Probing and capsasitors.. Whats up with Mosfet circuits?

[syndromet]

Just a quick queation...
I'm building AMZ Mosfet boost,and it doesn't work, so started probing it. I get no sound after the firs cap. Should I get sound here, no matter what is wrong later on in the circuit? How do I check if the capasitor isn't working? Almost all my resistors have somewhat different values fom the schem, as I'm using what I have laying around.

[GibsonGM]

Hi Syndromet,

I haven't really looked at the mosfet boost circuit, but assiming you're just running your guitar signal in thru the input capacitor, yeah, you definitely should have a signal at that point!  I would check my ground connections, make sure they're solid, make sure the guitar has output by going right to an amp (keep it simple, make sure of the little things first)...or the cap might be shot. Replace with same or close value. 

A good cap will show very high resistance on an ohmmeter which then decreases as the cap charges thru the meter, eventually looking like an open.  This works up to like 5uF...I'm sure your input cap is that size or smaller.   I use a DMM with a capacitor function to test mine, never fails.
Good luck!

[syndromet]

Hi Syndromet,

I haven't really looked at the mosfet boost circuit, but assiming you're just running your guitar signal in thru the input capacitor, yeah, you definitely should have a signal at that point!  I would check my ground connections, make sure they're solid, make sure the guitar has output by going right to an amp (keep it simple, make sure of the little things first)...or the cap might be shot. Replace with same or close value. 

A good cap will show very high resistance on an ohmmeter which then decreases as the cap charges thru the meter, eventually looking like an open.  This works up to like 5uF...I'm sure your input cap is that size or smaller.   I use a DMM with a capacitor function to test mine, never fails.
Good luck!

Thanks!
Seemed like the cap did what it should, but I couldn't get the rest of the circuit to work. I decided to try the SSMB from dragonfly instead, but could not get this to work either. At least I get signal through, but it's lower than unity...
That gives me some more q's:
What is the importance of the 9.1V zener diode in thees mosfet circuits? The biggest zener i had was 7,5V.
On the dragonfly layout, how important is the value of R2 (3m)? I don't have a 3m resistor, so I tried substituting it with a 3,9M. This is a feedback resistor, right? Could this be why the volume is so low? Would it hurt to try a 1M here instead?

Thats a lot of questions, but I someone would care to answer them. I promise you all a soundclipp or ten if I can get this to work...
Marius

[GibsonGM]

hi,

Looks like you got thru the capacitor problem...now to get you running!  I went thru a lot of the same stuff when I first started putting FX together, Syndromet....I found it most helpful to build 1 on a breadboard, and then bust ASS to find out why it doesn't work!  Started with a simple 2 tranny fuzz face.  It's the only way to learn.  In solving the prob thru forums, net searches & the like, you learn, so next time it'll be easier!  you start seeing building blocks that you will remember what they do when you see them again.  Most of my mistakes (and I still make them) have to do with putting a part in the wrong place, mistaking pinouts...zoning out, lol.

    If you post, you might include a link to the schematic you're talking about...I don't know what the SSMB looks like, so I can't really tell you why it's not coming together.  In general, a zener diode used for voltage regulation's GOTTA be of the right value...depending on how it's used, it will probably leave you short a few volts as you have it now; maybe keeping you below a threshold that's necessary to function.  As for the 3.3M resistor..can you add a few in series to get 3.3M?   A 2.2M + a 1M would get you close.  Sometimes you have to improvise, like putting caps in parallel to get a value you don't have at the time.
Try to post a schem or link to one if you can, ok?  Don't be afraid to mess around with values, but you may pop something...good to keep extra transistors, opamps around, etc!

Later!

[syndromet]

hi,

Looks like you got thru the capacitor problem...now to get you running!  I went thru a lot of the same stuff when I first started putting FX together, Syndromet....I found it most helpful to build 1 on a breadboard, and then bust ASS to find out why it doesn't work!  Started with a simple 2 tranny fuzz face.  It's the only way to learn.  In solving the prob thru forums, net searches & the like, you learn, so next time it'll be easier!  you start seeing building blocks that you will remember what they do when you see them again.  Most of my mistakes (and I still make them) have to do with putting a part in the wrong place, mistaking pinouts...zoning out, lol.

