Another bump of an old one, same problems though. This pedal drives me crazy - after fixing a bad trace between R6 and Q1 drain which took some hours. Don't know whether this is due to this specific board gone bad, but whatever.
I got the poor thing working and sounding just plain bad with no attack and mid-high, high frequencies at all. Did some head scratching exercise, and some googling 'till managed to find the issue, e.g. the supposed "fix" of 39k/10n that's setting what is rather extreme Low pass filter.
I took the effort to model the thing in Micro Cap and passed couple of analysis stepping the low pass cap value from 1pF to 10nF (through some common values - 220pf, 470pf, 1nf, etc.) and below are the results - green - captured at R12 and blue at the output). As one might see and definitely hear - there's quite a bit of difference - and it sounds nowhere near as good as when this 10nf cap is removed or at least decreased to a more sensible value.
Now, when it is removed the reason for it becomes obvious - the pedal sounds absolutely phenomenal, until you raise the gain/level - it oscillates like crazy - and after some time spent with matching all different varieties of J201 (SMDs and a bunch of NOS Fairchilds), changing few cheap ceramics that came with the kit purchased, I tend to think there's something with the PCB layout going on here.
I am not keen on purchasing a different kit with a different PCB (as at this point I well may have purchased a 2nd hand Diablo). I definitely can play with the biasing (and intend to do so - starting with Q3 source resistor), but this doesn't seem to be used in the original.
As it seems the current revision of the PCB needs a bit of tackling.
Will report back should I get somewhere further.
With respect,
Niki