So what if only ONE amp in a TL074 is fritzing out??

[stubish]

Hi,
   I'm almost done building this:


with this layout:


It's almost done. the only problem is I am getting a crappy fizz distortion out of pin 7 of the IC. I've audio probed the signal all the way into the input and it sounds fine. Checked the voltages to the IC, Ground and all the pins are right near 4.8v the VCC in is getting 8.9v All the other amps are working fine, output is as expected. However the output at pin 7 is distorted when I probe it (and consequently at the output)

Any ideas? Is it common to have a bad IC? I'm almost to the point of hauling tail over to the parts shop and getting two other TL074's and starting to desolder but I'm kind of at the end of my experience.

Is there anything I'm missing?

Any help is much appreciated, thanks in advance for the advice!

[stubish]

Gosh, sorry about the large image on top. I'll try and fix that!

[Taylor]

Always socket ICs on any circuit you haven't already built successfully.

As damaging to our egos as it is, the chance of a problem being a bad IC versus builder error is exceedingly small. Even more unlikely that you have 3 opamps working and a fourth not.

Make sure pins 6 and 7 are connected (use multimeter set to continuity mode).

Is output at pin 14 not distorted?

[stubish]

Hi Taylor, thanks for getting back to me. As a point of reference, this is my first build. As far as I'd gone before this was re-wiring a guitar.

I just double checked continuity on 6 and 7. Confirmed that it is present. I checked the resistance through the entire circuit. 45k to pin 5 and 35k to pin 12 would this make a difference? If there's a higher resistance, shouldn't this mean that pin 7 is receiving a smaller input signal and therefore should be less prone to distortion?

The only way I've managed to make pin 14 distort in the same way is to turn up the input gain to a point where it (to use a term that describes the noise quite adequatley) fritzes. However at a more reasonable gain, pin 16 is outputting clean guitar while pin 8 is fritzing. I can get it to stop fritzing out by lowering input gain, but it reduces output to a level that is all but unusable.

Thanks again. Hoping for some next steps!

[petemoore]

  The next steps are well defined and absolutely necessary for you to help us.
  Debugging [sticky thread], for results: read/follow the first post instructions.

[Paul Marossy]

Always socket ICs on any circuit you haven't already built successfully.

As damaging to our egos as it is, the chance of a problem being a bad IC versus builder error is exceedingly small. Even more unlikely that you have 3 opamps working and a fourth not.

Yeah, what Taylor said.

[stubish]

Here's the comprehensive help request.

1.   It takes the output of a guitar modeler. Applies a simple 2nd order low pass filter and then mixes some of the original signal into the output (via a POT).
2.   SJM harmonic converger (workalike)
3.   Source is another forum, not sure if I can mention it here. Auth is also a contrib to this board.
4.   No Mods, Exactly as the layout applies. Verifying as I go with the schem.
5.   + and – is not inverted.
6.   There is one TL074 Quad OP Amp. From top to Bottom as per the layout.
4.35   4.35
4.35   4.35
3.94   3.94
8.71   0
4.34   4.34
4.35   4.35
4.35   4.35
+ into the circuit board is 9.39 Volts
Ground rail is 0 Volts.

How am I looking?

Again, symptoms are at Pin 7 and forward from there, the output is tinny, harsh and fizzy. To note, R1 is a 10k resistor according to the schematic (am I correct in this?? It was one mistake I found tonight, I had a 10 ohm resistor in there).

Looking forward to your wisdom.

Stu

[Hides-His-Eyes]

Voltages look good.

[petemoore]

  I don't think you fried it, the voltages are comparing from stage to next stage look too 'balanced' for an OA fry.
  My earilier post was misposted in this thread btw...
  Audio Probe ! !
 

[stubish]

My wife is starting to think I'm obsessed. i think I dreamed about this last night  

Here's the correction. It's fizzing out at Pin 1 as well. Pin 14 is Fine, but I noticed this morning distortion at pin one, then amplified at pin 7.

I just re-soldered a suspect cap (C2 on the layout). No change.

Could the chip be receiving too much signal? I've tested the resistance going into the chip and it's sitting at 20k (what it should be to my eyes).

I've checked the cuts under the chips. I've checked contituity between pin 1 and 2.

Awaiting the sage advice

Stuart

[Hides-His-Eyes]

Are C1 and R6 going into the same hole?

[stubish]

Correct, R6 and C1 are in the same hole. I've checked resistance from Pin1 to Pin5 and it's 20K.

[stubish]

So if the voltages are right, the resistances are right. Does it come down to caps? (force of deduction, just call me sherlock). Right? I've double checked the values, but I've got no knowledge of how to test the caps apart from change them out...

Audio probe is clean on the right side, dirty on the left (the one with the filter). It's hard to tell if the distortion is present going into Pin 3. I can't get my probe loud enough at 20k of resistance. the output does tend to break up at Pin1+2 though.

UPDATE: Resoldered the R6/C1 pad as well as the Pads for C5. Checked over, turned it back on, and I can say it's worse, Squealing feedback from the filter side. May have been a short, becasue I can't replicate it. The clipping is still present, possibly worse.

[stubish]

I packed everything up yesterday, time to walk away for a bit.

I've been over the schem and layout and double checked everything. I think next course will be to go get a new pack of 10kr resistors, dump the electrolytic and start a new board all over again.... it's a Try, try again situation.

I'll let you know how it goes

Peace
Stuart

[JKowalski]

Just a note about the layout - why is R6 and C1 sharing a hole when you can just move the R6 pin down one strip (since pins 1 and 2 are connected) 

[antisolo]

Excuse my ignorance but what is V15?

[anchovie]

Excuse my ignorance but what is V15?

AC voltage source - represents your guitar signal.