Audio Probe Newbie Questions

[fishlore]

Hi I'm new here and new to all of this.  I'm using an audio probe to try to isolate why I'm losing distortion in a pedal.  The distortion (and pedal as a whole) was working intermittently as I wiggled things before the distortion finally stopped after I attempted a few "fixes".

Question:
Would I expect all components to generate a tone through an audio probe if the components and traces are working properly?
- I know resistors do, I *think* caps do(?), diodes are what I'm worried about.

https://3.bp.blogspot.com/-zsZWL4qYD0o/VeH8ZTwOGqI/AAAAAAAABjo/0ppbO2vTM7o/s1600/Klon-Centaur-schematic.gif

I have distortion at C9.  I have distortion at R13.  I have nothing at C10.  Dead silence.  I re-soldered the cap, but the problem persists.  One of my beloved germanium diodes looks cloudy and one doesn't.  Neither generates any tone at all on the audio probe.  I think I'm using a multimeter correctly and seeing about 0.2 on the 20K Ohm setting for each of these diodes.  Am I correct in assuming that somehow my distortion is being dumped to ground in this area somewhere, somehow?  Could it be something other than a bad diode doing this?  The last time I messaged my voltages on the ICs they seemed correct, but can redo if needed.

Thank you for your time.

[DIY Bass]

Do you have a diode measure mode on your multimeter?  You could measure forward voltage of your diodes to check.  If you are measuring something on the resistance scale though then you are unlikely to be dealing with a short.  Are you getting sound on both sides of R13 or just one?  If you are getting sound on both sides, but nothing at C10 the n I would be suspecting a bad connection in there somewhere.  Check for continuity.  If you get nothing on the C10 side of R13 then definitely check continuity from that point to ground.  If there is then you could have bad diodes, but there could also be a path to ground there through the footswitch for the LED if it is bad as well.  Maybe temporarily remove the diodes (or lift on end) and see if the signal then goes through.  If so you probably have a bad diode.  If still it doesn't then check the switch.

[idy]

I would also say look at that foot switch pole... That is a trick to kill all the signal that would normally go to those diodes and, after clipping, on to the mixer op-amp and the tone control. A short there would do exactly what you are complaining about.

The original design is to kill the distortion path of signal so it doesn't bleed into the buffered signal when you are in  "bypass."

[fishlore]

I don't see any continuity issues.  I can trace the circuit through this area and beyond.  I have sound on the audio probe on the C9 side of R13 but I don't have it on the C10 side of R13. Definitely nothing at C10 but do have continuity there. I feel dumb, but what does that tell me?  I'm not sure what D.SW is.  Is that the footswitch?  I hadn't thought about an LED issue.

Does a cloudy germanium diode mean it's bad?  Is a bad diode likely the issue?  Could I use a cheapo to verify?  If so anything in particular?

Thanks a ton for your input.

[fishlore]

I would also say look at that foot switch pole... That is a trick to kill all the signal that would normally go to those diodes and, after clipping, on to the mixer op-amp and the tone control. A short there would do exactly what you are complaining about.

The original design is to kill the distortion path of signal so it doesn't bleed into the buffered signal when you are in  "bypass."

Thank you idy.  When you say to check the footswitch pole, what exactly do you mean?  I have a pcb on the footswitch.  I also did pull a pad off on R28.  I put the resistor on the opposite side of the board, but I could have sworn I had distortion after that, but maybe not.  I guess I didn't put two and two together that my distortion would get eaten this way.

Thanks a bunch

[idy]

You could check for continuity to ground between C9 and C10, where your sound vanishes. That would equal a short to ground. If that switch is shorted that would be a path...

Figure out which of the rows of contacts on the foot switch is attached to the LED... It's job is either grounding the LED or the sound between C9 and C10.

The middle contact will be ground, the one above or below will be the path to the LED, the other will be that point in the circuit between C9 and C10.

If the diodes were shorted that would have the same result, a path to ground for the signal. More common for them to break and not conduct at all.

