Disconnected internal Preamp Wiring

[mcbain]

Hi guys, my first post so I’ll try my best to make it clear. I’m trying to fix the wiring on a Monterey LP which was brought to me. Guitar did work previously and I think it has what looks like an internal preamp with the 9v wires now disconnected. Preamp has no number or label of any kind on it. The wires don’t appear to have been connected to the preamp but to the jack which also has the main wire from volume pot disconnected.

So I’m not sure where to connect these 3 wires… the positive and neg from the 9v and the main signal from the volume. Every diagram I can find online has negative from battery to ring and positive straight to preamp. Normally I’d connect negative from 9v to ring and positive from 9v to preamp but there’s no available space on preamp where it would have been. I’m thinking positive from 9v and signal from volume pot to tip and negative from 9v to ring but Im definitely not sure and don’t want to damage anything. I’ll attach a few photos and I drew the best diagram I could…. Any help appreciated - Ben.

[newey]

mcbain-

Hello and Welcome to G-Nutz2!

I have no idea what a "Monterey LP" is. Is that the brand name? I assume you've looked at the web for more info and come up with zero. Without knowing the layout of the preamp, it may be tough to deduce things accurately.

You are correct as to how these things are typically wired. 9V negative probably goes to the ring, that's to disconnect the battery when no cable is plugged in. As for the 9V positive, I have no idea.

DO you know any of the history here?. If both the 9V wires were previously disconnected, that might suggest that the preamp went belly up at some point and someone disconnected it to use the guitar passively. In that case, the V pot + presumably would have been wired to the jack tip. Perhaps that connection then came loose, rendering the guitar silent.

Passive operation presumes that these are regular passive pickups with a preamp. But they could instead be low-impedance pups that require a preamp to operate. Checking the DC resistance of one of the pickups (has to be disconnected to test) with a multimeter would tell that tale one way or the other. Or, you might try connecting the V pot + to the jack tip and see if the guitar then works, meaning they are passive pickups.

I'm also mystified by your diagram of the pot wiring. I don't see any tone capacitors, you show all 4 wired as if they were 4 volume pots, which makes no sense. Are there tone caps on 2 of the 4, as I would expect?

[mcbain]

Hi, thanks for replying. Monterey is indeed the brand and it is a Les Paul copy. None of the 4 pots have capacitors on them so I’m assuming the preamp is adjusting the tone somehow?

[mcbain]

And Monterey is an extremely cheap guitar brand sold locally here….

[newey]

I suppose the tone caps could be internal in the preamp, but it is still odd that you show 3 of the 4 as wired with all 3 lugs (potentiometer-style) rather than with just 2 lugs wired (rheostat-style) as most tone controls are, and as the one with the disconnected wire seems to be. And, if the 4 pots are 2 volumes and 2 tones, then why do 3 of the 4 attach to the preamp while the fourth one (with the disconnected wire) is apparently wired differently?

If the company is still in business, perhaps an email to them would get you a wiring diagram?

[mcbain]

I think I’ve found something for sale online which is sort of similar and it is a 3 band eq. So 3 of the pots are for eq and the other pot is for volume? This would explain the 3 pots being wired differently to the volume pot…. I’ll try the volume pot wire direct to the tip when I get home tonight. I’m not having much luck contacting the manufacturer….

[Yogi B]

Aug 24, 2021 7:49:55 GMT -5 newey said:

I suppose the tone caps could be internal in the preamp, but it is still odd that you show 3 of the 4 as wired with all 3 lugs (potentiometer-style) rather than with just 2 lugs wired (rheostat-style) as most tone controls are, and as the one with the disconnected wire seems to be.

It really depends on the circuit, e.g. a typical amp tonestack doesn't only use pots as rheostats. Since the preamp is an epoxied literal black box, always connecting all three lugs of each pot separately (even if unnecessarily) also enables the preamp design to be entirely redone without changes to the external wiring.

And, if the 4 pots are 2 volumes and 2 tones, then why do 3 of the 4 attach to the preamp while the fourth one (with the disconnected wire) is apparently wired differently?

They're also the wrong 'side' of the 3-way toggle switch for the typical 2V-2T configuration so I'd have to say that the three 'other' pots control a 3-band EQ. The lack of external capacitors isn't too surprising: they'll just be on-board, gooped up with the rest of the circuit.

Googling hasn't found an exact match, but did turn up fairly similar looking bass preamps e.g. this listed on AliExpress. Though despite claims of being 3-band, in actuality they are only 2 band plus a volume pot & a pickup blend pot. From those photographs there's two things of interest:

• The red power lead from the preamp is joined with the positive lead of the battery snap and taped off.
• Although two separate black wires are soldered to the output jack (presumably the ring & sleeve terminals) they both appear to be soldered to the casing of the volume pot, thereby bypassing the trick of using a stereo jack socket for power switching.

If similar was true of the original installation in the guitar you have, the second of the above points could explain why someone previously tried to rewire it (the guitar would've been draining power even when unplugged).

---

Therefore my semi-educated guess would be to follow the conventional wiring, with which you're already familiar, specifically:

• desolder the red wire from the preamp to the jack & instead join it to the positive lead from the 9V;
• solder the loose red wire from the volume to the jack's tip
• solder the remaining negative 9V lead to the remaining ring terminal on the jack.

[mcbain]

Ok, so I quickly tried the conventional wiring as suggested. I just did a quick hookup to check it worked which it did! I got signal through my amp when plugged in, only briefly though which was to be expected as I haven’t soldered yet. So I guess it had been wired up incorrectly previously with the battery being drained all the time, even when not plugged in.

So I’ll solder it all up tonight and report back but it’s looking good….. Ben.

[mcbain]

Update: I soldered everything together, conventional hookup as mentioned in previous posts. Unfortunately, I get some very brief, very weak sound through pickups and if I play a string hard but it fizzles out almost instantaneously, doesn’t sound right. Battery is fine, I checked continuity between joins and connections I made and they are all fine. So I can only deduce that it’s still hooked up incorrectly. Volume pot is working to some degree, but signal from strings though either pickup and to output is fizzling out for some reason… Aargh!

So just to confirm - I connected positive from 9v to top red lead on preamp in drawing, negative from 9v to ring on jack, volume pot middle lug to jack tip, ground on volume pot to sleeve on jack…. Any suggestions? Thanks, Ben.

[mcbain]

Final update - I gave up and removed the wiring and installed a single master volume and master tone pot and a new jack. Working! So I guess either something was wrong with the internals of the active EQ or the wiring was incorrect but I tried several different ways and none worked well….

[newey]

Aug 31, 2021 7:07:50 GMT -5 mcbain said:

So I guess either something was wrong with the internals of the active EQ

That was my guess, just based on the fact that it was disconnected when you got it.