Callate 2 (MXR Gate)

[Rob Strand]

1.767V I am reading at the gate.  I am guessing that is within the OK range, right?

Yes it look OK.

You mean this one?
If so...  Actually, a whole LOT lower...  0V at the gate.  Source 3.88V, D 3.82V the drops to 3.53V.

Yes that resistor.
0V is correct.   
Sorry, I screwed up with the 4V number.   4V is what the JFET sees (gate to source voltage) but what you would actually measure is 0V.

So the important question, with the 100k removed does it pass signal?   and is it full level or down in level?

[jfrabat]

With JFET and 100K out, signal does go through.  Actually, even with JFET in, signal goes through, but no gating effect (regardless of pot position, sound is the same).

[Rob Strand]

With JFET and 100K out, signal does go through.  Actually, even with JFET in, signal goes through, but no gating effect (regardless of pot position, sound is the same).

FYI, pulling the 100k out is diagnostic step not a fix.   Also note there will be no gating as we have bridged out the collector and emitter of Q2 - another diagnostic step.

So just to be clear, we have to following results:
- with the JFET in and 100k in the unit doesn't pass audio
- with the JFET in and 100k out the unit does pass audio
Correct?

So that result would imply the JFET VP (Vgs_off) is too high for the circuit.    The circuit needs a JFET with a VP less than 2V.

So where to go from here:

It would be very useful to measure the VP (Vgs_off) of the JFET to *know* for certain what your particular JFET needs.   Do you think you could do that measurement? (the link at the end of my last post).   The problem is even though the unit is passing signal, ie. we have made an improvement, we still don't know if the JFET is fully off - even with the 100k pulled out.

As far as reaching a solution goes.   The problem is the VP (Vgs_off) parameter of your particular JFET isn't suitable for that circuit.  The reason is JFETs have tolerances and if you look at the datasheet it states VGS_off can be from 0.5 to 4V.   That circuit needs VGS_off less than 2V so you were unlucky enough to get one with a high VGS_off.   Now, while I didn't intend on pulling the 100k out to be a solution it can actually be used as a valid solution.   Pulling the 100k lets the gate voltage swing twice as much and that lets it handle VGS_off's upto about 4V.

[jfrabat]

So just to be clear, we have to following results:
- with the JFET in and 100k in the unit doesn't pass audio
- with the JFET in and 100k out the unit does pass audio
Correct?

Yes

So that result would imply the JFET VP (Vgs_off) is too high for the circuit.    The circuit needs a JFET with a VP less than 2V.

So where to go from here:

It would be very useful to measure the VP (Vgs_off) of the JFET to *know* for certain what your particular JFET needs.   Do you think you could do that measurement? (the link at the end of my last post).   The problem is even though the unit is passing signal, ie. we have made an improvement, we still don't know if the JFET is fully off - even with the 100k pulled out.

As far as reaching a solution goes.   The problem is the VP (Vgs_off) parameter of your particular JFET isn't suitable for that circuit.  The reason is JFETs have tolerances and if you look at the datasheet it states VGS_off can be from 0.5 to 4V.   That circuit needs VGS_off less than 2V so you were unlucky enough to get one with a high VGS_off.   Now, while I didn't intend on pulling the 100k out to be a solution it can actually be used as a valid solution.   Pulling the 100k lets the gate voltage swing twice as much and that lets it handle VGS_off's upto about 4V.

OK, the issue is my MM does not have a way to measure transistors/MOSFET/JFET/etc, so I would probably need a new one that has that feature (knowing the local market in Panama, it is highly likely it will be a Chinese brand, so quality is far from assured!).  But that I can fix by getting a new one (been thinking about it anyway). 

Now since the JFET is actually socketed, I could reproduce the circuit on the breadboard and we can try different things.  Now, I am a noob, so I need guidance on the fix, but reproducing the circuit should be simple enough.  Would that help?

[Slowpoke101]

Have a quick look to the image below;



Some MXR units came out with those component values. Changing the 1M resistor going from the JFET gate to Q2 collector can help with a JFET with a very low Vgss(off) characteristics. Also changing the zener to 6v2 can help.

