Bass Guitar Pre-Amp - Assistance required from local geniuses please!

[Ozyris]

Greeting fellow tinkerers!

So I have this bass guitar to fix. Someone tried before and was unable to to do it. l Between us I think we could make a darned good effort!

The preamp has been disconnected (I believe its a replacement that was not wired in properly). I have been unable to source a wiring diagram or even locate a supplier to ask for the connection schema. So its down to detective work.

First obstacle: The board itself is hidden within an impenetrable layer of opaque black epoxy. All we have to go on are the wires coming out of it and the understanding of what it is supposed to do.

It looks like this:



It is supplied by a 9V battery via a single wire. It receives a single pickup-level input signal. It connects to two tone controls (treble and bass), each with three wires. It connects to a single volume/gain control via two wires. It connects to ground via a single wire. It outputs to the jack via a single wire. Total of 12 wires. They may not be grouped correctly in the picture.

You may cheat and post the connecting schema if you happen to have one of these devices to hand. That would be good.

I have checked every combination of wires with a (cheap) voltmeter / ohmmeter and developed a tabulation of what I found. I shall post this up next as a table and a scribbled diagram.

Help, as previously will be deeply appreciated.

(bows in humble appreciation of the genuises/genii hanging around this forum)

[SteveNielsen]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

What's the make/model of the bass guitar?

edit: I'm not a genius but I know my way around electric guitars pretty well.

[Ozyris]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

Okay, so here is the table of measurements. As I suggested above, these are really very approximate. Where there is no number there is no measurable voltage or alternatively infinite resistance. To measure voltage I connected a 1.2V AA battery in series with my meter, then connected across pairs of wires.



I will post a scribble diagram of my interpretation of this table shortly.

Thanks!

[Ozyris]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

Hi Steve. Thanks for getting stuck in! Its a Crafter Cruzer four string (no snide remarks please = not mine!). No other identifying marks. I doubt this preamp is original.

[SteveNielsen]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

No snide remarks from me, I'm no guitar snob. What model number? There are lots of variations in pickup and switch/control wiring.

[Prop1]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

I repaired guitars and amps back in the '70s. Looking at your picture and what is sold on the internet; they all, but a very few, come prewired to the pots, output connector, and battery connector. You either have a really cheap one that is worthless, or someone removed the other parts. You would likely be money, and headache, ahead by buying better quality with all the parts and wiring instructions. They all use different wire colours, so without the actual manufacturer; you'll never find a drawing.

[Ozyris]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

Hello Prop1! Thank you for diving in. Yes indeed, it may well be not worth the trouble. But sometimes puzzles are fun. Check out my repair job on my Alesis M1 MK2 Active Monitors for a little insight into that.

I am afraid there is no further identification on the guitar.

Regarding the pots etc they are all there. I have good clean signal on the output of the blend pots. That's where the board will get its input signal from.

Here is my "scribble" layout of how the connections tie up. So each brown or grey line represents something that connects the wires together on the PCB. Brown for low resistance / higher voltage reading, grey for higher resistance / lower voltage reading.



I figure the "no readings" ones need something inside the board fired up or something connected to one or more of the other wires to have a signal or voltage.

Hmmm...

[Ozyris]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

...I am thinking that 10 might be the ground?

[Prop1]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

All blacks wires in this case will be grounds of some sort, but not necessarily to the battery for isolation purposes. Likely the 2 reds and the black grouped together went to the battery. I'm thinking the black, orange, yellow, and white are coming from the pickups. The rest would go to the pots and output connector. But that's just an educated guess.

[stj]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

one word - nitromors.

[SteveNielsen]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

Yeah, Prop1 is probably close but without knowing the guts of the circuit it's impossible to tell for sure. Not sure if it's worth trying to dissolve the potting for but I'm not a big fan of magnetic-coil pickup on-board preamps anyway. They raise the noise floor and cause distortion. Even cheap pickups usually have sufficient output for the preamp in a guitar amplifier. There are better ways to get an "overdriven" sound without actually overdriving the guitar amp input, IMO.

[Ozyris]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

Hello everyone and thanks for the input so far. Just had to dash off to Heathrow airport to fetch someone but back now.

Thinking in the car that my last diagram needs a tweak. Every one of the coloured junctions actually represents one of the wires coming out of the board. So here it is with that idea clarified.



It may not help anyone else's thinking but I think it helps mine.

It also occurred to me that it is probably only the hidden opamp that needs power, or am I wrong? As it is a dual stage preamp (treble and bass) there may be two power supplies to the opamp, but the division of power will be handled on the hidden pcb. Which might suggest that one of the "unconnected" wires is the power connection...

