Troubleshooting my Gristleizer build

[TheCowGod]

Hey guys. I recently got done building a Gristleizer (using the gaussmarkov PCB layout and bipolar power supply - schematic, project page, original PDF), and it sounds generally similar but not entirely as good as the examples I've seen here and on Youtube. I'm relatively new to audio electronics, so I was hoping you guys could help me troubleshoot. There's three main symptoms:

First, the LFO ticks pretty audibly even when there's no input signal. I tried to keep my signal and LFO wires separate, but I put it in a BB enclosure so everything's pretty close together. I see most of the enclosures used by other people are larger so there's more space to keep things apart.

Second, it seems like the range swept by the LFO (or its effect on the audio signal) isn't as wide as the ones posted on youtube. For example, in VCA mode, on the Square wave setting, with the Depth knob all the way up, I'd expect it to be alternating between signal and silence, but it doesn't get far enough to fully silence the signal. The same thing happens with the other wave shapes. It just seems like it doesn't have as strong of an effect as it does for others.

Finally, and most annoying, the VCF mode is nasally and unusable. It's like the bandpass frequency starts towards the top of the guitar's audio range (with the bias all the way up) and goes up from there (as the bias is turned down). From my limited understanding of RC networks etc, I believe that the VCF effect is obtained using C8 and C9 to shape the negative feedback on IC5, with TR2 acting as a variable resistor to alter the frequency at which feedback is lowest (and thus gain is highest). Right? In that case, it seems like this problem would be due to either the value of C8 and C9 (which I'm pretty sure are 6.8nF as prescribed) or maybe in the stuff between the Bias pot and TR2 on the schematic?

Actually, if it were the latter, and it were keeping TR2 from sweeping the correct range of voltages, I guess that could also be what keeps it from having as strong an effect as I'm expecting in VCA mode too. I wish I had a scope. I suppose I could try turning the speed all the way down and watching TR2's gate voltage with my multimeter. I'm not sure what range of voltages is "correct," though -- according to the original PDF, in square wave mode there should be about -8v to +8v on the output of IC3, which is dropped by the 47k R14, the depth pot (which is fully up for testing), and then the RC circuitry behind the gate of TR2, whose effect I'm not clear on. Do any of you have this circuit (operating properly) open or on a breadboard and could measure the voltages YOU'RE seeing at the gate of TR2? Do you guys agree that that seems like the place to start looking? Any other ideas?

BTW, I did the 1M input impedance mod (changing R19 to 1M, R21 to 1M, and R20 to 100K). I also followed someone's suggestion and increased the value of the input cap, C7 (from .1 uF to .220 uF), to pass more low end. I don't suppose either of those contributed to the issues I'm seeing?

I probably should have breadboarded this first. I recently got Taylor's PCB, so I want to get this working right before I build it again on that PCB. This time I'll definitely be breadboarding it first, but if you guys have any other ideas on what to try, I'd definitely appreciate it. Thanks.

Dan

[Taylor]

Have you tried tweaking the offset pot? Mine goes all the way off. I think it's probably a matter of tweaking the trimpots to get the right response.

As for ticking, mine ticks a little in some settings, but not too bad. Long wire runs can equal ticking problems, which is one reason I prefer board mounting pots.

The PCB I designed has a spot to add a cap from the LFO signal to ground. This definitely kills all ticking, at the expense of a slightly less choppy square wave. Try a 10uf cap from the wiper of the depth pot to ground.

[TheCowGod]

Yeah, I've played with both trimpots -- they can make the triangle mode sound pretty badass and distorted, though much louder than the other modes. It doesn't have much effect on the depth of the effect though. I imagine long wire runs could be my issue -- each of the wires from the board to a lug on the pots/rotary switches is about 3" long.

I experimented with the cap from depth pot wiper to ground -- 10uF eliminated almost all the ticking but curved out my square wave too much. 1uF was better, 0.47uF even better. That seemed like the best balance for me -- still a little bit of ticking but far less prevalent, and the square wave still sounded pretty square-y. Still not dropping to zero volume though.

The output of IC3 (LFO) is sweeping from -7.2 to +8.0v, so it seems about right. The voltage sweep I'm seeing at the gate of TR2, with the depth and bias all the way up, is -3.2v to -1.9v, which seems weird to me since the source of TR2 is tied to 0v and not -9v, but I can't really tell if it's wrong or not. From my work with robotics, I'm used to the base of a BJT sitting at a voltage between the collector's and emitter's, but I've been reading about tube amp circuits and I know that the grid of a tube is generally biased a few volts below the cathode voltage, so this looks like that. So basically I don't know what I'm looking for here, I guess

Since R22 and TR2 are functioning as an attenuator in VCA mode, it seems that the lower the resistance on TR2, the more of the signal will be sent to ground rather than to the output. So then if the signal isn't getting down to zero, that suggests that the resistance across the source-drain of TR2 isn't dropping low enough, correct? I measured that resistance across source-drain, in-circuit, and got a range of 113 to 335 ohms as the LFO swept up and down in square wave mode. I don't know if that's a valid measurement at all since it's still in the circuit, though, so I'm hesitant to draw any conclusions from that number.

Would someone with a working unit mind trying the same measurements? In other words, setting bias and depth all the way up, speed all the way down (in case that has any effect), 2P4T switch to square wave mode, and measure the voltage range swept at the gate of TR2, and I suppose also the resistance across the drain and source pins of TR2 as well. I'm not sure where I had my offset and shape trimpots set, but I'd just like to see what the numbers look like for a properly working circuit, to see if I'm even looking in the right place.

And as you can probably tell, I'm new to this and am only stumbling through the logic trying to figure out how the circuit works and how to go about debugging it. So if anyone wants to point me in a better direction, that'd be welcome as well Thanks for the help.

Dan