Leviathan delay feedback problem

[xorophone]

Hello! I've just assembled the Madbean Leviathan delay from Etcher's Paradise (http://www.madbeanpedals.com/EP/schematics/Leviathan.gif), but the feedback part of the circuit doesn't work properly. I've built this delay before, so I have some voltage readings I can compare with, but I've given the other one away so I won't be able to make any new measurements.

The problem I'm having is that I'm only getting a maximum of 3 audible repeats when the repeat pot is at max. When it's at 0, I'm just getting 1 repeat, so the pot is working as intended. I'm guessing there must be another part of the circuit that's responsible for the feedback in some way, but I don't know what to look for. I've measured the pins on both chips and all the voltages look perfect, except maybe pin 14 on the PT2399, which is modulating around 0.95v (On my other build I measured 2.39v). I've already audio probed the path to and from the repeat pot and I can't find any problems with it. I've used a 20k pot instead of a 25k, but I'm guessing that can't be the problem. I tried audio probing the pins on the PT2399 with all the pots at max and here are the results. Since the first 4 pins are power supply pins, I ignored them, so pay attention to the pin numbers.

5. Noise.
6. Quiet.
7. Weird sounding delayed signal with constant clicking sound. No dry signal.
8. Weird sounding delayed AND dry signal with constant clicking.
9. Dry and wet signal.
10. Quiet.
11. Quiet.
12. Wet signal.
13. Heavily distorted wet signal.
14. Loud wet signal.
15. Dry and wet signal.
16. Quiet.

Note that there's never more than 3 audible repeats where there's wet signal.

Weirdly I'm getting an extremely quiet delayed signal to my amp even when only the ground on the audio probe is connected. How does that work..? That might have made my measurements slightly more inaccurate since I got a signal in my amp even when the pin I was probing was quiet.

I've poked around quite a bit now, but I don't know where to look, since the path to and from the repeat pot seems fine. I'm guessing there's some other circuitry around the PT2399 that somehow limits the feedback.

[pinkjimiphoton]

try jumpering r19 maybe? are you getting signal thru the cap right before the repeat pot?
try a bigger feedback pot. check your cap and resistor values too to make sure you're not off somewhere by a factor of 10 or 100 or 1000. hey... it happens.

we could do more with voltages on the chip than sounds.  put your black meter lead to ground, and read the voltage at all the pins.

[xorophone]

try jumpering r19 maybe? are you getting signal thru the cap right before the repeat pot?
try a bigger feedback pot. check your cap and resistor values too to make sure you're not off somewhere by a factor of 10 or 100 or 1000. hey... it happens.

we could do more with voltages on the chip than sounds.  put your black meter lead to ground, and read the voltage at all the pins.

Thanks for the reply!

Jumpering r19 increases the feedback, but not by much. There's signal on both sides of c17. A bigger feedback pot would only be able to give me even less repeats, right? It does sound very likely that I've used the wrong value somewhere, so I'll take a look when I've got time.

I didn't post the voltage measurements earlier since they looked fine when I compared them to my old build, but here you go:

TL072:

1: 4.34
2: 4.28
3: 4.17
4: 0
5: 4.26
6: 4.28
7: 4.25
8: 8.58

PT2399:

1: 4.96
2: 2.48
3: 0
4: 0
5: 2.88
6: 2.47
7: 0.60
8: 0.62
9: 2.48
10: 2.48
11: 2.48
12: 2.48
13: 2.47
14: ~0.95 (this one worries me a little bit, since pins 9-16 were the same value on my other build)
15: 2.48
16: 2.48

[xorophone]

check your cap and resistor values too to make sure you're not off somewhere by a factor of 10 or 100 or 1000. hey... it happens.

I just checkad all the resistors and according to the color coding, they're all correct. I'll check the capacitors too, but I need a break first.

[ElectricDruid]

There's lots of stuff that affects the repeats (feedback) path. Both the pre-delay and post-delay filtering removes some of the signal and will decrease the number of repeats and propensity to feedback.

The pre-delay filter is R11-12-13, C8-9-10, and the post-delay filtering is R14-18, C11-16. A wrong value in any of those places could lower the cutoff (which is already set pretty dark, IIRC) and would make it impossible to get a decent number of repeats.

HTH,
Tom

[xorophone]

I just checkad all the resistors and according to the color coding, they're all correct. I'll check the capacitors too, but I need a break first.

All of the capacitor values are correct too.

There's lots of stuff that affects the repeats (feedback) path. Both the pre-delay and post-delay filtering removes some of the signal and will decrease the number of repeats and propensity to feedback.

The pre-delay filter is R11-12-13, C8-9-10, and the post-delay filtering is R14-18, C11-16. A wrong value in any of those places could lower the cutoff (which is already set pretty dark, IIRC) and would make it impossible to get a decent number of repeats.

HTH,
Tom

I tried probing these components. The signal is gone completely (from what I can hear) where R12 and C10 meet (pin 16 on the pt2399), but it's really loud at pin 15 (where R13 and C10 meet). I know you're not supposed to be able to hear any signal at an op-amps inverting input. Could this be something similar? There's signal where R11, R12 and R13 meet, but it's quieter than the opposite side of R11 and quieter than pin 15 on the pt2399.

On the post-delay filter there seems to be signal everywhere, but with varying volume. It's extremely loud at pin 14 on the pt2399, while it's much quieter at the positive side of C16.

[bean]

Pin14 does appear to be wrong as it should be close to 2.4v. Check your solder joint in that area, try a different batch of PT2399, etc. Audio probe C17 and C18. If you are getting signal to the LVL pot then C17 is probably fine and the problem area is narrowed to the RPT pot, C18 or R19.

[xorophone]

Pin14 does appear to be wrong as it should be close to 2.4v. Check your solder joint in that area, try a different batch of PT2399, etc. Audio probe C17 and C18. If you are getting signal to the LVL pot then C17 is probably fine and the problem area is narrowed to the RPT pot, C18 or R19.

Fixed it!

I don't know what was wrong, but I reflowed some solder joints around pin 14 and replaced the chip (which I've already done once) and it worked. Thanks everyone for the help!

[Plexi]

@bean
What would you tweak to get faster feedback oscillation, without distort the repeats?