Zvex SHO troubleshooting

[Lost_soul]

We call this "piggy backing", but I don't know if that idiom is used in your country.

I spent some time messing with the circuit and looking through datasheets for diodes. My best guess at the moment is that the "leaky" diode you're using is simply too good, not leaky enough. This would account for some of the funny time delay things you're seeing.

Do you have, or can you easily get some 1N4000 series diodes? They range from 1N4002 to 1N4007 part numbers, and are in black epoxy packages instead of the glass packages of the 1N4148/1N914 parts I think you're using. These are 1 amp rated power rectifier diodes, and they may have higher leakage currents than modern signal diodes like the 1N4148. The datasheets say they >>may<< have leakages up to 5uA. They probably won't be that big, but will probably be bigger than the signal diodes. They may work better.

What makes this hard is that the state of the semiconductor industry has made diodes so uniformly "better" that we are reduced to guessing which diodes are leaky enough for a use that is not what they were intended for.

Mike is an amazing person indeed!

I understand what you are saying with the piggy backing. You pack them like they are riding on the back of each other

I will see what i can do..

.............
You mean they (1N4148s) are not leaking enough to make this work? But when you made the Mil back then they were more leaky and that's what the circuit requires and based upon. Now i understand

Yes, i do have some 1N4007 diodes.
Should i use them instead?
And if yes, should i put it in place of the 1N4148 or parallel it?

[R.G.]

You mean they (1N4148s) are not leaking enough to make this work? But when you made the Mil back then they were more leaky and that's what the circuit requires and based upon. Now i understand

Yes, i do have some 1N4007 diodes.
Should i use them instead?
And if yes, should i put it in place of the 1N4148 or parallel it?

Yes, that is what I mean. Yes, try the 1N4007 diodes in parallel - piggy-back mounting if you can. It is not at all clear to me that they definitely will work, but at least they have a chance to work better. I think that the 1N4148 diode you have in the circuit leaks so little that it can be ignored and left in the circuit.

[GibsonGM]

Yes - please be careful. This is addictive.   

[Lost_soul]

Yes, that is what I mean. Yes, try the 1N4007 diodes in parallel - piggy-back mounting if you can. It is not at all clear to me that they definitely will work, but at least they have a chance to work better. I think that the 1N4148 diode you have in the circuit leaks so little that it can be ignored and left in the circuit.

Oh my god R.G.
It actually worked!
I used a 1N4007, mounted it on the copper side as i couldn't piggy back it because one leg of the 1N4148 was so close to the board.

I can't believe it actually worked finally!
I have been staring at it for 5 minutes now and it didn't shut down.


Thank you so much man. You, Mike, antonis, duck arse and everyone who helped me with this. I really appreciate it.

Only small problem left now is that there was a loud pop when you turn the switch on or off if i remember when i tested it (although the circuit has a pulldown resistor at the output).

[antonis]

Only small problem left now is that there was a loud pop when you turn the switch on or off if i remember when i tested it (although the circuit has a pulldown resistor at the output).

1 sec RC time constant (10μF X 100k) should be considered high enough..
More than 4 times RC is needed for output cap to be considered discharged..

http://www.learningaboutelectronics.com/Articles/Capacitor-discharging.php#:~:text=The%20Capacitor%20Discharging%20Graph%20is%20the%20a%20graph,after%20a%20given%20amount%20of%20time%20has%20elapsed.

What I mean is you can experience clicks/pops in case of frequent ON-OFF switching..
You could test it by reducing pull-down resistor's value significantly..
(of course, you'll alter circuit's gain 'cause pull-down resistor is effectivelly set in parallel with Drain resistor but you'll do it just to check if RC time constant is responsible for popping..)

[duck_arse]

Only small problem left now is that there was a loud pop when you turn the switch on or off if i remember when i tested it (although the circuit has a pulldown resistor at the output).

one test you need to do on popp fixing is to remove the mill and see if the pop varies. you hope it doesn't change.

[R.G.]

Good advice Antonis and Stephen. And that is the correct test, Stephen - see if the Millenium circuit is causing the popping.

I did notice that L.S. used the past tense "was" causing a loud pop. We know that there was something odd going on with the first circuit that did not work. It would be really good to know if the new, working circuit causes the same popping, in addition to to whether the now-working Millenium indicator circuit also causes it.

Lost Soul: can you try disconnecting the Millenium to see if it is causing the popping? And then try again with a lower-value output pull-down resistor? You can go as low as 10K with most effects, although 100K would be better if you can use it.

Long ramble on the workings of the Millenium Bypass follows.

For those of you who have not yet had Antonis' insight, the Millenium Bypass relies on a somewhat delicate balance of tiny leakage currents and the voltage those tiny leakages cause. When the effect is bypassed, the LED is turned off by the effect output resistor pulling the gate of the MOSFET to less than the threshold voltage for the MOSFET. For the 2N7000 that is 0.8V to 3.0v, and similar for the BSP170 and other small-package MOSFETs. But the leakage of the diodes pulls the output capacitor for the effect up by a voltage equal to the actual leakage current times the output resistor of the effect.

You're right, Antonis - reducing the effect pull-down resistor will reduce this offset if the leakage diode is what is causing the popping, and not an extra-leaky output capacitor on the effect.

The Millenium always causes an output offset. The reason to use the reverse leakage of a diode to bias the MOSFET on is to get the bias current to be very low so it does not pull up the voltage on the effect output capacitor "too much", which by definition is "enough to cause an audible pop".

