a little help with a mosface?

[skrunk]

hi,
I've just put a mosface fuzz together using the vero from dragonfly's gallery:

http://aronnelson.com/gallery/main.php/v/DRAGONFLY-LAYOUTS_0/album18/album144/AMZ_MOSFACE.jpg.html

and I'm having a problem.

everything works fine until I get to the 5.6k resistor at the BS170 (I used a 6.8k instead)..
using a probe I can follow the effected signal up to the BS170 side of the resistor, but loose the signal at the other side of the resistor, on the solder side of the board underneath.
I can't get a signal at the output cap at all.
shouldn't I be getting a signal at either pin of this cap?

I tried connecting a test lead from the 'hot' side of the resistor straight to the tip of the output jack and that works so it must be something to do with that output cap?

I can't understand where the signal is going.
I thought maybe it must be getting grounded somewhere on the solder side, but can't see where.
i can't get a signal at any of the volume pot lugs either.

I've checked and triple checked all trace cuts and for any bad connections, little connections across strips etc but no joy.
everything else looks good, polarity of caps, diode etc.
led and bypass work fine.

any help would be really cool, sorry i've no pics.
thanks.

[GibsonGM]

Well, at the other side of that resistor connected to the drain is 9V....that's the positive voltage rail!  Your probe should just 'click' when you hit that.  Hope your probe has a cap in series so you don't throw DC into your amp....

Try probing on the 3rd rail down, the gate rail, which has the break....that looks like the output side.  Follow the top lug of the purple volume pot wire backwards and see what you get!  ;o)

[skrunk]

thanks gibson, appreciate your help!
I can get a full signal with the probe on the second strip between the 2 trace cuts (really nice fuzz too).
I can't get a full signal on the third strip to the output.

I say 'full' as there is a very faint signal that I only noticed when I turned my monitors right up, but it's barely there and nothing like the signal I get on the second strip, which is where it's loudest (right after the bs170).

So I can't get a signal anywhere past the Q2 resistor, any of the pot lugs etc.
Maybe it's a dodgy cap? (its a wima mks2) Never had one before (a dozen or so builds) but there's always a first time I suppose.

I can't think what else it could be or how I can test it further. Gonna be a PITA changing it out. Prolly should have tested it first before I wired it up all neat and tidy

[skrunk]

So I changed the output cap for a new one, didn't solve the problem.
Went over everything again with the dmm checking voltages and continuity, everything checks out ok.

When I touch the 3rd strip after the output cap with the probe, instead of the signal I get a little click, like I do at the top 9v strip but I mesured the 3rd strip for voltage and there's nothing.

It has to be something to do with the power filtering maybe? The diode?
Really driving me nuts this, hate being stumped by a pedal.

[GibsonGM]

Hey skrunk,

I'm a schematic guy; I don't do Vero, so I'm trying to follow that layout, LOL. Found the schematic at AMZ, tho.

Just for grins, maybe try moving that 9V battery connection and the 47u filter cap to the LEFT side of the board (jumper its "-" to ground)? You don't have to move the big cap yet to test it if you want, it might just buzz some without it.   From the AMZ schematic, there might be an issue there...I'm not SAYING anything yet about the layout, just try that and see what happens, ok?  Looks like 9V oughtta be flowing thru the 1K resistor BEFORE the voltage divider formed by it and the 56k resistors, so I'd expect a voltage drop there.  And the 100k bias resistor needs to be coming off the 9V, not after it.  Your output is that top lug of the volume pot.  Listen there after removing 9V from right and putting it on the left.
Look here:      http://www.muzique.com/schem/fuzzface4.gif

And make doubly sure your BS170 is in the right way!!  Looking at the flat side, it is "S-G-D".  It probably is ok tho, if you get signal at the drain of the 'FET.  The problem is with how the voltage is applied and divided, I think.

