Fender Acoustasonic

[lowell]

This amp came in with no sound from the instrument input. The Line input channel works. I found two VERY hot op-amps. U2 and U3. Upon investigation I found that U2 pin2 (inverting input) has -6vdc on it, and the non-inverting input has 0vdc. From the looks of it, and I'm far from good at troubleshooting op-amp circuits, there should be 0vdc on pin2. There is a coupling cap from the previous stage, therefore blocking an DC from that previous stage. It's my understanding that pin2 should "match" pin3 because the op-amp is closed-loop. Therefore shouldn't it be 0vdc. So where is this -6v coming from? Finally, I replaced both op-amps (4560) with new 4558s that I have in stock. The channel now works, but is very low volume and distorted slightly...still same result in regards to the ICs getting really hot. I've measured every resistor related to these and all checkout. Also checked that everything is indeed wired as it should be - good circuit continuity between components.

Odd readings, or not, I don't know, are:

U3
pin1 to pin2 = 10k ohms
pin7 to pin6 = 20k ohms

Does this mean that the feedback caps C10 and C11 are leaky?

http://bmamps.com/Schematics/fender/...ice_Manual.pdf

[Enzo]

Pretty rare for DC leaking caps in these LV circuits. When teh IC is removed, what voltages are on the pins pads on the board? Where is the -6 coming from? usually from the output pin. Any chance it sits at -15v?

[lowell]

Haven't removed the ICs yet. I messed up in regards to polarity. It's +6v.

U2:
1 -1.2v
2 +6v
3 -.19v
5 0 v
6 0v
7 -14v

U3
1 -1.2v
2 -1.2v
3 0
5 0
6 -1.2v
7 +14.5v

[lowell]

Ok... very weird. Now, today, pin2 is -6v again. NO IDEA how the polarity could be changing like this.

[lowell]

Regardless... I went to pull the ICs and visually inspected the board on the solder side once more. The Notch pot looked like it had too much solder on its pins. Sure enough a few of the pins were all shorted together. Desoldered and resoldered it, the amp is now fixed.

Looking at the circuit I don't see how this could cause said problem... but they're not getting hot anymore and the amp is nice and loud. The pot is wired as a variable resistor, not a voltage divider, so not sure HOW in the world this affected anything. If the pot is fully one way of the other, it is either IN circuit or OUT (shorted). So if the pins were shorted how is that different. UNLESS - R29 A/B were shorted. Ok now that could make sense.

Opamps circuits are so touchy. I have Walter Jung's book - IC Op-Amp Cookbook. It seems as though U3 is an integrator. As I understand it an integrator changes it's bias overtime because of the charging of the Fb cap. Or that's what I gather. Please correct me or refine what exactly an integrator does.

[MarkusBass]

It seems as though U3 is an integrator. As I understand it an integrator changes it's bias overtime because of the charging of the Fb cap. Or that's what I gather. Please correct me or refine what exactly an integrator does.

It is an integrator but you should look at a group of opamps. U2A, U3A, U2B and U3B form a state variable filter: https://en.wikipedia.org/wiki/State_variable_filter and instead of trying to understand how a single opamp works, you should rather read about the state variable filter and think how they work together.

Mark

[Enzo]

Frankly I wouldn't worry WHAT the circuit is called, signal goes in one end and comes out the other, or should. That is what to look for. With your DC voltages all screwed up, it won't work. And any DC problems are not the result of the name of the circuit.

[g1]

The Notch pot looked like it had too much solder on its pins. Sure enough a few of the pins were all shorted together. Desoldered and resoldered it, the amp is now fixed.

Looking at the circuit I don't see how this could cause said problem... but they're not getting hot anymore and the amp is nice and loud. The pot is wired as a variable resistor, not a voltage divider, so not sure HOW in the world this affected anything. If the pot is fully one way of the other, it is either IN circuit or OUT (shorted). So if the pins were shorted how is that different.

There are probably other traces near that pot. Due to the excess solder (maybe even flux) any resistive or capacitive coupling to unrelated traces could cause issues, or perhaps oscillation.