Edit: Have now uploaded the correct picture.

[Rob Strand]

Yes

OK cool.  I think we are on the right track.

OK, the issue is my MM does not have a way to measure transistors/MOSFET/JFET/etc,

No multimeters will test JFETs.  To check the VP (Vgs_off) all you need is a volt-meter.

Wire-up the test circuit on the left.  I recommend adding a 220 ohm resistor in series with the power to prevent damage if you connect the JFET incorrectly.   Connect your multimeter (set to Volts) where it shows the "V".

Changing the 1M resistor going from the JFET gate to Q2 collector can help with a JFET with a very low Vgss(off) characteristics. Also changing the zener to 6v2 can help.

Changing the zener is OK we should get 1V or 1.5V extra drive that way.  Changing the 1M fixes one problem and creates another smaller problem.  The 1M helps linearize the JFET.   

Pulling the 100k doesn't affect the linearization however it could have an effect on the dynamics.  It's possible the cap needs to be decreased but making the cap too small also negates the linearization.

[jfrabat]

OK, been busy with work and stuff (had a business trip to Costa Rica, and since I had to stay Friday night, I took advantage and played with my old band there on a gig they had).

Battery: 9.22V
Vgs Measure: 3.60V

I recommend adding a 220 ohm resistor in series with the power to prevent damage if you connect the JFET incorrectly.   

Now I wanted to add this for safety, but I have NO IDEA what you meant, so I just double checked all the pinouts.  If you could educate me for the future, it would be greatly appreciated!

[Rob Strand]

OK, been busy with work and stuff (had a business trip to Costa Rica, and since I had to stay Friday night, I took advantage and played with my old band there on a gig they had).

No problem.

Battery: 9.22V
Vgs Measure: 3.60V

OK great.  So clearly your VP value is high and that's the problem.   To add to the problem your zener voltage is 3.93V, which is a little low.

So if you replaced the 5.1V zener with a 6.2V zener, as SlowPoke suggested, it would certainly help.   In fact that change alone will probably get it going.   See if you can get 0.5W zeners not 1W zeners.   If you still aren't happy after the zener change, the next mod would be to replace the 100k I mentioned before with say 1M.    You probably don't need to do that until you try the zener, it's another trick up our sleeve we can use if we have to.

[jfrabat]

So ideally I want a 1N5234?  If unavailable, 1N4735 would be the second choice?  Is this correct?

Just to double check (I do not think any of these will work), let me tell you what I have in my stash as far as diodes go:

Germanium

• DJ025 (I understand they are interchangeable to the 270 below)
• 1N270
• 1N914

Silicon

• 1N4148
• 1N4004

Zenner

• 1N4733

Schottky

• 1N5817
• BAT42
• BAT46
• BAT41

[Rob Strand]

So ideally I want a 1N5234?  If unavailable, 1N4735 would be the second choice?  Is this correct?

Yes, we can get the 1N4735 to work.

Just to double check (I do not think any of these will work), let me tell you what I have in my stash as far as diodes go:

No can't use any of those.

I have a feeling we might *need* to change that 100k to at least 680k also (the one connecting to the 1M and the 47n, near IC1b pin 6).  More drastic changes would be to change the 1M to 1k8 or 2k2, as in Slowpokes picture.

We *can* get it to work.

[Rob Strand]

Now I wanted to add this for safety, but I have NO IDEA what you meant, so I just double checked all the pinouts.  If you could educate me for the future, it would be greatly appreciated!

Sorry, I didn't get back to you question.   In the diagram I posted  the 9V rail connects to the JFET drain.  You "cut" that wire and place a 220 ohm resistor between two points.     The idea is if you connect the JFET incorrectly the current is limited to about 40mA (9 V / 220 = 40mA),  which is going to cause a lot less damage than "shorting" the battery.

[jfrabat]

OK, I got some news, and some decisions to make; I called a local electronic store, and asked them if the had any 2N5485 and 1N5234 and they said yes.  I asked my wife to get me 5 of each (you know, to keep in stock).  The diodes are in fact 1N5234 but the JFET I got actually reads K161 on top and GR 6K on the bottom.  I am thinking this is a 2SK161, which is NOT the 2N5485 I asked for...

