Mosfet booster problem

[leorock]

Hi there.

I have a Mosfet booster that a guy made for me years ago (I think it's based on AMZ Mosfet) and its knob is acting like a tone knob of a guitar, but backwards. Not like a clean booster that it would should be. I'm a newbie at pedal electronics, but what I can do to solve it?

[GibsonGM]

Hi...welcome.  "Tone knob", like when you go to turn the volume UP, the tone gets darker, or brighter? Does the volume change, too?

I'm assuming the thing worked ok when you got it? 

It could be as simple as a wire broken free from the pot, or the pot itself could be toast.   Or it could be a bigger issue.    Are the jacks *ok*, as in not able to spin freely?  Can you open the back, to see if anything looks strange, like broken wires....

If a reasonable amount of time doesn't find the problem, you could easily start your pedal-building career by just rebuilding the circuit - it's pretty easy! 

[leorock]

When I turn knob up, the tone gets darker and also there's a volume drop. When I turn all the way down, I get almost the same volume as bypass but with some mosfet-type sparkle.

It was working ok, but sometimes was failing to turn on. I've put a new switch and new jacks and that problem seems to gone, but now the effect doesn't work properly.

There's no visible broken wires and ohm resistance in all conections i've made is 0.





1

[GibsonGM]

That's a weird set of symptoms, bud...what comes to mind, kind of a guess...sounds like maybe there is something wrong with the source bypass cap.  If it's an electrolytic and was put in backwards, maybe it is failing, or has just dried up.    Again, just a guess.

You're going to have to get at the board and do some voltage measuring.....do you have a multimeter?   I guess the next logical question is - do you want to learn how to build stuff like this, or just to repair this one.    Like I said, you could build one in the time it will take to figure out what's going on! 

[JerS]

If it worked ok before you wired the new switch, and after the switch replacement you are experiencing the issue - my bet is that the wiring of the switch is not correct. Double check your switch wiring and see how it goes.

[highwater]

It's hard to tell for sure, but in the pictures you posted, it looks to me like the output is coming from the middle lug of the pot - like a perfectly normal volume pot. In the AMZ MOSFET Booster (link), the pot is wired as a variable resistor, increasing gain by reducing the resistance to ground seen by the source-bypass cap. Without pictures of the circuit board (both sides), it is impossible to determine which function the pot on your pedal is intended to serve - if at all possible, you should tear-off that yellow tape and take pictures of what's underneath it.

Also, that switch looks a bit melted in one corner... I don't know if that would cause the symptoms you are describing, but it can't be good.

[antonis]

Also, that switch looks a bit melted in one corner...

And an amount of melted solder wire on pot's white wire jackets (close enough to power input)...

[GibsonGM]

Yes, all connections have to be traced/assured to be good before you get into the board.  I should have pushed that part of it more, I think.  The most common errors are the ones WE make...like wiring things up!  A DMM and continuity tester should help you trace the switch wires to be sure they're ok....