Klon Centaur Problem

[GeorgeG]

I am on my 10th guitar pedal and finally have an issue. I built the General Guitar Gadgets Klon Centaur clone, completely stock:

http://www.generalguitargadgets.com/pdf/ggg_kc_sc.pdf

The pedal sounds pretty good but as you increase the gain control once you get past around 11 o clock, the output starts pulsing between zero and something more than zero output with increasing speed as the gain pot gets increased.

DC voltages:

Batt      9.5

V+        9.4

IC1:
Pin 1     4.7
Pin 2     4.7
Pin 3     3.7
Pin 4     0
Pin 5     4.7
Pin 6     4.7
Pin 7     4.7
Pin 8     9.4

IC2:
Pin 1   14.7
Pin 2     4.7
Pin 3     4.7
Pin 4    -8.7
Pin 5     4.7
Pin 6     4.7
Pin 7   -5.3
Pin 8   16.5

IC3:
Pin 1     9.4
Pin 2     4.7
Pin 3     0
Pin 4   -4.3
Pin 5   -8.6
Pin 6     4.9
Pin 7     7.3
Pin 8     9.4

D4 cathode       9.4

D5 anode/cath 9.4/13.0

D6 anode/cath 13.0/16.4

D2/D3                0/0

Any help would be appreciated.

Thanks,

George

[antonis]

Your buffer/voltage follower IC1a should have indentical DC values at all 1,2,3 pins..!!
(maybe 3.7 at pin 3 is a typo or is "loaded" by your mutimeter impedance, 'cause all other Vbs are 4.7..)

Check for V2..!!
(it shouldn't be at 16.4 Volts..)

Something happens between D5 anode & D6 cathode..
(it seems like a voltage dubler..)

Your IC2b Out (pin 7) is unacceptable..
(IC2b is too close to hit it's negative supply rail..)

[GeorgeG]

I found the problem. Actually the only voltage that was way off was IC2 pin 1. Turns out that R20 is tucked away between some components with very narrow spacing and I missed it so the gain of that op amp was too high due to the lack of feedback resistor. hence the pin 1 voltage being too high. I fixed the problem and now all controls work through their entire range and there is no unstable function.

Unfortunately even though I could not use the gain past 11 o clock before the pedal became unstable, I like the sound way better when it was "broken". One of the best pedal sounds ever. Any thoughts on why? I guess that negative feedback loop of resistor and capacitor affects the gain of the stage but that's also an integrator/low pass function. With only a capacitor I guess its a low pass filter with uncontrolled gain near DC so low frequencies were preferentially enhanced. That explains why it sounded fatter before but now its also buzzier. The distortion is not as mellow.


George