Unwanted Oscillations Dr. Boogey

[TheNixon]

Hi,

I am newbie here and probably this topic was discussed already and I need some help. I made Dr Boogey(gaussmarkov) with custom layout for SMD components. At higher gain I get oscillations (squel) in last branch between last Q4 and Q5 JFET transistors. (one is amplifier section, other is follower). Oscillations are present only at higher gains and output volume. I checked all GND connections, try to add 100nF cap to power supply but without success. I shortend all pot connections and connect shield to ground. Oscillations are still present. There still must be some loop and I thnk the only possible is back through supply.

Do you have any suggestions how to prevent or at least try to prevent oscillations.? I am thinking about to increase elco-caps from 1uF to 10uF on FET sources, try to add 100nF at different loactions? Do you think it is anything wrong with SMD layout while the components are prety close one to each other?

Thank you in advance.

Best regards,
Nix

[TheNixon]

Schematics: http://gaussmarkov.net/layouts/drboo/drboo-schem.png

[DUY1337GUITAR]

I'm having the same problems too. 
Here's a quote from dschwartz:

the dr boogie it´s also sensitive to the way you layout parts inside the box..
the rules i follow (well, they are pretty standard for high gain pedals anyway):
- NEVER put the input jack too close to the output section of the pedal (volume pot, last stages, too close to the PCB or over the PCB)
- NEVER put the output jack too close to the input section (input jack, first stages, gain pot)
- gain pot should be away from volume, treble and presence pots.
- input jack isolated from the box. Ground it from the input of the board.
- Use only (yes, only) regulated PS. (sometimes i include the regulator inside the box)
- Enlarge filter caps (i use 220uF)
- input-->switch-->board  wiring allways shielded and as short as possible
- Board-->gain pot wiring shielded if it crosses over the PCB

[TheNixon]

Hey,

thanks i found a solution. After all coupling capacitors probes and everything the solution is very simple. (as always...) Big problems, simple solution.

A time ago I made a small box with two jack connectors and output connectors to PCB (just simplified guitar in and amp out and connector for cables from effect)... Well the bigest mistake was.... that I made twised pair of Input and Output connector   (instead with GND)... Yes that is solution... i removed all those cables and replace it with shielded microphonic cable and now everything is ok (i guess it would be ok just to untwist them... so no mistake on layout, mistake outside. Can't tell how happy I am now.... Where were my brains when I made that box. 

Thanks for your suggestions, they are simple and sound basic but in fact we offten forgot about them and many times is this the only reason for problems. Thanks again. 

[TheNixon]

To that list I would add:
-- if you are making custom layout try to solder POTS, jacks etc on PCB directly (PCB components), that would shortend the connections and offers less noise.  I always try to avoid cables and wires if possible. It is really less noisy.

BR,
Nix

[DUY1337GUITAR]

LOL the closeness of the input and output wires is what causes much of the oscillation and you had the them together the entire time XD
I'm glad you got your solutions fixed, I'm gonna build another Dr. Boogey soon

[TheNixon]

Yes it is LOL!  I was already about to make new layout.... 

Br,
Nix