ikay

How it affects the solo'd P sound depends  on how it's wired. If a P/J is wired as a regular VVT then the additional vol pot will change the load on the P pickup which does affect the tone. In the same way that swapping 250k pots for 500k makes the sound brighter, adding an extra pot (in parallel) reduces the load and makes the sound a little less bright. Soloing the P pickup in a VVT setup will sound similar to running a standard P with the volume backed off a bit. A smidge less grunt. You might not notice it unless you really listen for it but it does make a difference.

This TB thread is about a similar problem - https://www.talkbass.com/threads/peavey-millennium-bxp-ac-preamp-cuts-out-when-treble-control-turned.1098452/

What make of bass is yours? As a first step it might be worth giving the iffy pot a squirt of contact cleaner to see if that makes any difference.

It would also be possible to add a switch to completely remove the added J pickup (and vol pot if it has one) from the circuit allowing you to switch between a P/J config or the original unmolested P config.

This article on fixing a B2A with dodgy wiring contains a wiring diagram which might be useful - https://richsstudioproject.wpcomstaging.com/2015/08/19/hohner-b2a-active-electronics-repair/

Crikey, that's a bit of jumble!

Have you tried tweaking the trim pots for the mid control in the control cavity? You can adjust the mid freq and the level of cut/boost if you're not happy with the factory settings. Might be worth having a play with that before replacing the preamp.

If it doesn't work in active mode, even after changing the battery, then just switch it to passive and use it in passive mode. To connect it to a software package like GarageBand you will need an audio interface like a Focusrite Scarlett or something similar - https://www.amazon.co.uk/Focusrite-Scarlett-Solo-Audio-Interface/dp/B07QR6Z1JB

Here's the Ric wiring. I've tweaked it to take the switch out of circuit. The 4.7nf cap in the bridge pickup signal path cuts some bass which gives the characteristic Ric clank. Just remove it if you don't want that. Again you might want to change the pots to 500k for your Spector humbuckers. This circuit uses a stereo jack but you could use mono and just connect both hot wires to tip. It has no isolating resistors so there will most likely be some interaction between the neck and bridge pickup controls. Standard Ric wiring though so I guess it works!

1 hour ago, prowla said:

What are the series 220k resistors for?

I believe they're there to stop the tone controls interacting with each other. If you remove them, the hot wire from each pickup has a clear path to both tone pots. A side effect is that they also attenuate the signal somewhat which isn't ideal. I'll see if I can dig up a schematic for a Ric as an alternative.

Jazz stack-knob VTVT wiring is essentially the same, just use 4 separate pots rather than 2 stacked. If the Spector has humbuckers you might want to make the vol pots 500k instead of 250k. See diagram below. Tone is on top, vol underneath. 

Depends whether the piezo element is bonded into the saddle or just a push fit. I have a Hipshot piezo bridge and the elements just lift out (pic below). If the hole where the wire comes out is large enough, I'd try giving it a gentle prod with a stiff bit of wire like a paper clip.

The TC Electronic RH750 head has a built in tuner but no aux power supply.

https://www.tcelectronic.com/Categories/Tcelectronic/Bass/Head-Amplifiers/RH750/p/P0CIV#googtrans(en|en)

Looking more closely at the El Torro active circuit (see both links below), I think, if you ignore the active/passive switch (S1) and the preamp, what remains is the wiring for the original passive version. The preamp appears to just be a signal boost to present a stronger and cleaner signal to the amp. The treble and bass controls are provided by a passive circuit. 

Active circuit schematic - https://www.bassesbyleo.com/images/el_toro/tech/el_toro_wiring_diagram_drawing.pdf

Active circuit description - https://www.bassesbyleo.com/images/el_toro/tech/el_toro_wiring_diagram_description.pdf

The passive wiring would then look like the schematic below. See the description link above for the function of S2 and S3. 

This passive L-series wiring diagram may provide some clues  - https://www.bassesbyleo.com/passive_l_series.html

The Stingray pup position actually overlaps a 60s J bridge pup to quite a degree. It might just about squeeze in with a 70s position J-style bridge pup without mounting lugs. Here's a comparison pic.

8 minutes ago, hooky_lowdown said:

I miss that big, fat, warm single coil tone. 😫

Exactly. Mine purrs like a contented cat - an original '57 (with Clive Brown refin). It just makes me smile 😊

These are great workhorse basses, the best value Stingray you can buy IMO. GLWTS

10 hours ago, franzbassist said:

Well you are most welcome to try mine when we get out of lockdown, assuming it hasn’t sold by then.

Oh and the Baers are front-ported, not sealed.

Ah yes front ported, don't know why I thought sealed. And thanks, if we ever do! 

3 hours ago, chris_b said:

Anyone can make a mistake! I'd put out feelers for another Berg 112. You know how good two of those cabs will sound.

Indeed I do!

3 hours ago, franzbassist said:

If it's any help I tried my Baer with a Markbass TRV112 and it sounded terrific.

Thanks, that's interesting. I guess odd pairings do work sometimes. And yes, it was your ML-112 that prompted the question!

Thanks for the replies and I'm glad I asked! That all makes perfect sense. I generally play small venues with backline only and am quite happy with my amp/cab combination (LMIII + Berg 112). Used to have two Berg 112s but rather foolishly sold one. That will teach me!

Specifically I'm considering pairing my Bergantino EX112ER (1x12 plus tweeter, front ported cab) with a Baer ML-112 (1x12 plus 6" mid driver, sealed cab). The idea being that the punch and upper mid detail of the Baer might complement the extended bottom and sweet top end of the Berg. I love the Berg on it's own but have also heard good things about the Baer and I'm wondering how well they'd work together. Or would their individually excellent sound qualities just cancel each other out and sound meeh? Is pairing up cabs from two different makers in any case something that's generally best avoided? Google didn't yield any useful insights so I'm hoping the combined experience of all you excellent and helpful BCers will! 😎 

I think the biggest challenge will be finding a way to do it without sanding the existing finish back, at least to some extent, particularly if there's a build up of polish on top of the old finish. Personally I think it looks great in your pic, that sort of naturally aged and well worn look is a beautiful thing!

Excellent, well done!

This may help - https://www.redstarguitars.com/how-to-decode-cts-pot-numbers/

EP885 would seem to indicate a CTS pot made for Allparts.

For the neck (J) pickup, the white wire (hot) and the black+shield wires (cold) should just go where the original J hot and cold wires went which should be easy enough to spot.

For the bridge (MM) pickup, the white and green (hot) wires should go to the switch where the original black and red wires went. It will work whichever way round they go (eg. white for black and green for red, or vice versa) but the order does determine which coil is tapped. Wire it up and if you're not happy with which coil is being tapped just swap the wires over. (Bartolini wiring code says red is neck coil and black is bridge coil)

The brown+yellow+shield wires go to the pot lug where the original white+green+shield wires went.