Abyss With No Clean Signal - SOLVED

[debrad]

Hi everyone!

I have an Abyss build that seems to function in every way EXCEPT the signal is incredibly clipped/distorted.

The board came to me with no functionality and I got it this far by rotating the TL074 into the correct orientation, replacing a 1M resistor at R14 with the correct 100k, and swapping the mixed-up C10 and C13 capacitors.

Plugged in, I get all the pulsing, warbling, and volume/speed adjustment but the output is spitty, gated, and distorted no matter how much I dial back the Level, Intensity, and Voice controls.

I haven't audio probed the circuit yet but I did measure the following (and I feel like Pin07 on the TL072 seems a little suspicious):

AC Adaptor = 9.15V

TL072
Pin01=4.43V
Pin02=4.45V
Pin03=4.04V
Pin04=0.00V
Pin05=4.40V
Pin06=4.44V
Pin07=8.29V
Pin08=8.88V

TL074
Pin01=4.45V
Pin02=4.45V
Pin03=4.36V
Pin04=8.89V
Pin05=4.40V
Pin06=4.45V
Pin07=4.45V
Pin08=4.45V
Pin09=4.45V
Pin10=4.39V
Pin11=0.00V
Pin12=4.40V
Pin13=4.45V
Pin14=4.45V

Q1 (MPSA18)
C=1.90-1.92V
B=0.61V
E=0.00V

Q2 (MPSA18)
C=1.91-2.02V
B=0.66V
E=0.05V

Any advice on where to start looking for the cause of the clipping would be MUCH appreciated!

- brad -

 

[debrad]

I got a chance to do some audio probing this morning...

There is a nice clean signal all the way to Pin01 of the TL074 where a bit of distortion is noticeable. There is also a tiny bit of distortion noticeable on the side of R10 connected to Pin01 and then a little more on the side of R9 that is connected to C7 which has some distortion on both the "+" and "-" leads.

R11 has some distortion on the side connected to C7 but sounds quite clean on the side connected to Pin05 of the TL072. On TL072, Pin05 is clean, Pin06 and R12 have NO sound and then Pin07 is WAY louder than any other signal in the circuit and REALLY distorted.

 

[debrad]

Reassuring to see that my troubleshooting skills continue to improve. I saw the evidence leading me towards an issue at Pin07 of the TL072 and whatever controls the voltage at that point but still wasn't 100% sure about the true cause so...

I asked for some help on the Facebook Pedal Builder's page and someone suggested I confirm the values for R9 and R12. I had already done that but decided to confirm and, in doing so, noticed a little TOO much wiggle in R12. Turns out it wasn't soldered completely on one side!

After adding a bit more solder, Pin07 immediately changed from 8.29v to 4.45v and the "Abyss" began vibing beautifully with NO harsh clipping.

Yay...problem solved!

All the other misplaced components aside, it's interesting that my last two troubleshooting sessions came down to disconnected resistor...