Help Wirring Bass Guitar on board preamp

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Whoops
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Help Wirring Bass Guitar on board preamp

Post by Whoops »

Hi,
I'm repairing at the moment an Oakland & Co Bass. I'm having a lot of difficulties in trying to wire the active preamp circuit.
There's not much info around on these bass guitars, and no schematics.
The Active Bass preamp circuit uses 2 IC's, the TL064 and a TL061.
It seems a similar layout to a MEC bass preamp, although the MEC uses TL064 and TL062, couldn't find a schematic on the MEC unfortunately.

Someone in the past messed around with the wiring of the active circuit and some wires were cut also.

I did a reverse engineering on the circuit and drawn a schematic of it (attached) , with the color of all the wires that come out of the board.
I would like to ask if someone could help me out of point me in the right direction of where to connected the wires.
The active part has a DPDT 3 way switch and a Dual gand 56K Pot.
The passive part has an 250K volume pot and an 500K Tone pot. The passive circuit is wired and working (picture of the current layout attached).

Image

Image



I just don't really know where to connect the wires coming out of the preamp board and how to integrate this circuit into the passive circuitry already in place.

I know how to read schematics, and I'm able to build fx pedals and other audio electronic projects although I'm still learning and I don't know electronically how an amplifier, buffer or active tone circuit works. That's why I'm asking for your help.

The wires coming out of the preamp board have this colors:

Yellow
Green
White
Blue/Black
Grey
Black (ground connection to VOL Pot case)
Brown
Yellow Stripped (ground connection to Tone Pot case)
Orange stripped
Orange
Violet
Blue Stripped
Blue
Red (+Voltage from the Batteries)
PAD??? (pad on the pcb , it seems a wire was cut from here)

Some of them should connect to the 3 way DPDT switch (the switch that turns the active circuit on or of)
Others to the Dual gang pot
and the rest to the others part of the circuit like the pickup selector switch, volume pot, tone pot, output jack

The 3 Way DPDT Switch has already a Jumper soldered between 3 pins, picture attached.

Image

It would be great if someone could have a look into the schematic and give some tips and direction

Thank you so much
Last edited by Whoops on Sat Jan 31, 2015 8:55 pm, edited 2 times in total.
Whoops
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Re: Help Wirring Bass Guitar on board preamp

Post by Whoops »

I redrawn the schematic with Eagle and replaced the hand drawn one in the first post.

Here are also some pics of the PCB top and bottom:


Image

Image
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mediatechnology
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Re: Help Wirring Bass Guitar on board preamp

Post by mediatechnology »

Clue #1:
R14 appears to be the preamp input from the pickup selector.

I'd be tempted to re-draw that on the left hand side and what's going on might make more sense.
It took me forever starting at the schematic to find it.
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JR.
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Re: Help Wirring Bass Guitar on board preamp

Post by JR. »

I didn't even try to follow that original schematic, this version doesn't hurt my brain box as much.

#! the Tl06x is a low current consumption version of the TL07x family so reasonable for 9v battery operation.

I will start by telling you what I do see.

#2 IC1B appears to be a unity gain inverting stage that also mixes in two padded down non-inverting inputs. So R5 and R6 appear to be signal inputs to that stage with a couple ten dB of attenuation before a gain of 2x.

#3 IC1A appears to be an inverting stage but without a feedback resistor it will not work as drawn unless there is another component or feedback path between the output and - input of IC1A. The fact that IC1C has similar input R and feedback C makes me suspect that these are two integrator sections from a compound multiple opamp 2 pole state variable filter. The DC feedback is connected from the output of the LP section back to the input of the HP section. This SWF topology generally uses 3 or 4 op amps. The two simple integrators are (IC1A and IC1C), The HP stage is either IC1D or IC1B, my guess is IC1D...

IC2 is most likely the front end preamp. So pick-up feeds the input at the node you have labelled pad?? That input stage has some frequency shaping but is nominally a unity gain buffer. The output of this buffer feeds the input (HPF stage) of the compound 3 or 4 op amp SVF.

IC1B may be inverting the band pass output so it is correct polarity to blend with HP and/or LP outputs that are normally opposite polarity in standard SVF topology.

It is not immediately obvious to me why all the SVF stages are wired to each other through off the PCB wires and switches. If their intent is to confuse copy-cats it is working.

I would suggest researching a typical SVF to see if you can make sense of the off board switching.

It would be helpful if your wiring diagram included wire colors, something must still be attached that we can learn from.

My head hurts too much to speculate further from this limited information. It looks like a SVF but that does not immediately suggest how to connect everything.

JR
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mediatechnology
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Re: Help Wirring Bass Guitar on board preamp

Post by mediatechnology »

Clue #2:

R1 and R7 likely have a dual pot in a "rheostat" (variable resistor) connection in series with them to set the state variable SV filter frequency.
This pot may be dual linear or anti-log (rev-log).
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JR.
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Re: Help Wirring Bass Guitar on board preamp

Post by JR. »

Yes a dual pot could tune the center frequency of the SVF. I typically use dual potentiometers not rheostats in SVF for better accuracy, but this is a bass guitar preamp? I didn't see any dual pots in the wiring diagram.