That leads to another imponderable, almost un-measurable item: how big an offset voltage causes a pop. I tested this when I came up with the leakage-diode bias stuff. A resistor could be used instead of a diode to bias the MOSFET on. I messed with this a lot and came up with my personal answer/opinion: 15-20mV. A slow-ish rise of that amount is not noticeable even in high gain amps with distortion pedals except in very unusual cases. Smaller is obviously better, but engineering often tends to provide answers of "this is enough to work OK".

So the offset voltage on the output of the effect is the MOSFET bias current times the pull-down resistor. If the pull-down resistor is 1M, a 10M pull-up resistor as in the original Rat bypass makes the offset be a 0.909... times the power supply, or ab out 0.8V. A 100M resistor would get this down to 80mV. Changing to a 100K pull-down gets this down to 8mV, and that should be inaudible. This is the line of thought that led me to come up with the diode leakage thing. 100M resistors are kind of special things, although you can find them now.

For a 9V supply, a 100M resistor lets through 9/100M = 90nA, and that 90nA in a 100K pull-down gets the offset to 9mV. Most brands of 2N7000 and BSP170 have input gate capacitors in the range of 20pF to 60pF, although I did see one brand of BSP170 with a gate capacitance of ~ 350pF. The time to turn on the MOSFET is the time for the bias current to ramp up, say, 60pF to 3V, or t = 60pF*3V/I; for 90nA, this is
t = 60E-12*3/90E-9 = 2E-3 or 2mS. for 9nA, the time is 20mS, which is still instant to a human watching the LED. Well, assuming I didn't drop a decimal place while doing this in my head. 

So smaller is better for the leakage bias; and you need BIG resistors to do the pull-up. I first tried germanium diodes, and they worked great for turning on the MOSFET and LED, but caused audible pops. I went to ordinary silicon diodes, and their leakage was not enough, really. Enter the gold-doped signal diode.

Twenty five years on, we can't always get gold-doped signal diodes, and the leakage currents in diodes we can get will still bias the MOSFET on eventually, but the turn on can be very slow. For many users, adding a second signal diode doubled the leakage bias, and made it work. Lost Soul's setup may be having problems with this. Using a power rectifier diode that as been optimized by the manufacturer for different things than signal diodes seems to have added leakage bias back in. We just need to test how it needs the pull-down resistor to be juggled, and whether that is OK in the context of this effect.

It's a juggling act.

[Lost_soul]

Twenty five years on, we can't always get gold-doped signal diodes, and the leakage currents in diodes we can get will still bias the MOSFET on eventually, but the turn on can be very slow. For many users, adding a second signal diode doubled the leakage bias, and made it work. Lost Soul's setup may be having problems with this. Using a power rectifier diode that as been optimized by the manufacturer for different things than signal diodes seems to have added leakage bias back in. We just need to test how it needs the pull-down resistor to be juggled, and whether that is OK in the context of this effect.

It's a juggling act.

i am sorry but i just don't fully understand mose of this
this is some head spining stuff
but until i do....
i should try to just desolder the control wire from the DPDT switch and try again to see if the pop persists, right?

also a thing i noticed is that after like 10 minutes or something of plugging the board and see how it sounds, (just noodling) The LED went gradually more dim until it finally went out again!
i disconnected the 9v battery then after like an hour i reconnected it and same thing happened (after 10 minutes the light became dimer and dimer but i didn't see it go all out. maybe it was just about time)

does that mean the additional diode isn't providing the leakage bias the MOSFET needs?
should i try to add another diode? maybe a 1N4148 this time or what? and does this have something to do with the pop?

[GibsonGM]

Pops basically occur when you ask two circuits that are sitting at different DC voltage levels to connect. If there is a difference in DC voltage between them - and there almost always is - then you will hear them equalize to a steady state very quickly, and it is a "POP".  The pulldown resistor on an FX output is there to shunt any leakage from the output cap to ground (caps leak DC), but some circuits will still have a difference.  I bet if you check for DC voltage on the booster output you may find a few mV of DC there.

The other most common way we get pops is from the mechanics of the switch. A 'cheap' switch just does not work as smoothly or quickly as a 'quality' one, and that can make mechanical thumps.    Most pops are a combination of these things.  Remember, you are amplifying the switch noise in your amp, too!  So a small pop becomes a big, annoying "POP".

Does the pop become smaller if you switch the boost in/out, then do it again a few times right after?

If you can't resolve this, there are a few ways to make the pop smaller, let's get everything working right first, though.

I think if I had the LED problem you describe, I would add another diode in parallel and just see what happens (1N400x type). 

[Lost_soul]

Does the pop become smaller if you switch the boost in/out, then do it again a few times right after?

If you can't resolve this, there are a few ways to make the pop smaller, let's get everything working right first, though.

I think if I had the LED problem you describe, I would add another diode in parallel and just see what happens (1N400x type). 

No it doesn't become smaller if i do what you said unfortunatley

Nothing seems to work with this cursed build...

I added another diode (1N4007)



Nothing changed. I noodled around for about 20 minutes. Light was natural at first then dimmed until it was very very dim. I unplug the battery and plug it right away and it doesn't light up.

Idk if it's important to mention but when i touch the pin where control wire goes to on the switch the LED lights up!

I am sorry guys for bothering you with my stupid issue... I know it took a lot of your time and i appreciate your help.

[bluelagoon]

Time to get some new components
Check the original schematic, and rebuild it,
Then if that doesn't work,
Do it again and again until it does.
It is a pretty basic east circuit, where I am sure with a decent breadboard and the right parts you would get it going easy enough.
Understandably it isn't so easy to source your required parts where you are.

I do give you and the good folk who supported you here  A+ for trying everything to fix it,
But I think you may have trouble shot the legs off this one,
If possible might well be time to try a rebuild.
Could solve all your problems.
Good Luck Jim.