Callate 2 (MXR Gate)

[jfrabat]

Hey, guys.  Some years ago I started building an MXR Gate (PCB from Tonepad), and I never did get it to work.  I just ran into it, and started looking it over.  And while I normaly would do things different now (like socket the IC!), I cannot find why it is not working.  I checked all the values of the resistors, caps, pot (A500K), transistors, etc.  Diode is 1N4733, which is a 5.1V Zenner...  I am using the 2N3904 for Q1 and Q2, NTE451 for Q3 (should work, as specs are almost identical; see here - oh, and it will be cut down; legs are left long for testing), and solder joints seem good.  Any ideas?













I am not sure what they mean by Ra in the resistors, but I used the same value it states in the build docs.

[Slowpoke101]

This noise gate can be a bit fiddly to get working but it is not too difficult. A quick look at your pictures doesn't show any thing wrong with construction that I can easily see. Good workmanship.
One thing to keep in mind is that the gate may actually be working correctly but it does not seem to. With the sensitivity control turned fully counter-clockwise (which one would would expect to be minimum ), the gate will be fully muted (closed ). A very large input signal would be needed to unmute (open ) the gate when set this way. Turn the sensitivity control fully clockwise and the gate should open, or open with a very small input signal. Basically the sensitivity control seems to work backwards to a volume control.

However, you may have a component problem or some error that I haven't noted yet, so some voltage measurements would be helpful - the IC, transistors and the voltage across the 5.1V zener (it won't be 5.1V, it will be less ).

[jfrabat]

This noise gate can be a bit fiddly to get working but it is not too difficult. A quick look at your pictures doesn't show any thing wrong with construction that I can easily see. Good workmanship.
One thing to keep in mind is that the gate may actually be working correctly but it does not seem to. With the sensitivity control turned fully counter-clockwise (which one would would expect to be minimum ), the gate will be fully muted (closed ). A very large input signal would be needed to unmute (open ) the gate when set this way. Turn the sensitivity control fully clockwise and the gate should open, or open with a very small input signal. Basically the sensitivity control seems to work backwards to a volume control.

Thanks for the workmanship comment!  I did move the pot to different positions, so it is not that...  In fact, most where around the middle, but I did go to both ends trying to get it to work.
 

However, you may have a component problem or some error that I haven't noted yet, so some voltage measurements would be helpful - the IC, transistors and the voltage across the 5.1V zener (it won't be 5.1V, it will be less ).

OK, some voltages:

Battery (at the board 9V input): 9.3V

IC (TL072CP):
1: 3.91V
2: 3.94V
3: 3.90V
4: 0.00V
5: 4.71V
6: 4.63V
7: 4.63V
8: 9.28V

Q1 (2N3904):
E: 3.12V
B: 3.54V
C: 9.24V

Q2 (2N3904):
E: 0.00V
B: -3.88V (weird, right?)
C: 3.43V

Q3 (NTE451):
D: 3.92V
S: 3.92V
G: 3.49V

Zener:
Cathode: 3.93V
Anode: 0.00V

[blackieNYC]

The voltages on pins 8&5 appear to be swapped.   Pin 8 should be the 9volt.  The pins from 1 to 8 wrap around in a U shape.  If you got the pinout wrong when you took those measurements you're probably OK at the TL072. 
Edit-He has corrected the posted voltages

[Rob Strand]

The voltages on pins 8&5 appear to be swapped.

I went to post that as well but I posted on the wrong thread.

One thing that's weird is most of your measurements show a your multimeter loading voltages but pin 5 doesn't show this effect!    Are the resistors on pin 5 1M?

FYI, see this thread,
https://www.diystompboxes.com/smfforum/index.php?topic=116755.msg1157922;topicseen#msg1157922

[Rob Strand]

This isn't the issue but, FYI, see updated schematic,
https://www.diystompboxes.com/smfforum/index.php?topic=122689.0

B: -3.88V (weird, right?)

The negative voltage could be due to rectification.

Maybe IC1 is oscillating.   People have had this happen in the past.  I saw this in my notes.  One fix was to add a 22pF to 100pF cap across the 1M ohm resistor on IC1;  see the notes on the updated schematic.

[Slowpoke101]

OK. I see from your voltages that you have mixed up the numbering of IC1's pins (5 to 8 ) and this has been pointed out previously. I also not that Rob has started a thread covering some errors that appear on some circuit diagrams of the MXR noise gate. Keep an eye on that thread as some interesting info is covered there.

The main issue at the moment is; Does the noise gate pass any audio at all or is it totally silent?
If you take the JFET out of the circuit, does the circuit now pass audio?

Things to note: On the Tonepad version the 10uF capacitor going to the drain of Q3 is shown with the correct polarity.
                      The 5.1V zener should be a 0.5W version and not a 1W version. Zener diodes do need some current to work correctly. Don't worry about this yet.
                      I see that you do have a small ceramic capacitor across pins 1 and 2 of IC1. This is good as it can help calm IC1 down a bit - not so inclined to oscillate.
                      There are some other modifications (component substitutions ) that would be a good idea when the circuit works correctly.

Rob does note that some people have experienced a sort of fizz noise with the circuit. I have experienced it myself with versions that I have built and some that I've had in for repair. It's nothing to worry about and can be controlled without too much stress.

[Rob Strand]

Things to note: On the Tonepad version the 10uF capacitor going to the drain of Q3 is shown with the correct polarity.

Actually, I think tone-pad's cap is flipped as well.

