's price point and the Sentell pickup is gloriously rich and sensitive to playing touch. I love it! Anyone else out there with something rather 'special' at the budget end of the GAS richter scale? Post pics if you can!

Ah, sorry I didn't realise the yellow wire joined the two diagonal lugs. Looks like that's just a jumpwire which hardwires the two coils in series then. Which means the two connecting points you need are as in pic below. Give that a try that and see if it works.

This is only an educated guess but looks to me like the linked brass plates are there to ground the pole pieces and the two lugs at either end are hot and cold for each coil with the yellow wire going to the hot terminal as in pic below. This gives you a four-wire hook up with options for wiring the coils in series or parallel. I stand to be corrected!

Not tried it myself but here's a short thread on the subject - http://www.luthiertalk.com/threads/neck-re-profile-to-asymmetrical.360/

From 12th fret to centreline of bridge pickup on my '72 J is 377mm. To front edge of casing is 368mm. This is the early '70s position which is closer to the bridge than standard 60s position.

I had one of those too, took me ages to work out what it was!