Of course the two SVF poles could be tuned separately, in effect changing the Q/bandwidth by how close or far apart the poles are spaced.

Still a bit of a mystery.. Does anybody know what this is supposed to do? That might be useful information.

JR
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mediatechnology
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Re: Help Wirring Bass Guitar on board preamp

Post by mediatechnology »

Yeah, what are the knobs on this gizmo labeled and how many are(were?) there?
The second drawing doesn't seem to relate to the board or schematic.
Whoops
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Re: Help Wirring Bass Guitar on board preamp

Post by Whoops »

mediatechnology wrote:Yeah, what are the knobs on this gizmo labeled and how many are(were?) there?
The second drawing doesn't seem to relate to the board or schematic.
Hi Thanks for all your help, Im reading everything you wrote, trying to understand, and learning.
I will address the issues you pointed and will redraw and presenting new layouts as you asked.
Just as a quick answer for now about the Pots,
its written in My first post:

"The active part has a DPDT 3 way switch and a Dual gand 56K Pot.
The passive part has an 250K volume pot and an 500K Tone pot. "

The passive section also as a 3 way On On On Pickp selector switch

The Dual gang pot has in one part connected the GREEN Wire and the Wiper has the Blue-Black Wire.
The other part i not connected, it would make sense to connect there the YELLOW Wire, but then I dont know what wire should go on the wiper.

Thank you so much once more, will read once more, and will report back to you.

Also The supply need to have both positive and negative voltages, I dont see any virtual grounding on board, so probably there was a wire coming from the middle of the two 9V batteries and connecting to ground. Its just a thought.
Last edited by Whoops on Sat Jan 31, 2015 8:56 pm, edited 1 time in total.
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JR.
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Re: Help Wirring Bass Guitar on board preamp

Post by JR. »

Whoops wrote:
mediatechnology wrote:Yeah, what are the knobs on this gizmo labeled and how many are(were?) there?
The second drawing doesn't seem to relate to the board or schematic.
Hi Thanks for all your help, Im reading everything you wrote, trying to understand, and learning.
I will address the issues you pointed and will redraw and presenting new layouts as you asked.
Just as a quick answer for now about the Pots,
its written in My first post:

"The active part has a DPTP 3 way switch
DPTP?? don't know what that stands for "double pole toilet paper" perhaps a 2P3T (two pole triple throw).
and a Dual gand 56K Pot.
dual gang? OK the wipers will connect to Yellow and Green to shift the center frequency of the SVF. for proper operation the bottom of both pots are grounded through an end limit resistance (to be a variable divider) or left floating to be a variable resistance.
The passive part has an 250K volume pot and an 500K Tone pot. "
the volume pot probably connects to the orange striped wire (best guess) .

the tone pot is less obvious. In your drawing it looks like the tone pot is connected to the volume pot, that might work or not, passive guitar tone wiring is sometimes pretty crude. Or tone pot could connect to gray wire? (guessing).
The passive section also as a 3 way On On On Pickp selector switch
are there 3 pickups?
The Dual gang pot has in one part connected the GREEN Wire and the Wiper has the Blue-Black Wire.
The other part i not connected, it would make sense to connect there the YELLOW Wire, but then I dont know what wire should go on the wiper.
This is painful to make sense of. but best guess for frequency pot is violet wire goes to top of one section of frequency pot with the wiper of that section going to green wire. The top of the other pot section connects to the blue -black wire with it's wiper to the yellow wire. I will guess for now that the other two leads of the dual pot are left floating since I do not see two same value resistors to ground.

So I see 3 inputs. the line labelled pad?? is a high impedance input perhaps from a pickup. THis gets buffered and summed direct into the output amp (?). The blue and blue striped wires are inputs to the SVF HPF section which is first section of SVF.
Thank you so much once more, will read once more, and will report back to you.

Also The supply need to have both positive and negative voltages, I dont see any virtual grounding on board, so probably there was a wire coming from the middle of the two 9V batteries and connecting to ground. Its just a thought.
Yes PS is incomplete. Brown wire may be -9V and +18V connections are actually +9V but it is not obvious how to turn off split supply with switching jack that typically only has one pole. 27k and 22uFd may be part of a V/2 supply... is 22uF polar? is there a + lead connected to ground side?


This whole exercise is like a bizarre IQ test... Those are my guesses for now.

JR
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Whoops
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Re: Help Wirring Bass Guitar on board preamp

Post by Whoops »

Hi JR,
There are 2 pickups on the bass. The 3 Way (on-on-on) Pickup Selector switch positions are Bridge Pickup, Brige and Neck Pickup, Neck pickup.

Image

I re-drawn the schematic with Mediatechnology's layout sugestion.

I re-traced the board also 2 times, looking for mistakes of my original tracing.
I found an important mistake there was a resistor missing, it's 2K Resistor and connects IC1B Pin6 to R12 and C5.
IC1D and IC1C were wrongly placed in the first schematic, they are swapped now.
And the C8 capacitor is 22nf and not 22uF, JR this cap is a normal non-polarized film cap.

I fixed all these mistakes in the schematic, it makes much more sense now, it's much clearer and easy to follow.

Image
Last edited by Whoops on Mon Feb 02, 2015 4:41 pm, edited 2 times in total.
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