Debugging SansAmp gt2

[rvargas]

Hi everyone,

Yeterday I finished my build of the sansamp gt2 (using tonepad version), however it did't work the first try, totally dead. I then started following the signal using an audio probe. First thing I noticed is that the output of the first opamp stage (either one of the selectable ones) was VERY weak, I heard the signal but very soft. Given that one of those first stages has at least gain = 1, I suspect that could be a problem. In fact, when I first breadboarded the circuit before doing the PCB, this part was fine and got a decent signal out from there.

Now my suspicion is that it could be related to the bias voltage Vb. I measured a low voltage, around 2 volts.

This morning I started the debug again, started redoing some solder points around that seemed weak or that could be better. Started testing again and no signal at all :? at the same point I probed yesterday.. I probed the input of the opamps (pin 5 and 3, red circle in the schematic below) and I got no signal there! But I got signal immediately before the 10K resistor (blue circle). Changed the opamp (of course it was on a socket!) with no avail. I took out the chip and I got no signal either - well without chip I dont know, should I get a signal, between the 10K and 1M resistor?



Any help is appreciated.

[Govmnt_Lacky]

Check for a solder bridge between the the ground input at Pin 4 of IC1 and the input to the 10K resistor. Your signal might be grounding out there. You may have accindentally created a solder bridge when you " started redoing some solder points around that seemed weak or that could be better."
If you are getting an input signal before the 10K resistor but NOT after it. That should tell you what the problem is. Are you sure it is a 10K and not a 100K or 1M??