The diodes clip the signal, *any* diode will work, but the designer of the original circuit thought he had some magic ones. The circuit is tuned around using low fv diodes (you would use the diode setting on the meter and it would say something like .2 or .3.) They clip a signal which may be already distorted by the OpAmp. Many would try other diodes (originals are Germanium) to get slightly different flavors.

[idy]

And R28 doesn't do much (along with R26 and 27.) This pedal has both the clean and dirty side always connected to the output, but when you switch, one gets a resistor in the way dropping its power. That and the "muting" of the distorted side where I described above accomplishes the switching. By having both sides "kind of" attached all the time it makes the switching quieter. Not an abrupt back and forth, just shorting out a R in one or the other path. Fancy.

[fishlore]

You could check for continuity to ground between C9 and C10, where your sound vanishes. That would equal a short to ground. If that switch is shorted that would be a path...

The C10 side of R13 has continuity to ground.  Each leg of each of the germanium diodes had continuity with itself, and with ground, as well.  I can't even imagine trying to unseat that switch from the pcb if the issue is there.  I suppose I'll reflow the solder on each pin and hope for the best.  A few pins might be a bit on the light side, but I was worried about using too much and making a short underneath, and yet that's probably what I did.  Ugh.

I should add, that I tested continuity to ground with the switch in both positions, but concentrated on the on position.

Thanks a million. 

[idy]

Are you clear about "diode/continuity mode" on your meter? It usually has a picture of a diode and sometimes a symbol for sound so it will beep when the probes are shorted. If they are across a diode (in the right direction) they will tell you voltage at which the diode 'turns on.'

Between C9 and C10 should have continuity to ground but only in bypass mode. When you hit the foot switch the LED should light and  there should be no only a path to ground through the diodes, the meter should say (in diode/continuity mode) .2 or .3V. From that point to ground. Probably best to disconnect power to the pedal before these tests. There should not be DC voltage at that point, but still.

The diodes should not show continuity when tested individually, they should show  (in diode/continuity mode) .2 to .3 volts (with the pedal turned off.) You can test them in circuit.

You still haven't confirmed that the switch is working, showing continuity between either top or bottom row and middle row, switchable.
If it comes to replacing the switch you would probably want to replace the switch PCB (if is a separate board.)
Yes the switches get damaged by too much heat.

[fishlore]

Thanks again Idy.  TIL, that the beep mode had a picture of a diode too.  I do have a diode/continuity position.

Starting with the switch:

1-2-3
4-5-6
7-8-9

Power provided in State 0 (no led):
continuity between -
1-4, 2-5, 3-6 ** but also 1-7.  Testing 2-8 makes the Led glow dimly with no beep.

Engaging switch to State 1 (lit led):
continuity between-
4-7, 5-8, 6-9 ** but also 2-8.

Verifying with LED light on: I have continuity to ground at C10.  I have the same continuity with the switch off.  Both states have C10 going to ground.

As far as the diodes:
Power plugged in, not plugged in, pedal on, pedal off, all results are the same for D1/D2 (ge)
the diodes always beep no matter how I try to test them.  The number on the multimeter always quickly drops to zero.  The behavior of the germanium diodes is vastly different than the other diodes on the board.  With the others I don't get a beep and a three digit number stabilizes when I test them.  Under a jewlers loupe D1 is clear and see through all the way to the board.  D2 is cloudy and charred looking and I can't even see into it, let alone through it.

I don't know if the pedal switch behavior is normal.  I'm really concerned D2 is ruined, but I'm not sure if that alone would create the problems I'm seeing.  My initial thought is changing diodes is a lot more appealing to me than changing the switch so I think maybe I'll sacrifice the ge diodes and throw in some schotkey's or something.

Thanks!

Edited: Removed the Ge diodes D1/D2.  Replaced with Si.  D2 was definitely shot.  Either way problem still exists and resigned to the fact that it's the switch.  The Aion kit has quite a few things on that PCB so I'm not sure what I'm going to do.

Either way thanks to everyone.