Need Help: Debugging Hotcake clone

[thedavidbeam]

Hey guys. I built a hotcake clone using a verified schematic and vero layout. It works fine, but when you max the gain out, it starts... oscillating. I guess that's the right word. Any idea on what the problem may be? I used all of the correct components, and the only change I made was switching two 10 uF electro's to 1 uF electro's. I've already tried replacing all the caps, the op-amp, and I've made sure everything is grounded. Any help is greatly appreciated. I built another one a while back and it sill works perfect without this issue.

Thanks!

[EATyourGuitar]

it is a high gain pedal and you did alter the resonant frequency slightly with that cap change. did you use shielded cable? are your in and out wires to the board zip tied together?

[thedavidbeam]

what do you mean by shielded cable? the wires are not zip-tied together. Would changing the cap(s) to the original 10uF fix the issue?

[thedavidbeam]

also forgot to mention that I used a 9.1v Zener in place of the recommended 8.2v.

[EATyourGuitar]

I'm not sure why you have that problem but it could be a combination of things. different parts, different layout, different board. try this shielded cable to the board in and out. and see if it helps at all

http://www.smallbearelec.com/servlet/Detail?no=86

you need to ground all your shields to one location

[Eric.nail]

I'v built this one with with this layout:
http://1.bp.blogspot.com/-83H21h8zGLw/Typ4FGlqB8I/AAAAAAAAAgs/fVC9BYXJHCo/s1600/Crowther+Hotcake.png

Generally speaking, You can usually substitute values just as long as their within a 5-15% range of the original value(so i'v heard, and successfully done). BUT...It's not recommended. especially if you're trying to stay true to the schematic/original device. I'd shy away from swapping caps of that value. I doubt it's a grounding issue, though it definitely can be.

Also, What's the purpose of the diode in the schematic? i'm curious.

[kupervaser]

I built this one too:
http://1.bp.blogspot.com/-83H21h8zGLw/Typ4FGlqB8I/AAAAAAAAAgs/fVC9BYXJHCo/s1600/Crowther+Hotcake.png

Also had the same oscillation when the gain is maxed, never used it but instead built a different one from this document:
https://dl.dropbox.com/u/3131065/Pedalen/Crowther%20Hotcake/Pancake%20Project2.pdf

I have no more oscillations, however I have other issues now, there is some unpleasant note decay.

[therecordingart]

Put a small cap (try 47pf) in the NFB circuit. Get it oscillating then touch the cap to pins 2 & 6 of the IC.