Regarding the idea of how the wires might be "grouped" (they are indeed grouped with shrink tubing as per the original picture), it is worth noting, I think, that the two tone pots have three wires each. One to each terminal and one for the sweep. As its an active tone preamp I suspect at least two of them will be connected to the opamps internally. There is a centre detent on each tone pot, which suggests cut below centre, neutral at centre and boost above centre for each one. Logical?

The blending is done external to the pcb. The signal is delivered to the board on one wire after the blend pot. As I mentioned,there is good strong clean signal at that point.

The volume pot may or may not get gain from the pcb. It has its own external ground in the normal way and only had two wires previously soldered to it. Both I think went to the pcb.

It may be a point to start at by assuming the existing grouping might have some value and go from there? It looks like this:



Hmmm...remember it could easily be wrong and what caused the other tech to give up...

[stj]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

given that you dont even know or have not said if it's single-rail or split-rail power input,
i think nitromors is the best option.

[Prop1]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

It may be a point to start at by assuming the existing grouping might have some value and go from there? It looks like this:



Hmmm...remember it could easily be wrong and what caused the other tech to give up...

1,2,3 are clearly power.
4,5 I suspect are output.
6,7,8 I think are tone related.
9,10,11,12 I think are inputs from the pickups.

I suspect there are more than 1 opamp on the pcb. One for amplification, and one for tone.

[Ozyris]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

Hello stj. I am wondering as to the meaning of the word "nitromors"?

Prop1. Thanks for taking the time to think about this. Just to recap - there are two tone controls, one for bass and the other for treble, each needing three wires. The unit is a 2-band EQ preamp.

The pickups are connected to the blend pots which are not involved in the preamp other than supplying it with a pre-blended signal through one wire.

Two wires were previously connected to the volume pot suggesting there might be some gain activity in the board.

There will be a ground wire and a power wire, plus there will be a signal output wire. That's twelve wires accounted for.

[Ozyris]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

So, as the power wire is often red on such circuits as your post suggests, then 1 or 2 may well be the 9v+ connection. Looking at my diagram it seems unlikely that it would be 1 as it doesn't make sense to me to have substantial voltages all over the pcb (relatively speaking). It seems more likely to me that 2 would be 9v and would go only to the opamps.

I agree that it is probably a dual opamp similar to the TLC074 or LM741.

Connection 1 may well be the signal output or input. Red is often used for the hot wires. Plus it is connected strongly to 10 but also through some resistance to 3, 6, 8 and 12. As we know, on passive circuitry the signal at the volume is connected to the two tone pots grounded via the caps and the pot resistance, and my tests in such a case would have revealed that. So perhaps that is what the diagram is telling us? That one of 3, 6, 8 and 12 will definitely be going to the treble and bass pots, maybe even the volume pot too?

Not sure how much more time I can afford to spend playing with this...got some replacement units on the way...

[Prop1]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

So, as the power wire is often red on such circuits as your post suggests, then 1 or 2 may well be the 9v+ connection. Looking at my diagram it seems unlikely that it would be 1 as it doesn't make sense to me to have substantial voltages all over the pcb (relatively speaking). It seems more likely to me that 2 would be 9v and would go only to the opamps.

I agree that it is probably a dual opamp similar to the TLC074 or LM741.

Connection 1 may well be the signal output or input. Red is often used for the hot wires. Plus it is connected strongly to 10 but also through some resistance to 3, 6, 8 and 12. As we know, on passive circuitry the signal at the volume is connected to the two tone pots grounded via the caps and the pot resistance, and my tests in such a case would have revealed that. So perhaps that is what the diagram is telling us? That one of 3, 6, 8 and 12 will definitely be going to the treble and bass pots, maybe even the volume pot too?

Not sure how much more time I can afford to spend playing with this...got some replacement units on the way...

This is just a guess, I've been out of electronics for years, but the 9v maybe going to a voltage divider to create a simulated +9v and -9v. That's why there are 2 red leads. If my memory serves me correctly; opamps require a + and - power?

[stj]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

nitromors is a gel based paint stripper that can be used to break down epoxy-based potting compound.

it's used a lot to remove resin from chips in laptops and "security" modules.

[Ozyris]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

Aha. Thank you stj. I learn so much here. Sounds expensive. Relative to the preamp...

[stj]

Re: Bass Guitar Pre-Amp - Assistance required from local geniuses please!

it's about £6 a liter i think from diy shops.