Callate 2 (MXR Gate)

[jfrabat]

Why the hum with 6.2V zener (V=?) and not with 5.1V zener (3.93V)?.

Important note; when I tried the pedal with the other zener, the amp was VERY low (kids were asleep) so I am not so sure the hum was not 5here before.  I just noticed it now...

[Rob Strand]

Important note; when I tried the pedal with the other zener, the amp was VERY low (kids were asleep) so I am not so sure the hum was not 5here before.  I just noticed it now...

Ah OK.   I guess the important thing is it is there.   You can get weird stuff like that happening.   If you want to check the previous configuration again it's up to you.

[jfrabat]

Important note; when I tried the pedal with the other zener, the amp was VERY low (kids were asleep) so I am not so sure the hum was not 5here before.  I just noticed it now...

Ah OK.   I guess the important thing is it is there.   You can get weird stuff like that happening.   If you want to check the previous configuration again it's up to you.

What do you recommend?  You know more than me, so I will follow your lead...  It's not like the soldering iron is not out (working on a Super8 looper anywaY!).

[Rob Strand]

What do you recommend?  You know more than me, so I will follow your lead...  It's not like the soldering iron is not out (working on a Super8 looper anywaY!).

BTW what are you powering the effect from?

The idea is to find out where how the hum is getting in.  One important thing try to get an idea in you mind how loud the hum is now before we change anything.  Some of the tests might reduce the hum but not remove it but that might help work out what is wrong.

Try this sequence of tests:

Short the collector and emitter of Q2 again like we did before.
Set the sensitivity pot in different positions min, 12 O'clock, max
and see if it changes or removes the hum.   

Leaving the Q2 collector-emitter short  in place.
Change the 680 ohm to something like 3.3k  (anything from 2k2 to 4k7).
See if there is any change in the hum with different settings of the
Sensitivity pot.

If you didn't get any hum at all in the above tests.  Remove the short on
the collector and emitter of Q2. Then again see if there is any change
in hum with different settings of the Sensitivity pot.

You could also try putting a 100uF cap across the power rails.

See how you go.  What to do next depends the above results.

[jfrabat]

BTW what are you powering the effect from?

The idea is to find out where how the hum is getting in.  One important thing try to get an idea in you mind how loud the hum is now before we change anything.  Some of the tests might reduce the hum but not remove it but that might help work out what is wrong.

You know, now that you mention it, it could be a noise power source.  Mine is a regular 9V source (not for pedal, but regular electronic one) with a long cable (I spliced 2 together to make it about 8 feet or so in length).

Currently, my pedal box uses this power source, but I just ordered this one yesterday because I am getting a bit of hum from the first one, and I have read it is not really isolated (and when I run the looper, all pedals will be on at once).

Try this sequence of tests:

Short the collector and emitter of Q2 again like we did before.
Set the sensitivity pot in different positions min, 12 O'clock, max
and see if it changes or removes the hum.   

Leaving the Q2 collector-emitter short  in place.
Change the 680 ohm to something like 3.3k  (anything from 2k2 to 4k7).
See if there is any change in the hum with different settings of the
Sensitivity pot.

If you didn't get any hum at all in the above tests.  Remove the short on
the collector and emitter of Q2. Then again see if there is any change
in hum with different settings of the Sensitivity pot.

You could also try putting a 100uF cap across the power rails.

See how you go.  What to do next depends the above results.

OK, will try to do this tonight (at least advance some).  Will let you know how it goes...

[Rob Strand]

You know, now that you mention it, it could be a noise power source.  Mine is a regular 9V source (not for pedal, but regular electronic one) with a long cable (I spliced 2 together to make it about 8 feet or so in length).

The MXR noise gate has some design issues that lets hum through from the power rails.
The main one is the 1M + 1M divider which feeds the last opamp (pin 5 on Tone-pad's schematic).  FWIW, the cap here actually stuffs-up the linearization of the JFET.  The second is the zener is running at low currents.

A fix for power supply hum is to put a 100uF cap in place of the 10uF cap across the power rails.  Then add say a 47 ohm in series with the +9V lead between the power inlet +9V and the PCB +9V.

If that's the problem it's much easier to solve than some obscure circuit behaviour.

