hello,
I recently built a mad professor stone grey distortion clone from a kit by a company called puzzlesounds, and, while it sounds great, it has a weird problem. When the gain is maxed out, a high pitched oscillation comes in when the volume knob is turned anywhere passed unity. This problem goes away when a buffered pedal is placed before the sgd. I have checked to make sure I have used all the correct value parts in the right spaces and have messed around with the input section a little bit with no luck. Here is a link to the schematic: http://3.bp.blogspot.com/-P2RCqMsl__A/TxLM0MEtCJI/AAAAAAAAAio/rVtTwB3v83g/s1600/Mad+Prof+stone+grey+schem.gif
Is this the nature of the pedal, or can this problem be fixed? If so, how?
Thanks
Perhaps your input and output wires are too close together or try using shielded cable for them. Could also be other wiring within the pedal as based on the pictures it looks like the layout is sort of willy nilly.
Check out this thread too
http://www.diystompboxes.com/smfforum/index.php?topic=29816.0
Thanks mth5044,
Unfortunately, the jacks are board mounted, so there aren't really any wires to replace unless the oscillation could be occurring from long wires to the volume pot. I took voltages from all the pins of the chips and transistors following the directions of the article, I'm not sure if they are correct because the post mentioned that the ground wire to the input jack had to be cut, which I was unable to do because, once again, the jacks are soldered directly to the pcb. Here's what I got, though:
TL071:
1: .13v
2: 3.9v
3: 1.98v
4: 0v
5: .13v
6: 3.91v
7: 7.87v
8: 0v
CD4069UBE
1: 3.74v
2: 3.85v
3: 3.36v
4: 3.81v
5: 7.85v
6: 0v
7: 0v
8: 0v
9: 7.85v
10: 0v
11: 7.85v
12: 0v
13: 7.85v
14: 7.85v
Q1 (J201):
D: 3.89v
S: 3.9v
G: 3.9v
Q2 (j201):
D: 3.9v
S: 3.89v
G: 3.89v
I am assuming this is an impedance issue as the problem goes away when a buffered pedal is placed in front of the sgd. thanks for any help.
What happens if you remove C6?
Removed c6, oscillation got worse, so I tried a higher value and the oscillation still occurred and the gain was lowered, so I put the 15pf cap back in. Any other suggestions?
Ok, now the LED doesn't work, I replaced it, reflowed all solder joints that have to do with the led circuit, and still nothing happens. The pedal effects the signal how it should (except for the oscillation), the led just doesn't turn on when the pedal is on anymore.
Nevermind, fixed the led, but does anybody else have any ideas as to how to fix the oscillation?
Wait, the kit came with j201s and the schematic says the transistors are 2n3819s, could this be causing the oscillation?
They're certainly not pin-compatible. J201 is DSG and 2N3819 is DGS. Some of the characteristics on the datasheet (e.g. VGS(off)) are also different. Not sure if this would explain the oscillation.