The diode to ground looks like protection - if you hook a battery up backwards, it will forward-bias, short out, and keep the MOSFET etc. from blowing (hopefully).  That would be a dead short if it was bad, and you'd get NO fuzz anywhere....

The 'click' is just a voltage differential...don't worry about that for this test...

Let's see what moving that 9V will do! 

~MJP

[skrunk]

cheers gibson  

yeah I was thinking I might try that, I had been looking at the amz schem too earlier.
I'm getting 9v at that side of the top strip anyway so I figured it was ok.
I'll give it a go anyway..

ok I tried it and it's the same story. (just moved the 9v connection, not the cap or diode)
I just get the effected signal as far as the 5.6k and no further.
Nothing on the third strip or any pot lugs.

Isn't the BS170 'D-G-S' (left to right, as you look at the flat side)?
I got these ones:
http://www.musikding.de/product_info.php/info/p343_BS170.html

so as you look at that vero layout, its flat side is facing to the left.

btw does it matter where I take a 9v connection from the 9v strip to the + of the LED.
I had it coming from between the 47uf cap and the diode, tried taking it out but it made no difference.

fwiw i'm getting 8.24v on each side of the trace on the top strip with no continuity between each side.

[GibsonGM]

You're ok on the mosfet orientation....some are different (PITA!!).   

Your LED connection (do that after it's fixed, lol) doesn't matter much, that strip is the same along its length....

8.2V each side - That is VERY strange, man....you should have some voltage drop simply due to the 1k resistor bridging the top strip.  Must be getting swamped by the high resistance of the silicon devices...

I really think something is bridged near that point of the circuit, or the BS170 got damaged due to static or heat (?).  Do you have signal at the gate of the mosfet?  What are your voltages at each pin of the mosfet?  Is your footswitch already hooked up?

Filtering won't have an effect on this kind of thing - only if the cap was bad, then it wouldn't work at all or would be hissy...this is part misplacement, solder bridge, blown part, backwards parts...all things we all do, he he he.  Physics says if we put those (working) parts in the right order, the thing WILL work.  Sometimes it just takes more eyes to find the problem!!!   Happens to everyone - keep looking, and it will get finished!!!

~MJP

[skrunk]

Finally got this working today, thanks to gibson for his help!
I just made up a new vero layout going by the schem and put the 47uf elek and the diode on the other side.
I love the sound from this and highly recommend building it.



btw would it be worth putting a 9.1v zener from Q2 gate to source for static protection or would the 1N4001 do the job?
am I confusing polarity protection with static protection?

[WhenBoredomPeaks]

http://aronnelson.com/gallery/main.php/v/DRAGONFLY-LAYOUTS_0/album18/album144/AMZ_MOSFACE.jpg.html

I've built this and had the same problem and after a hour of debugging i abandoned the project that some of my transistors must be bad.

But this topic "inspired" me to suspect a problem in that vero layout and i think i have found the problem.

The red 9V connection from the battery should have connected to the left side of the top row not the right side.
(i know this is an useless bump but at least we know that the layout is wrong in the gallery )

[LucifersTrip]

...and that's why it's always cool to check out and understand the schematic first.  this one has amazing sustain.

[WhenBoredomPeaks]

I should add that if i connect the positive battery lead to the left side then i should remove the power filtering plus the diode because the filtering cap will work like a lowpass filter on the output signal i think.

I will replace the positive lead in 1-2 days and will update the topic with the results.

[WhenBoredomPeaks]

I will replace the positive lead in 1-2 days and will update the topic with the results.

Ok, it works. (with the 9v in moved to the other side of the row and the power filter cap and diode removed)

[WGTP]

If you like messing with Hy-Brid Fuzz Faces, here are some more for you.  I spent many hours messing with this stuff.

http://web.archive.org/web/20050513200126/www.geocities.com/tpe123/folkurban/fuzz/fuzzyfaces.html

http://www.home-wrecker.com/multiface.html