Klon Centaur Klone - Low volume in bypass or engaged
I'm trying to help a friend debug a Rimrock Mythical OD, a clone of the Klon Centaur with an added switch to change the clipping diodes. He's tried to get the builder to take care of this under warranty, but long story short, no love.
The issue that's occurring is that when engaged, there's no headroom. The gain and volume both need to be cranked to get any drive out of it and it sounds a bit fizzy when you do. Moreover, in "bypass" (this design is not TB) it still cuts more than half from the volume; plugging straight into the amp is 2-3 times louder than going through this pedal in bypass mode. From my own experience building a Klone, unity volume occurs at about 9 - 10 o'clock on the volume knob and that famous sustaining drive really gets going at about three on the gain, but as I said this has no guts at all and kills volume in every setting.
So far, I've tried:
1) Putting a jumper between the ground lugs of the input and output jacks. Checking continuity of the output jack at first suggested that paint overspray might be interfering with the ground, as this design seems to rely on the case for complete grounding. This did not help.
2) Checking the output voltage of the charge pump (TC7660). It sits right at 18.25V when plugged in to my Pedal Power II.
3) Swapping the TL072's. I put a new op-amp of my own in each of the sockets; this did not help.
4) Checking the operation of the switch. It appears to be a decent DPDT unit that gave some odd resistance readings when I checked its operation across the 68K resistors on the signal side, but I haven't yet removed the switch from the board so it's likely to have some parallel resistance.
From the schematic here https://buildyourownclone.com/products/silver-pony-1 it's clear that "bypass" still runs through the first stage of the first 072, so the fact that the issue is occurring both off and engaged tells me that this stage is not getting enough juice ot provide proper gain, but I'm unsure where to really start digging. Could it be the resistor and/or cap on the input are fried? What's my first step from here?
Thanks in advance.
If it ain't broke, it can probably still be optimized...
I would use an audio probe to confirm where the drop in volume occurs. If it is indeed after the first op amp buffer, then check all of the resistors and perhaps reflow some of the solder joints. Check all of the resistor values around the buffer and the output (e.g. 68K, 100K). It's possible that a wrong resistor value was used during construction.