probing

[orange418]

Hi~ I just assembled an audio probe recently and I was testing it out with a schematic except I'm not sure what I should be hearing..I get audio through in non bypass mode using my guitar as a signal generator( probably a bad idea) and I get normal volume at my coupling caps, but at my second NB4558 on various pins I get a high pitched squealing noise..and also when I probe the legs of the rate pot... I understand I'm not supposed to hear anything for the filtering caps( that includes tantalums right?) but how would I know if they're bad.. or why my pedal is squealing..There is no occillation anywhere like, you know, if you had you pedal plugged in with no guitar and can still hear whooshing noises.. Thanks, Im not an engineer..=( Any tips would be great..

[GibsonGM]

Hi orange,

I think the squealing you're hearing with the 4558 depends on which pins you probe.   Power supply pins would make some noise like that.  GND pin should not (as long as your probe is grounded...it is, right? ;o) 
What effect are you probing?   You should follow the input signal thru the input cap, up to the 4558 input, and then check the IC's output pin...then on to the next stage, all the way to circuit output.   Your probe has a capacitor on it, to stop DC voltage, right? ;o)

#1 is check that the IC and everything else has proper voltages...this often will tell you if your caps are good (the ones not in the signal path).   A short could cause the 2nd opamp section to squeal at +,- or output....depending on what the pot is supposed to do, it might make noise if it has V+ on it, or you could have some components assoc. with the 2nd OA section that are in wrong or are defective....probing will point you to where to trouble is, but sometimes you have to play detective when something's not working.  See the thread  "Debugging" "What to do when it doesn't work" for more info!  The IC could have blown due to a short, but I'd go there last, after having found what's causing the trouble.  Sometimes you really have to get in there with a magnifying glass & strong light to find solder bridges...

In the future...please tell what the ckt is, link to the schematic, what your IC, FET or BJT voltages are, and what pins are making what noises.   Keep at it, you'll get it to work, it's usually something simple 

[jonathan perez]

THE ALIENS ARE COMING!


NO PROOOOBING!!!! 

[MikeH]

A good DMM with a continuity meter (tells you if two points are connected or not) is possibly the best debugging tool to have.  It will tell you when components are connected that shouldn't be because of minuscule invisible solder bridges, and also if components that should be connected are not, even when the solder joints look perfectly fine.  90% of my debugging solutions come from continuity checking.