Maestro filter sample hold slow onset fault

[nocentelli]

After several years of on and off tinkering, I built a cut down version of the maestro FSH using this schematic - Based on the tonepad 13600 version, i removed the envelope detector section, used the unused OTA buffer instead of a discrete jfet buffer between OTA sections, and used different pins of the noise transistor which gave better noise (advice provided by aquataur from this website):


This iteration sat on the breadboard for several weeks, with regular bouts of lengthy playing without problems until I was satisfied it was worth building. I knocked up this vero layout, checked it carefully before building, and it fired up first time.


HOWEVER..... after a few minutes, it was clear all was not well. The random voltage blips and LFO section continued to function, but the guitar sound under the filter slowly became more distant and distorted and the distortion was of an unpleasant intermodulation type (i.e more noticeable with chords). I unplugged the boxed up effect, and checked the chips were seated etc, and after messing around and reconnecting the power, the fault had vanished. However, after a few minutes the pattern repeated - Guitar sound slowly fades in level and becomes more grainy and distorted until the effect is unusable. After a couple of weeks of occasional experimentation, it is also clear that shorting the power supply clears this fault (as I discovered by accidentally letting the adaptor plug shield [+9v] touch the casing of the effect).

Obviously, my next step is to measure and record voltages at all the pins and significant DC points a) when it's initially working, and b) as it starts to fail, but since this is a fairly involved process i will try to make time for next weekend, in the meantime I was hoping someone might spot a difference between my schematic and vero that I have overlooked, suggest a significant point in the circuit where this kind of fault might arise, or perhaps point out how my slight changes may have fatally injured the functionality of this circuit.