Troubleshooting Dead End FX Charonium/Last Gasp Misty Caves Reverb

[mdcmdcmdc]

Agreed, the modulation section isn't particularly interesting on its own. The demos of the stock pedal sound pretty cool, though—I think the freaky deaky reverb is the star of the show so it'd be cool to figure out what's going on.

There must be some critical component that's very hit or miss? Seems weird that 5 people have all had the same issue.

[Ry]

I spent about an hour messing around with the circuit and haven't solved anything.  I tried upping the gain of the opamp stages by 20k, thinking that maybe the feedback loops needed some help, I tried swapping out the transistors, I tried different opamps (both brand and type).  Nothing made a difference in the operation of the feedback trimmer (other than modifying the range at which a signal could be heard).

I'm having a little trouble figuring out what exactly comprises the feedback circuit.  It might be easier if I could print out both pages of the schematic...but that's not happening any time soon.  I'll continue to think about it, but I'd be happy if anyone can identify how the feedback circuit should operate.

[mdcmdcmdc]

I comped the two half images into a single page on the previous page of this thread if that's helpful at all? I appreciate you putting some thought into this!

[Ry]

Yes, that helps, I didn't notice that it was stitched together. 

[ElectricDruid]

I'm having a little trouble figuring out what exactly comprises the feedback circuit.  It might be easier if I could print out both pages of the schematic...but that's not happening any time soon.  I'll continue to think about it, but I'd be happy if anyone can identify how the feedback circuit should operate.

I think partly the reason you're having trouble is because that schematic isn't laid out very logically. It wanders around quite a bit, and there are lots of connections back to a Vref in the centre which isn't marked as such, and isn'T even obvious because the cap is miles away from the divider.

Here's the audio path. First the signal comes in (in green) and goes through some filters to the delay line. Then it comes out of the delay line (in red) and does the feedback path (in red) via the vactrol's LDR and meets up with the dry signal (hence it is feedback).



HTH

[Ry]

Thanks!  I'm guessing that the resistance value of the LDR isn't being driven in a way to cause the feedback.  I'm planning on seeing what other vactrols/LDRs I have on hand to see if a change of that part helps.

[Ry]

Bingo.  A VTL5C2 works like a charm.  Resonance for days.  I think the issue is with the 5C3 clones not having the correct resistance range for this circuit.

[ElectricDruid]

Bingo.  A VTL5C2 works like a charm.  Resonance for days.  I think the issue is with the 5C3 clones not having the correct resistance range for this circuit.

The circuit is not well designed is more it, in my view. If it's highly sensitive to something variable like vactrol resistance, that's a design flaw. I'm not even sure exactly how using a massive series resistance ahead of an op-amp buffer is supposed to work as a level control. Maybe that's what that random R34 100K resistor is supposed to do? And then that's followed by the passive mix of the dry and wet signals, which doesn't include any resistors at all and therefore probably depends on the output impedances of the things feeding it to determine how much of each you get. The circuit is hacky.

[mdcmdcmdc]

Bingo.  A VTL5C2 works like a charm.  Resonance for days.  I think the issue is with the 5C3 clones not having the correct resistance range for this circuit.

AYYYYYYYY thank you!
I'll give this a try.

[mdcmdcmdc]

Just dug up this photo that DFX sent a while ago and it looks like they used the LCR-0203 in the verification build, so that one should probably work as well.

[Ry]

I have some of those on order.  I'll try one out when I get them and report back.

[dirtyuncleleo]

are they using the 2n5088s in the verification build?

[mdcmdcmdc]

Not sure, but whatever they have in there I guess they've had to twist the legs to match the 2SC1815/ECB pinout.

[Ry]

Yes, I had to twist the legs on m 5088's to match the original pinout.

The issue isn't with the transistors, it's with how the feedback loop is driven into resonance.  The LDR portion of the vactrol is modulating the frequency of the phase shift, which causes resonance.  The LDR in the 5c3 Vactrols that a lot of us have used isn't suitable for this purpose, it must not be providing the correct resistance value to modulate the phase shift of the filter into an unstable response (resonance).

[BetterOffShred]

I just built one of these and it fired up right away with some known 3207/3102 combo and a Chinese 5C9 .. 

I ended up removing the vactrol from the on board LFO and building a shoot the moon LFO, then matching LDR to the circuit with an orange LED..  I think it was 5537 LDR.  It won't reverberate at all if the LDR isn't correct from my observations.

The shoot the moon LFO has the wave shape and better speed from my testing, and then with the bias and the depth you can get all kinds of subtly that I didn't see in the original LFO, or on the videos.

[dirtyuncleleo]

hey @mdcmdcmdc

so i had some simliar problems to you: swapped out the vtl5c3 with an LCR-0203 and it worked great. something about the VTL knockoffs (no originals for me) causes weird fluctuations especially in the resistors around IC5. i used the freq function on my dmm, which showed that nothing was coming through the octocoupler.

hope you haven't given up yet. it's a pretty fantastic sound, once it works.

L

[jmasciswannabe]

A little discovery on my debugging end I thought I'd share here. Vactrol LED wont work unless D4 is in. I don't like rate indicators so left it out. A lot of double checking swapping all kinds of stuff later and had the DOH! moment after paying closer attention to the schematic.

You can also swap the 1uf capacitor in the lfo to increase the speed and/or use a momentary switch to short the ldr for resonant "bursts."