[jfrabat]

You know, now that you mention it, it could be a noise power source.  Mine is a regular 9V source (not for pedal, but regular electronic one) with a long cable (I spliced 2 together to make it about 8 feet or so in length).

The MXR noise gate has some design issues that lets hum through from the power rails.
The main one is the 1M + 1M divider which feeds the last opamp (pin 5 on Tone-pad's schematic).  FWIW, the cap here actually stuffs-up the linearization of the JFET.  The second is the zener is running at low currents.

A fix for power supply hum is to put a 100uF cap in place of the 10uF cap across the power rails.  Then add say a 47 ohm in series with the +9V lead between the power inlet +9V and the PCB +9V.

If that's the problem it's much easier to solve than some obscure circuit behaviour.

Then let me start with that, as it is a known issue (and even if I solve the other one, I will need to look into this one!).  So if I understand you correctly, this means replacing the 10 uF electrolytic cap on the top right (that's the one that connects the power rails, right?).



with a 100 uF one?  And add a 47 Ohm resistor somewhere between the power jack and the board?  If this is so, it is simple to do (the pedal does not yet have a board for it, so I can easily add the resistor and heat shrink wrap it).

[Rob Strand]

with a 100 uF one?  And add a 47 Ohm resistor somewhere between the power jack and the board?  If this is so, it is simple to do (the pedal does not yet have a board for it, so I can easily add the resistor and heat shrink wrap it).

Yes that's it.  Should be easy to do.

[jfrabat]

with a 100 uF one?  And add a 47 Ohm resistor somewhere between the power jack and the board?  If this is so, it is simple to do (the pedal does not yet have a board for it, so I can easily add the resistor and heat shrink wrap it).

Yes that's it.  Should be easy to do.

OK, that's done.  I messed up in my description, though; I remembered using the power supply, but I used it on the Green Russian I connected with the gate to try the gate function (that pedal is noisy!).  But the gate I actually powered through a 9V battery.  Here is the issue I have:

Especially at the end of the chord, you can clearly hear the hiss I mentioned...

PS: Pardon my narrative; English is not my first language...  and I have not practiced it in some time!

[Rob Strand]

Especially at the end of the chord, you can clearly hear the hiss I mentioned...

Ok that's helps a lot.

I am fairly certain it is coming from IC1a (on tonepad's schematic).  So when you have the Sensitvity pot full IC1a is clipping very hard somehow the clipped signal is finding its way back into the audio.  These problems are tricky to identify and solve.

The question is how is is getting in!   

So here's the possibilities:

- Through the power supply.   Lucky you have the 100uF on the power supply because the cap should help reduce the effect.   For now we will assume this is not the cause.

- Coupling between the tracks.  When I look at the layout the clipped signal from IC1a passes through the 47nF capacitor then that track runs right next to all the audio signals.  So that is asking for trouble.  Without re-laying out the PCB there's a some things to try.  Put a 220ohm resistor in series with the 47nF at the output of IC1. 

- Oscillation of IC1a.  The input of IC1a (pin 3) runs right next to the output on pin 1.  That's also asking for trouble.  The thing is through, you problem gets worse when the sensitivity pot is full.  In this case, I'd probably expect oscillation to get worse when the pot was at the center.   The oscillation can be from loading the output with a capacitor (the 47n+1uF in the circuit) and the 220 ohm should fix that.

So try the 220 ohm resistor. See what happens then we will take it from there.  Don't take it out because we might need it in the next steps.

PS: Pardon my narrative; English is not my first language...  and I have not practiced it in some time!

Your English is great.  Really clear.

[jfrabat]

So try the 220 ohm resistor. See what happens then we will take it from there.  Don't take it out because we might need it in the next steps.

OK, will try it tomorrow and see how it goes...  Just to be clear, I would be taking one leg out of the cap and the resistor would be going from the newly freed leg to the old pad, right? 

I am fairly certain it is coming from IC1a (on tonepad's schematic).  So when you have the Sensitvity pot full IC1a is clipping very hard somehow the clipped signal is finding its way back into the audio.  These problems are tricky to identify and solve.

The question is how is is getting in!   

So here's the possibilities:

- Through the power supply.   Lucky you have the 100uF on the power supply because the cap should help reduce the effect.   For now we will assume this is not the cause.

