MEC WARWICK 3 BAND EQ RF ISSUE

[darmstat]

I have a MEC / WARWICK 3 band bass on board eq and it has an issue. when i plug in my bass guitar the active system has a lot of noise and RF interference. the signal sounds weak when compared to the passive sound? I replaced the battery and i do not see any wires disconnected from the circuit. Any help on this would be appreciated.

[bajaman]

first check the jack socket, before replacing op amps

[darmstat]

here are the voltages arcoss the 2 op-amps

tl064
pin 1 4.52 v
pin 2 4.45 v
pin 3 3.78 v
pin 4 9.70 v
pin 5 4.48 v
pin 6 4.48 v
pin 7 4.47 v
pin 8 4.58v
pin 9 4.57 v
pin 10 4.54 v
pin 11 0
pin 12 4.30 v
pin 13 4.54 v
pin 14 4.54 v


tlo62

pin 1 .24 v
pin 2 .44 v
pin 3 .66v
pin 4 0
pin 5 4.30 v
pin 6 4.52 v
pin 7 4.52 v
pin 8 9.70 v

[bajaman]

the voltages on the TL064 are good - the voltages on the TL062 do not look right to me but without seeing the schematic - hmmm. i would replace the TL062 and remeasure the voltages you should be getting approx 4.5 volts on pins 1,2, and 3.

[darmstat]

I did the best i could to trace the circuit. Here is a schematic that i came up with.

[darmstat]

I replaced the tlo62 with another one and still the same voltage readings? I tried yet another op-amp and still the same readings?

[bajaman]

ok maybe that side of the op amp is not being used in the circuit - can not see the schematic

[darmstat]

schematic

[ggedamed]

You should double check the schematic for R1 - it doesn't make much sense to bias the + input through R3, then connect it to the ground.
Also, pins 10 and 5 of TL 064 should get some bias from somewhere.
Double also, you may have missed a resistor from pin 14 to pin 6 of TL064.

mw3_1.png

Other than that, nothing beats the old audio probe in debugging.

LATER EDIT: try to use easier to edit/view formats like png/gif/jpg, instead of pdf, when posting things not meant to be definitive.

[darmstat]

Maybe this one is a little better.

[darmstat]

Better

[ggedamed]

As Bajaman said, the first op amp has a problem. I would take out the TL062, then measure the voltage at pin 3 (in the op amp socket) and resistance from each of the pins 1 and 2 to ground. The voltage should be VR and pins 1 and 2 should have no connection to the ground. I'd also test the electrolytic caps around the op amp (C12 and the 1uF between R2 and ground).

[darmstat]

Thanks for the help. I bread boarded the last schematic and it works as it should.

[darmstat]

I replaced C 2 and it works fine now?

[Serge75]

Hallo, could someone please, post the pots for the circuits, where they connect and what value they are? I m not finding anything on google at the moment for a 3band mec preamp. I play on a warwick fortress one 5 and have always liked the sound, but I play it passive. I'd like to give it a try a the active middle cut. Thank you,
Sergio

[modman]

Looks like some updated schematics were deleted?
Anyone got those?

I've added fixes based on some pretty poor pictures.
It really needs a recheck on a real board or some better pictures.

[darmstat]

Updated

[george giblet]

Good stuff. Thanks for the update.

I can see the following differences between my schematic your update:
- Where R4 and R12 connect, R12 on my schematic connects to the other side of R4 compared to yours.
* From a technical perspective on how the circuit works I'd go with my connection.
- C2 connects to the other other side of C1.
* Both will work, hard to know what is correct without a PCB.
- R11 value: Mine 20k, Yours 22k.
* Not much to worry about there. Again, would need to be checked against a PCB.
Maybe this part varied in production?
- C4, C8, C9 values. Mine 10pF, Yours 100pF.
* No idea what is correct. I'd have to go over my info and see how clearly I could read the values.
EDIT: I can confirm these caps are 10pF. It is also clear these caps are different to C2 on the PCB.
Personally I'd probably prefer the sound of 100pF.
- I've got a diode on the power.
* I believe this is correct as you can see it on the PCB pic I've posted.

[george giblet]

I just realized I actually made a proper schematic and hadn't posted it!

[Dirk_Hendrik]

Pretty!! Thanks!