    If you post, you might include a link to the schematic you're talking about...I don't know what the SSMB looks like, so I can't really tell you why it's not coming together.  In general, a zener diode used for voltage regulation's GOTTA be of the right value...depending on how it's used, it will probably leave you short a few volts as you have it now; maybe keeping you below a threshold that's necessary to function.  As for the 3.3M resistor..can you add a few in series to get 3.3M?   A 2.2M + a 1M would get you close.  Sometimes you have to improvise, like putting caps in parallel to get a value you don't have at the time.
Try to post a schem or link to one if you can, ok?  Don't be afraid to mess around with values, but you may pop something...good to keep extra transistors, opamps around, etc!

Later!

I was a little hasty when posting last night.
here is the layout.

Guess I could trace the schem if necessary.

Something is definitely wrong with my build. When measuring the voltages at Q1 i get:
D: 7,5V
G: 0V
S: 0v

I guess It's just the zener and the 3.9M resistor thats causing these problems. What are hey doing in this circuit? Would it work with zeners with a rating higher than 9.1? What happens if I put diodes in series?

And GibsonGM, Thanks for all your help

[GibsonGM]

No prob for the help, Syndromet...I'm in here reading up on all kinds of stuff, it never ends!  I have questions posted here, too.   

It's hard to see what the prob. might be based on just the layout...I know what you mean now.  I'd check each & every component for the right values. R2 is probably biasing the gate, so it's important.  R1 sets how much current can flow thru the unit...check out transistor biasing in here & on the net, you'll learn a lot.   Almost always, when something I do doesn't work, it's just that I made a stupid connection somewhere...and it happens all the time no matter how hard you try!   The #'s on the FET seem odd, but I haven't used them either!   I use bipolar transistors...

I'm suspecting a backwards diode, but you need the right zener in there to eliminate that as the cause.  I wouldn't use a higher rated one, it might be involved in discharging the big capacitor & could zap your fet.  Check polarities & for shorts.  A bad ground on the fet could also be involved, so check that all the way back!  Trying series diodes could work, but...at .7v per drop, that's 13 diodes ;o)  Final thought, a zapped fet maybe?

The reason I tend to stick with 'regular' transistors before I decide to build something is because it's hard to get stuff (I live in Maine, no electronics stores anywhere near so it all comes off the net).   Simpler to buy a bunch of transistors like 5088's for use later.  The best booster I've made is the Muff Boost...very simple, works awesome!!!  I just pulled it out of the switch setup, and removed the clipping diode section.   If you are still stuck later, I could post it this evening after work....the best thing is to get all the right parts before you try it, tho!  Unless you really like troubleshooting, LOL!  ;o)

Later,

Mike

[syndromet]

The Fet was broken..  I'm getting so used to doing mistakes, that I never suspect it to be a broken part. Now I have a nice clean boost with a lot of volume, just what I needed for this project.

[hairyandy]

Try that broken FET but put it in backwards.  Sometimes the pinouts on those things are not what you think and they'll still pass a little signal when in wrong.  Just a suggestion...

[petemoore]

The Fet was broken..  I'm getting so used to doing mistakes, that I never suspect it to be a broken part. Now I have a nice clean boost with a lot of volume, just what I needed for this project.

"a nice clean boost with a lot of volume"
  This sounds like a depiction of a working Mosfet Boost...
  Mosfets are even static sensative, I don't remember having one fail on me, just reading about that others have and that they can.

[syndromet]

Try that broken FET but put it in backwards.  Sometimes the pinouts on those things are not what you think and they'll still pass a little signal when in wrong.  Just a suggestion...

Yeah, I've read that too, somewhere. However, both fets came from Aron's shop, and they came in the same batch. They are both Fairchild, and the booster worked as it should.

I've hooked the booster up after a JD Easy Sim, and I'm gonna use this little circuit for direct recording on my PC's line-in. I'll give you a soundclipp as soon as I've boxed it all up.

Thanks for all the replays, by the way.