- Coupling between the tracks.  When I look at the layout the clipped signal from IC1a passes through the 47nF capacitor then that track runs right next to all the audio signals.  So that is asking for trouble.  Without re-laying out the PCB there's a some things to try.  Put a 220ohm resistor in series with the 47nF at the output of IC1. 

- Oscillation of IC1a.  The input of IC1a (pin 3) runs right next to the output on pin 1.  That's also asking for trouble.  The thing is through, you problem gets worse when the sensitivity pot is full.  In this case, I'd probably expect oscillation to get worse when the pot was at the center.   The oscillation can be from loading the output with a capacitor (the 47n+1uF in the circuit) and the 220 ohm should fix that.

One of these days you will need to point me to some text to study a bit more.  That explanation is still over my head!  I kind of follow you, but I still don't get the logic of how it all works (or why).  But I CAN follow instructions!  LOL!

[Rob Strand]

OK, will try it tomorrow and see how it goes...  Just to be clear, I would be taking one leg out of the cap and the resistor would be going from the newly freed leg to the old pad, right? 

Yes, that's correct.

One of these days you will need to point me to some text to study a bit more.  That explanation is still over my head!  I kind of follow you, but I still don't get the logic of how it all works (or why).  But I CAN follow instructions!  LOL!

To tell you the truth some of these pedal problems are very tricky.  At this point I don't know what the problem is,  I only know where it might be (and still could be wrong).   The idea is to try a few things to narrow down the possibilities.   Debugging via forums takes a bit of patience and perseverance.

[jfrabat]

Resistor is in.  No change in hiss.

[Rob Strand]

Resistor is in.  No change in hiss.

OK, no problem, just leave it  we might need it later.

So the next step would be to increase the 47pF to 100pF.  It's the ceramic cap next to the IC.  You could solder a 47pF in parallel with the existing 47pF it that's easier.  It's not actually shown on the tonepad schematic but it goes in parallel with the 1M resistor on IC1a.

[Fender3D]

You may add a bigger cap @ Vb also....

actually the issue is the dual op-amp: IC1a has a huge amount of gain... MXR used 2 singles back then, and kept them far apart each other

[jfrabat]

You may add a bigger cap @ Vb also....

OK, so whats is Vb again?  Sorry, I am a newb and do not yet understand electronics...

[Fender3D]

Vb is Vb on schematic

[jfrabat]

Vb is Vb on schematic

Ah, that makes sense...  So you mean the 10uF electrolytic that is in the same line as the diode?  If so, that would be the one marked below, right? (trace marked in yellow)





If that is correct, how much bigger?  I have 22uF, 33uF, 47uF, 100uF, 220uF...

So the next step would be to increase the 47pF to 100pF. 

Done.  Still there is hiss...

[Rob Strand]

You may add a bigger cap @ Vb also....

Certainly worth trying.

Another thing worth trying in this area is to reduce the 22k on the zener.  As a test try 2k2.   

Done.  Still there is hiss...

In your video  you increase the sensitivity to full, then play the chord you hear a hiss at the ends.   There's two things that is not clear to me about that test:
- Does the hiss stay there forever after the note dies down?  For example if you hold the strings or turn down the guitar volume to minimum.
- When you increase the sensitivity to full does the gate stay open without any notes being played?

So one test worth doing at this point is to lift one end of the 10uF going to the JFET.   Then see if you hear noise with the sensitivity on full.   The gate will not longer work but it will pass audio at full level.   The idea behind this test is to see the noise is getting into the audio path via the JFET (actually the JFET modulation) or via the power rails or PCB tracks.

[Rob Strand]

I tried to determine the nature of the noise from the audio in your video.  I took the tail part of the audio, heavily high-pass filtered it at 1kHz and amplifier it.  I head a constant noise which has an unnatural character.   

So the bigger Vb cap and the stuff I mentioned in my last post should help narrow down:
- oscillation
- modulation from the JFET.  It's possible that noise from the guitar or input is triggering the gate circuit and modulating the audio.
- noise from the zener.

Something I wasn't clear on as well,  it's important to know if the noise *suddenly* appears at a certain pot setting, and, if it gradually gets worse as you rotate the pot to full.