Sorry for being MIA for the past couple of days--busy times for us right now as my daughter just gave birth to our first grandchild last week and we've been doing a lot of running back & forth (they live very close by, fortunately).
I'm a bit confused about exactly how/where you're signal probing, so let's take a step back and look at the schematic:
The "Drive" control is a misnomer--it's actually a blend control, as one half of it (VR1a) sends some portion of your incoming signal through the IC1a gain/clipping stage and the rest through a bypass path to the IC2b stage. As you turn it CW for more gain/distortion, it also increases the gain produced in IC1a by increasing the resistance of the other half (VR1b) of that dual-gang Drive pot located in the IC1a feedback loop. Since you've shown that (to my surprise) the IC wasn't causing the problem, we need to try to pin down where it's coming from. So I would suggest probing some points up in that first section of the effect circuit and see what we can learn.
I would suggest initially setting the Drive control all the way CCW so that the signal is going exclusively through the clean section of the circuit. The first thing we want to rule out is that the fuzzy feedback you're getting isn't arising from the JFET input buffer. So use your signal probe to confirm clean signal at the input of the effect circuit on the eyelet 5 solder joint just above the footswitch, where your signal enters the PCB circuit. Assuming that checks out, now probe the gate (middle leg) of the 2SK246 JFET, followed by the source (upper leg). You should hear the
same clean signal at the same volume at both points that you heard on eyelet 5. If not, report back here with what you're hearing.
If the JFET checks out, please check the clean signal path next. Probe the bottom lead of resistor R9 (20k)--you should hear that same clean signal at the same or just slightly reduced volume. Now turn the Drive pot to full CW and probe the bottom of R9 again--the clean signal should be greatly reduced or even inaudible.
Now probe the top lead of resistor R11 (56K). You should hear a much louder and quite distorted signal there coming out of IC1a, but without that fuzzy feedback thing going on. Now turn the Drive control back to full CCW. Re-probe the top of R11 and you should have a fairly quiet clean signal, quieter than the input signal.
Please run these signal checks exactly as I've described and report back with your findings. See probe points circled on PCB diagram below.
Yeah, I know this stuff is tedious, but I don't know a better way to pin down the source of your problem, since we've eliminated the easy stuff. I do have my own GP right here to be able to cross-check your findings against a verified working unit.
Attachment:
GP_front_of_circuit_probe_points.jpg [ 128.3 KiB | Viewed 937 times ]
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