Now, the question is, what to do.  Do I try the K161, or do I keep the NTE and change the diode?

[Slowpoke101]

It is a 2SK161-GR which should work but so should the NTE device.
Measure the Vgs(off ) voltage as you did for the NTE device - If it is lower than 2.8V it is worth trying.
We know that the NTE JFET has a Vgs(off ) of about 4V which is too high and will not work without modifications.

[jfrabat]

Measured Vgs is 2.41, so I figured I try it.  Checked pin-outs, and it is the same also.

Re-soldered the resistor in, de-soldered the jumper, and tried it.  WE HAVE LIFT OFF!!!!  (obviously, this is AFTER I remembered to take out the jumper!  Because, you know it, I tried it first without taking out the jumper and signal just went through!  LOL!)

Thanks to everyone for their help!

[Rob Strand]

Re-soldered the resistor in, de-soldered the jumper, and tried it.  WE HAVE LIFT OFF!!!!  (obviously, this is AFTER I remembered to take out the jumper!  Because, you know it, I tried it first without taking out the jumper and signal just went through!  LOL!)

Good stuff. 

You also changed the zener to a 1N5234 (6.2V 500mW), yes?

[jfrabat]

No, kept the one that was there.

[Rob Strand]

No, kept the one that was there.

With your zener at an actual voltage of 3.9V it might not be passing the signal at full level.
For VP = 2.4V you want the zener at more than 2*2.4V = 4.8V.   At a zener voltage of 3.9V
the JFET won't cut-off fully and you will get a bit of attenuation.

[jfrabat]

Let me test it this weekend and lets see how it goes.  I have the other zenner on hand, so it would be a quick replacement if I do need to replace....

[jfrabat]

Swapped out the Zener for the 1N5234.  Pedal works (gate function), but there is a slight "hum" when I play the B and high E strings (unless the gate is less than halfway).  It could well be the JFET (it is Chinese, and just looking at it, it looks a bit shady), so I tried the NTE, and that one is not gating; it lets signal through, but does not gate (at least not noticeably).

[Rob Strand]

so I tried the NTE, and that one is not gating; it lets signal through, but does not gate (at least not noticeably).

OK for this.  Don't worry too much.  I had a feeling the 6.2V zener wouldn't be quite enough to get that JFET working.  To use that JFET we would need to do further mods.  So the reason for that is understandable.

Swapped out the Zener for the 1N5234.  Pedal works (gate function), but there is a slight "hum" when I play the B and high E strings (unless the gate is less than halfway).

So that's more of a problem.

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

We know:
1) A 6.2V zener makes the JFET turn off more and hence the gate is open more.

2)  The "hum" reduces when the Threshold is turned down.
This will cause the JFET to stay on more often.

3)  From the previous result, the 2SK161-GR & 5.1V Zener (3.93V) didn't have a
a hum problem.  Here the is JFET probably not turning off completely.

All three cases are consistent from the JFET's perspective.  It seems the fully turning off the JFET is a factor.

However, it might be a symptom more than a cause!

So possible causes might be:

- Hum or noise is getting into the Threshold pot.  This circuit is high gain could pick-up hum/buzz.  The thing that add difficulty is you don't hear that signal directly you only hear it's affect on modulating the gain.   That's not really helped by the high value 500k pot.

- Oscillation of the Threshold amp.   That could cause also sorts of weird behaviour.

- When the Threshold amp is saturating it might be putting  junk throughout the ckt.  Perhaps a 100uF on the power rail will fix it.

I guess one question I have for you is, are you sick of debugging this or are you happy to see it through?

Some things I would try at this point would be:
- Try to make the wiring to the sensitivity pot neatly as possible, in particular the wire going to the pot wiper.
  Try twisting a grounded wire around the wiper wire.
- 100uF on the power rail.
- See what happens when you lift the 1uF cap.  Possibly replace it with 10n.
- Try replacing the 680 ohm resistor.  Try say 2.2k.

The aim is to see what fixes the "hum" problem.