Active Pickup circuit problem

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This circuit is acting up. It is functioning almost like a gate. There is no signal unless I hit the strings hard. Then it switches on. If I supply power to only the active pickups, it works fine so I am assuming the problem is in the preamp mid boost) circuit. My circuit is the Active 612A circuit.

Thanks
 

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This circuit is acting up. It is functioning almost like a gate. There is no signal
\unless I hit the strings hard. Then it switches on.

One stage could be saturated, coming out of saturation temporarily,
when there is a large input signal. Check the DC levels at each stage's output,
before any coupling capacitors. The values should be roughly halfway between
the power supply rails.
 
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Wrong forum, this belongs in I&A.

I looked at the manual, quite infuriating as the wiring diagram, wiring schematic, and circuit board schematic do not match each other.

In any case my tip would be to check all mechanical elements first: switches, jack, battery connector, wire solder joints. Hitting the strings hard also imparts a mechanical force to the guitar. If you think this sounds stupid i can show you a guitar with a pickup switch that has a mind of its own.
 
Thanks for replies. I figured it belonged in the instruments forum except that active pickups and circuitry have more in common with preamps than 9 out of 10 electric guitars.
It is definately not mechanical although it has been on more than one occasion. The wiring was a non functioning rats nest when I got it. I worked my way through and had it working beautifully for about a week.
I double checked today thinking battery voltage was sagging but it is still over 9 while under load. The schematic is accurate it is just displayed terribly in the manual.
 
The schematic shows a single IC. It runs on a single 9v supply. Make sure your battery is alive first. But then the IC must have a voltage reference. Over on the left are two 47k in series across the 9v and ground. the center point between them is your 4.5v reference. Note the 10k resistor to each IC input from that 4.5v.

DO you indeed have 9v and ground at the IC and about 4.5v on the other six pins? If the reference is missing so will most of your sound. otherwise, if one or both output pins of the IC are at some DC other than 4.5v, then swap the IC.
 
Are you trying to fix a broken Yamaha built guitar, or did you DIY the circuit from the schematic which has at least one ugly error. In fact some of it makes no sense at all.

Follow the positive (right most) end of the battery. It is connected to the positive end of C4 which it the audio input to the opamp circuitry. These can not be connected. R3 and the other 47K resistor above it create the mid supply (4.5 volt) reference for the opamps, which is connected to the input through a 470K resistor (correct). Then there is a 10K resistor to ground that messes the DC reference all up. There is something drawn wrong with that whole area of the schematic since the 10K is actually wired directly across the battery and performs no function other than to drain the battery, or possibly perform an orderly discharge to avoid a thump.
 
It is just like this. All stock. Does not match original schematic
 

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I wasn't able to located the source of noise yet. I did replace the 4558 opamps.
I have removed the preamp circuit from the guitar to isolated the pickups from the diagnosis. The noise is still present only when 9v is applied and volume control is at least 60 % clockwise. The noise sounds like a dark white noise with crackling and here is a picture on the scope.
 

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Back to this again. I replaced a couple of components and ran it in series externally. It seemed to work until I connected it back inside the guitar. Here is another picture of the schematic without my drawing of the F30A that exists on the PCB but not in the schematics. In the original schematic, there is a capacitor in brackets between R1 and C1. It is absent from the PCB in my guitar. What would it be for and what would be a typical value? Thg
 

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