Zvex '59 Sound- it worked and now it doesn't.

[00Stevens]

I built a '59 sound clone using this layout- http://guitar-fx-layouts.42897.x6.nabble.com/ZVex-59-Sound-Verified-td41289.html - but with the SHO boost after it, and it sounded great for a couple of weeks. Then, all at once, the signal became weak and the lows dropped off. The sound came back fully after messing with the footswitch for a bit, but the signal hasn't been right since. I tried replacing the footswitch, and there was no sound at first when i hooked it back up, so i tried swapping the transistors and that brought it back to the same weak signal for some reason. But since then, i've tried the old transistors and it doesn't make a difference. And it's way quieter than it should be. Also, here's a link to the offboard wiring i used, in case it helps: https://3.bp.blogspot.com/-T8OdqPmP_2Q/UG3WZyt28aI/AAAAAAAAC5U/lTz0RyZXAG8/s1600/!Offboard+wiring+-+dual.png

[Slowpoke101]

I see that you have used sockets for the transistors.
Sometimes those sockets do not grip the legs of the transistors tightly and only make contact intermittently.
If you were happy with your original transistors (sounded good to you ), put them back in and solder each leg into the socket (not just one leg but all three ).
This may help restore function or it may make no difference. But solid connections give less problems (usually ).

[00Stevens]

Yeah, sockets can be finicky, but i meant to say that regardless of which transistors are in there now, or how well they're seated, the signal is still weak. I'm at a loss as to what it could be, since it was working perfectly for a while and nothing seems to have changed. It almost seems like signal is being lost to ground via a short or something, but everything seems to be wired up fine. I might try swapping some of the pots. Maybe the sustain pot is leaking more signal to ground than it should be, i don't know.

[00Stevens]

 I'm pretty new to this stuff, so i don't know what to do with these numbers, but here are the voltage readings i got- both with the effect On and Off.

ON 

Q1
D =5.20
G =2.58
S =.39

Q2
D=5.72
G=2.36
S=0 to .17

Q3
D=3.99
G=2.62
S=.35
.
.
.

D1
A =.38
K =2.58

D2
A =9.77
K =9.54


OFF

ON 

Q1
D =5.24
G =2.60
S =.40

Q2
D=.47
G=4.73
S=0 to .46

Q3
D=4.02
G=2.64
S=.36
.
.
.

D1
A =.40
K =2.6

D2
A =9.77
K =9.55

[antonis]

I'm a little bit of confused about On & Off readings  but Q2 Drain & Gate voltage readings on second batch of measurements aren't OK..
(MosFet seems to be defective or heavily "bottomed" by false NFB bias..)

[Mark Hammer]

This may or may not apply in your case so feel free to ignore.

A common-enough occurrence among newer pedal-builders is that they apply too much heat to their stompswitches.  Inside the switch is a dab of thick grease, used to both seat the contacts properly during assembly, but also to damp any "chatter" in the contacts.  When the lugs are heated too much or for a prolonged period, the heat is conducted internally and that can make the grease liquify, coating the internal contacts.  The grease, in turn, acts as an insulator, turning what was a nice shiny gold contact into a whenever-I-bloody-well-FEEL-like-it contact.

The good news is that you CAN safely disassemble such switches, rehabilitate them, and re-install them.

[00Stevens]

I really don't think it's the footswitch; i took great care in replacing it not to overheat the lugs, and a test with an audio probe confirmed that it's not affecting the signal. I also tried pulling the pots out (thinking maybe something was shorting to the chassis), and i desoldered the drive pot completely, because the one lug looked like it could possibly have been shorting to the casing or the back of the pot, but none of that worked.

And for those readings i posted, i accidentally measured everything with the footswitch disengaged (the second set of numbers), and then again with the effect engaged (the first set). I'm gonna do some more poking around now to see if i can't figure out what's going on. If i eliminate all the offboard components as culprits, i guess the problem has to be within the board. Maybe a diode or something, i don't know.

[00Stevens]

I'm a little bit of confused about On & Off readings  but Q2 Drain & Gate voltage readings on second batch of measurements aren't OK..
(MosFet seems to be defective or heavily "bottomed" by false NFB bias..)

Oh, i misheard my brother when he read this, lol. I though he said the second batch of measurements "are" OK. I'll try swapping that one again, but if it is being messed up by a false NFB bias (i don't know what that means, by the way), how would i go about fixing that. Like i said, that was with the effect off, but i did think it was strange that only Q2 was affected drastically by the footswitch being on or off.

[antonis]

I'll try swapping that one again, but if it is being messed up by a false NFB bias (i don't know what that means, by the way), how would i go about fixing that.

R7 & R6 set the Gate bias voltage, which should be roughly about halfway the Drain voltage..
In reality, it can have any value, strongly depended on specific MosFet (Drain current down to Gate-Source voltage) but NOT lower than Drain voltage..!!

So, there are 3 options related to your measurement:
1. Defective MosFet..
2. Wrong value ratio of R7/R6..
3. Soldering/wiring issue around the above circuit..

[00Stevens]

Thanks for all the help, fellas. I got it working again. I was poking around with an audio probe, and the signal was strong up until the 10n capacitor coming off of Q1. On one side of the cap it sounded strong, and the other side sounded weak and thin. I went to confirm this and i bumped the 82ohm resistor next to it, and all of the sudden the signal was strong throughout the circuit. And it would stay that way until i pushed the footswitch off and back on, then it would cut back out (most of the time). So either the cap was bad, or there was a bad solder joint around there. Either way, i replaced the capacitor and it's working perfectly. And i can't get it to malfunction, so i'm very happy with it. Thanks again for pointing me in the right direction; i learned a lot.

-Ben

[Slowpoke101]

That is good news 
The most likely problem probably would have been a faulty solder joint. We all have had that problem - still do