When I'm audio probing a circuit, should I be getting signal on the output side of the resistors?
depends of what part of the circuit
Circuits have output sides.
Resistors do not.
i think he means output as "the other side" of resistor... kinda strange question
yeah sorry. let me clarify. When I audio probe circuits and I am tracing the signal path, it is fairly common that I get to a resistor and the signal doesn't continue through it. Is this common or does it mean there is something wrong with the resistor or the solder point?
Usually that'll mean either a bad solder joint, or a burnt resistor... Just double check...
If it still doesn't work, it might be some sort of loading issue, and you should check further down the circuit to see if the signal still doesn't come through... If it doesn't... see above...
Your question may seem simple, but to answer it correctly, more information would be needed.
While the signal does travel from the input of the circuit to the output, it does not necessarily appear on every component. When using the audio probe (a great debugging tool), you need to know the 'signal path' - that is, the parts of the circuit that actually carry the signal.
Some of your components will connect to +9v, while some will connect to ground. The audio probe will not show any signal on the ends of the components connected to these points. If one end of your resistor is connected to 9v or ground, then no, you should not hear any signal. That is normal. If we knew the schematic you were working from, folks here could point out the signal path, and tell you what components should have signal present on them.
A little tip I've found helpful when audio probing. (i've built two pedals so far, and neither worked the first time cuz I'm a complete noob....so this is one area I have unfortunately had to gain some experience in.)
I look at maybe 4 or 5 main areas in the circuit and probe them first. For instance, The inputs and outputs of the circuit, to make sure your signal is getting to the board and if the board is working....that your signal isn't getting lost on the way out (like wiring a switch incorrectly, or a jack.)
I also like to start at the op amps and transistors. Just probe the ins and outs, to see if your piece isn't orientated incorrectly (i put the transistors in wrong on the tubescreamer at first). When I had sound going in and not going out...I figured something was up.
I know many people just trace it all the way through....but I for some reason find that difficult. I tend to find a point where it works and a point where it doesn't, and then I know my problem is somewhere in between. And then I pick a spot in the middle of that...and same thing. Eventually I move closer to the problem.
As for your not getting sound on resistors....somebody already said it. Not all components are part of the signal path. Sometimes they are there for other purposes such as to manipulate the power, and so if you touch any component that is carrying straight DC, you shouldn't get any sound (assuming you put a cap on your audio probe, otherwise you'll get a nice pop/scream/wail of death.)
good luck
yeah. I have a cap on the audio probe :icon_lol: So let me just give a specific example. One of the circuits I'm working on is a Rebote 2.5 from Tonepad and using the Tonepad layout which is beautifully color coordinated to show the different source paths. When I trace the signal path from the input jack, I get to a certain resistor and the signal just stops. I can pick it up again further down the line but that source could be coming from somewhere else. I've found this to be the case with other circuits too but maybe with those, I just wasn't paying attention to the layout and was probing resistors on the +9V path. So on my Rebote though, could that problem be a burnt resistor?
Quote from: studiostud on August 27, 2008, 02:16:39 PM
yeah. I have a cap on the audio probe :icon_lol: So let me just give a specific example. One of the circuits I'm working on is a Rebote 2.5 from Tonepad and using the Tonepad layout which is beautifully color coordinated to show the different source paths. When I trace the signal path from the input jack, I get to a certain resistor and the signal just stops. I can pick it up again further down the line but that source could be coming from somewhere else. I've found this to be the case with other circuits too but maybe with those, I just wasn't paying attention to the layout and was probing resistors on the +9V path. So on my Rebote though, could that problem be a burnt resistor?
I'll expand on that "resistors don't have outputs".
The correct answer to your question is "Yep, that could be a burnt resistor. But it also might not be a burnt resistor, too. No way to tell without knowing what the circuit and the resistor is doing right there. It may be perfectly normal for the signal to drop to zero or not." You happen to have come up with a question with no definitive answer.
Any time you think you have a burnt resistor, the next step is not to reach for your internet keyboard, it is to flip the DMM to measuring ohms and measure the resistor value. If the resistor is OK, it will measure something like the value printed or painted on the resistor. While the audio probe is a good instrument for tracking signals, it is not perfect. You have to use all of the tools available to you.
QuoteAny time you think you have a burnt resistor, the next step is not to reach for your internet keyboard, it is to flip the DMM to measuring ohms and measure the resistor value. If the resistor is OK, it will measure something like the value printed or painted on the resistor. While the audio probe is a good instrument for tracking signals, it is not perfect. You have to use all of the tools available to you.
Make sure your circuit is NOT powered up when you use your DMM to measure resistance.
Quote from: demonstar on August 27, 2008, 05:33:02 PM
Make sure your circuit is NOT powered up when you use your DMM to measure resistance.
Now that is something I didn't know. I'll definitely remember that. Thanks for all the great advice!