[Slowpoke101]

Things to note: On the Tonepad version the 10uF capacitor going to the drain of Q3 is shown with the correct polarity.

Actually, I think tone-pad's cap is flipped as well.

Rob, you are correct when it comes to the circuit diagram, but the component overlay shows the correct polarity (square pad being positive ). This did annoy me a few years ago when I first built the Callate 2 and it didn't initially work.

[Rob Strand]

Rob, you are correct when it comes to the circuit diagram, but the component overlay shows the correct polarity (square pad being positive ). This did annoy me a few years ago when I first built the Callate 2 and it didn't initially work.

Ah, now I see, the layout is different to the schematic.
Thanks!

Another one of those simple circuits that always has a bug somewhere!  So hard for the poor buggers building these things.  (I'm not bagging Tonepad here.   Francisco does a good job smoothing out a lot of bugs.)

[jfrabat]

The voltages on pins 8&5 appear to be swapped.   Pin 8 should be the 9volt.  The pins from 1 to 8 wrap around in a U shape.  If you got the pinout wrong when you took those measurements you're probably OK at the TL072.

You are right: I did not go in the U shape when I measured.  I started the second row from the top...  Corrected the original post to show correct pins.

[jfrabat]

One thing that's weird is most of your measurements show a your multimeter loading voltages but pin 5 doesn't show this effect!    Are the resistors on pin 5 1M?

Resistors are 1M.  They are the one under and the one to the right of the IC in this pic:



Brown Black Green gold = 1M
You can just make out the trace to pin 8 and pin 4.

The main issue at the moment is; Does the noise gate pass any audio at all or is it totally silent?

Completely silent

If you take the JFET out of the circuit, does the circuit now pass audio?

No

Things to note: On the Tonepad version the 10uF capacitor going to the drain of Q3 is shown with the correct polarity.
                      The 5.1V zener should be a 0.5W version and not a 1W version. Zener diodes do need some current to work correctly. Don't worry about this yet.
                      I see that you do have a small ceramic capacitor across pins 1 and 2 of IC1. This is good as it can help calm IC1 down a bit - not so inclined to oscillate.
                      There are some other modifications (component substitutions ) that would be a good idea when the circuit works correctly.

My Zener is a 1N4733; I think those are 1.0~1.3W, right?  Tried to check the spec sheet, but did not find it.

[jfrabat]

Things to note: On the Tonepad version the 10uF capacitor going to the drain of Q3 is shown with the correct polarity.

Actually, I think tone-pad's cap is flipped as well.

In my case, negative is going to ground, so I don't think that is the issue.

[Rob Strand]

Completely silent

I think this is the problem to address.

You should try to put a short across the collector and emitter of Q2.  Just solder a wire.
If the JFET is working then the sound should come through.
(I can see you have the links across AB and CD which you do need if you don't have the mods.)

My Zener is a 1N4733; I think those are 1.0~1.3W, right?  Tried to check the spec sheet, but did not find it.

Don't worry at this point.    I'm pretty sure the original used a 0.5W but the 1W should at least work.
The reason why the 0.5W is preferred is because zeners are only 5.1V at a particular current.   That exact current varies but for a 5.1V zener it could be anything from 5mA to 50mA.   The noise gate circuit operates the zener at *much* lower current, like 200uA.   What this does is it makes the zener voltage somewhat lower than 5.1V.   If you look at this pic there is an "upside down"  curve on the left.  Normal 5.1V is between the Imin and Imax points.  If you operate at very low currents you might get below the knee point and the zener voltage is somewhat lower than 5.1V.  The reason a 0.5W zener is preferred is it is more likely to have a knee point at a lower current than a 1W.  Anyway this is all technical details.   From your measurements we already know the zener operating at about 4V which is fine.

In my case, negative is going to ground, so I don't think that is the issue.

For the cap, don't worry about this also.  The layout has the cap around the right way.  The schematic shows the cap around the wrong way.  It's not the cap you mentioned going to ground it's the cap going to the JFET.
In your pics, it's the 10uF to the left of the 1uF NP cap.

[Slowpoke101]

Rob's suggestions are very good. The first step is to make the circuit pass audio and then move onto getting the gating function to work.

Having had another look at your pictures I have noted some possible solder bridges. Shown in the picture below are three arrows pointing towards some possible solder bridges. Check them and remove them if they are bridges.

[Rob Strand]

BTW, sorry for butting in on the thread.

[Slowpoke101]

Rob, there is no need to apologise for butting in...I welcome it and I am sure that everyone else does. The more info the better as far as I'm concerned.

[jfrabat]

BTW, sorry for butting in on the thread.

You are sorry for helping?  No apologies needed, my friend!!!

[jfrabat]

Having had another look at your pictures I have noted some possible solder bridges. Shown in the picture below are three arrows pointing towards some possible solder bridges. Check them and remove them if they are bridges.

Just checked; they are reflections.  No bridge (and checked with the MM also; no signal going through, except the middle one, but that signal is coming around the resistor and cap.

You should try to put a short across the collector and emitter of Q2.  Just solder a wire.
If the JFET is working then the sound should come through.

Will try this today

(I can see you have the links across AB and CD which you do need if you don't have the mods.)

So take them out then?

[Slowpoke101]

Leave the links across AB and CD (and leave the EF link in too ). You only remove them if you are wanting to use the attack and release modifications - don't bother with those mods at the moment.

I really did think that there was a definite solder bridge where the middle arrow was pointing. Oh well.

Try the short across Q2 and see if audio can get through. If it doesn't it is time to do some checks with an audio probe to see where it's getting lost.