ROG Condor - Weird U1a op amp voltage - help requested

[5thumbs]

I've been working on this issue off and on for a few days now.  Everything else about the build seems to be working fine, but I get a highly-distorted output from the Condor cab sim.  The distortion is occurring in the U1a op amp.  The output voltage is way off what it should be, but after checking all the resistors and trying a new IC and socket for U1, I'm still having this problem.

Audio probing confirms the distortion centering around the U1a op amp.  The voltages after U1a are what I'd expect to see (but of course, the sound is distorted, being downstream from the U1a distortion generator.)

Here's the schematic:




I used Gringo's layout, which I also downloaded from ROG:




Finally, here's the solder-side of my PCB:




OK, enough pictures, onto the details...

1.What does it do, not do, and sound like?            Heavy op amp distortion at U1a output

2.Name of the circuit =                  ROG Condor Cab Sim

3.Source of the circuit (URL of schematic or project) =      http://runoffgroove.com/condor-pcb.pdf

4.Any modifications to the circuit?             Yes.  Added Marshall/stock notch mod, as described at http://runoffgroove.com/condor.html

5.Any parts substitutions? If yes, list them.            No

6.Positive ground to negative ground conversion?      No

7.What is the out of circuit battery voltage? =>         9.61V

Voltage at the circuit board end of the red battery lead =   9.61V
Voltage at the circuit board end of the black battery lead =   0.3 mV

Q1      J201
G = 0.4mV
S = 0.24V
D = 4.75V

IC1 (or U1)   TL072IP
P1  8.99V
P2  8.99V
P3  0.57V
P4  0.3 mV
P5  4.29V
P6  4.33V
P7  4.33V
P8  9.61V

IC2 (or U2)   TL072IP
P1  4.33V
P2  4.33V
P3  4.31V
P4  0.3 mV
P5  4.31V
P6  4.33V
P7  4.33V
P8  9.61V


Any suggestions y'all might have would be greatly appreciated.  Thanks in advance for your time and assistance.

[GibsonGM]

Check the continuity from pin 1 to pin 2...check that the chip is in fact grounded. 
Have you played with that trim pot? Could be causing too high a level in U1.  Audio probe at the output of the trim pot, then thru the resistors/caps, to U1 input. Check Vref at the junction of the 100k R's.   If it's ok to there, problem is with U1 and you need to take further steps...

[5thumbs]

Check the continuity from pin 1 to pin 2

Check.  Beeps w/the continuity beeper and shows 0.0Ω resistance between those two points.  I also pulled the U1 op amp and tested the resistance between Pins 1 & 2 through the socket, which read 0.0Ω resistance.

...check that the chip is in fact grounded.

Check.  Beeps w/the continuity beeper and shows 0.0Ω resistance between Pin 4 and the ground wire solder joint.

 

Have you played with that trim pot? Could be causing too high a level in U1.  Audio probe at the output of the trim pot, then thru the resistors/caps, to U1 input. Check Vref at the junction of the 100k R's.   If it's ok to there, problem is with U1 and you need to take further steps...

I read several other threads here about biasing Q1.  Most folks biased the trimpot so Drain was 1/2 of supply voltage.  I'll only use my Condor with a regulated wall wart, so I set Q1 Drain to 4.75V, which is just under the 9.61V supplied by the wart.  The only time the buzzing/distortion goes away is when the trimpot is buried clockwise, which has the effect of killing all signal, not just the distortion.

Voltage reading at the junction of R5/R6/R7 (100K resistors in Bridged T) is the same as ground readings, 0.3mV.  Vref on the "Vref side" of R7 (100K) is 4.31V, which appears to be normal.

I've actually audio probed all the way from the PCB Input through to the non-inverting input of U1a.  It changes tone and volume level due to the JFET and various caps/resistors, but remains clean...what I'd characterize as expected tone and behavior.  The signal remains in its expected state even on the U1a non-inverting input.  It's not until the U1a inverting input/output (Pins 1 & 2) that the high voltage/distortion is observed.

From Q1 Drain to the trimpot and C1, I'm getting 4.70-4.75V.  Between the C1 and U1a non-inverting input, I get 0.567V.

Normally, I'd suspect some sort of short from +9V to Pin 1 or Pin 2 -or- some sort of unknown large resistance between U1a non-inverting and inverting inputs, causing the op amp to boost the signal until it bangs into the 9V supply voltage.  I'm going to trace the feces out of the +9V short option tonight.  As for the "ghost resistance" issue, I figured that might be due to either a bad op amp or bad socket.  I tried a spare NJM4580 I had sitting around and it behaved the same way as the TL072IP.  I also tried a new IC socket and got the same behavior.  I'll probably give several other op amps a try as well, but I suspect any op amp will boost until it hits the supply voltage and distort like crazy.

So now that I've had a several-hour break from my previous trouble shooting, I'm going to give it another shot.

Thanks for your help.  I'm definitely looking for new ideas and angles on this one, as I've gone through all the normal steps and my Condor is still persistent in this U1a behavior.

-Brett

[5thumbs]

OK, fixed it.  TL072IP = odd U1a voltages.  NJM4580 = odd U1a voltages.  MC1458 = better U1a voltages, but still distorted.

I finally gave up and used my old standby, the OPA2134PA for U1.  Once I put in that chip, the voltages became "normal" and I could bias it properly to get a variety of clean to slightly gritty sounds.  I put another OPA2134PA in U2 and the distortion did not change, but the EQ did flatten a bit, so I kept the pair of OPA2134PAs installed.

First Impression: the ROG Condor is a neat little device.  I'm going to enjoy playing it and getting to know it better.

Thanks Mike for your help.  I appreciate you taking the time to respond.

[Lelik]

I'm also building my Condor and mine has a very low (weak) output with the trimmer in an extreme position (and with the volume and gain pots set at the max). With the trimmer in other positions I get no sound at all or a big distortion.
Anyway, I don't understand what are the correct voltages I should get at the IC and transistor pins. Can you help me with this?
I'm using NE5532 op amp.
Thanks for any information.

[sushyoshi]

I´m also building one but it has a problem. When i connect it to the amp i get a loud "hiss" like sound so high and when i play the chords on the guitar it stops, but then 2 seconds later it starts again. My ICs are well powered and grounded.
So it would be great if anyone who has it working would please post some on the voltages at different points so i could check what where the problem is.
Could anyone supply the voltage outputs ate the JFET, ICs and other important points? I´ll later post mine ( run out of battery on the multimetter  )

[sushyoshi]

Ok i got it working again, its my on/off switch it doesnt work well. Maybe it got burned inside when soldering. Im going to try put a LED on that to know when its working or not.
Anyway nice sound from it!