Zvex Distortron stripboard, not distorting

[Ozsi]

Hi Guys, my first post and first attempt at building a pedal (aside from a BYOC several years back).  Lots of fun so far, but I've hit a roadblock and hoping for some help.

From the debug checklist:

Circuit - ZVex Distortron. 
Source - http://tagboardeffects.blogspot.com.au/2013/10/zvex-distortron.html
Which is essentially this circuit from the thread linked from the above page, with the changes mentioned also on the above page (C5:56n -> 22n and C7:10n -> 100n)


Modifications - none (or none intended...)
Substitutions:
   1N5819 Diode instead of 1N5817
   2N7000 MOSFETs instead of BS170 (and I learnt the hard way that the pins are reversed on the 2N7000!  )
   2.2nF caps instead of 2nF
   Gain pot is 5K linear, instead of reverse log.

So, the problem - after one write-off, a rebuild and some more debugging, I've got the circuit passing through my guitar signal, but cleanly with no distortion at all as far as I can tell (which still felt like a bit of a win after several failed attempts ). 

The volume pot works, and the gain pot works insomuch as it also reduces the volume of the signal.  The tone pot seems to also bring the volume right down.

But, the guitar is coming out completely clean...  I know very little about guitar pedal electronics, but from what I understand the MOSFETs should be boosting the signal until they are overloaded and it clips?  But that's not happening for some reason?  I don't know how I'd go about confirming that or fixing it though - so hopefully someone here can help!


Voltages:
(this is with the OD/Hi-Lo switch running through R13 to ground, and with the SUB switch open/off)

9v Battery: 8.9v

MOSFETs
Q1
D=6.29v
G=3.05v
S=1.86v

Q2
D=2.13v
G=1.58v
S=0.22v

Q3
D=4.19v
G=1.58v
S=0.27v

Diodes
D1
A=8.9v
K=8.67v

Z1 (zener)
A=1.86v (connected to source of Q1)
K=3.03v

I went through and measured the voltages all across the board too, and nothing jumped out as being obviously wrong (e.g. grounded where it shouldn't be etc).  But again, I don't know enough to detect anything much more subtle than that.

Would any more information help?  e.g. pictures of the board itself?

Thanks,
Simon

[duck_arse]

hello and welcome, ozsi.

we always like to look at pedal pictures. sometimes we can even help.

[Ozsi]

Hah, thanks, OK here are a couple of quick pics, front and back.

As you'll see, this is barebones at the moment, and not pretty...  The battery and in/out jacks wired direct to the board.  My first attempt had a lovely footswitch, led, working bypass etc, but the circuit itself was stone dead.  For the rebuild, I decided to focus solely on getting the circuit to work, then will go about turning it in to a pedal later on.




Simon.

[duck_arse]

I won't say check across all your vero cuts to make sure they really are cut, because I'm sure you've done that already. I will say the orange-wired pot looks like it might be a candidate for shorts to the body. I'd move the wires out of the rivet holes, as there seems to be some stray strands about.

there are a few places that look like you have two wires stuck into the same hole. have you? the vol pot also looks to have the top and bottom wires to the same place. does it?

looking at the circuit, the 3 gain stages are identical except for their source resistors. so, if you set the gain pot to 180R (use your meter, power off), the voltages on Q1 and Q2 should be almost the identical. reset the gain pot to 330R and Q1 and Q3 should be the same, with the switch open. switch closed, pot to 0R.

I don't know this will fixxe, but it might show up some clues.

[Ozsi]

I won't say check across all your vero cuts to make sure they really are cut, because I'm sure you've done that already.

Yep I checked them at the very beginning and it all was OK.  I checked after soldering each component too to make sure I hadn't connected two adjacent lanes.

I will say the orange-wired pot looks like it might be a candidate for shorts to the body. I'd move the wires out of the rivet holes, as there seems to be some stray strands about.

Ta, I've cleaned this up now.  I'm still experimenting with the best way to do everything, so the orange pot (tone control) was the "through the rivet holes" one

there are a few places that look like you have two wires stuck into the same hole. have you? the vol pot also looks to have the top and bottom wires to the same place. does it?

Yeah well spotted, pin 1 and 2 on the gain control (yellow wires) I'd put in to the same ground hole, as with pins 2 and 3 on the OD switch.  I think I could have just left them unconnected? Again, more experimenting just to get a feel for it all.

The volume pot (blue wires) doesn't though, it has pins 1 and 3 going to adjacent rows on the board, with pin 1 grounded.

looking at the circuit, the 3 gain stages are identical except for their source resistors. so, if you set the gain pot to 180R (use your meter, power off), the voltages on Q1 and Q2 should be almost the identical. reset the gain pot to 330R and Q1 and Q3 should be the same, with the switch open. switch closed, pot to 0R.

I don't know this will fixxe, but it might show up some clues.

OK, hear are voltages with the gain pot at those levels -
with gain pot at 180:
Q1 S=0.17 G=1.52 D=3.05, Q2 S=0.09 G=1.38 D=5.46

With gain pot at 330 and OD switch open
Q1 S=0.32 G=1.57 D=3.33, Q3 S=0.27 G=1.65 D=4.10. 

With the gain pot at 0 and OD switch closed:
Q1 S=0 G=1.35 D=2.71,  Q3 S=0 G=1.39 D=0.11


Now, what I have also done is build an audio probe.  Using a sine wave fed in from the PC, I found the output from Q1 was actually amplified a little (though still clean).  The signal drops almost to silence at the gate pin of Q2 after passing through C3.  From that point, the output from Q2 is a little louder, and a little louder again from Q3 if the OD switch has R13 in the circuit (though it's still much quieter than the input volume at this stage).  Then it's on the output side of R14 where it's back up to it's final output volume, around about the same as the input volume. 

That seems to go against what I'd intuitively expect.  Does it shed any more light on things?

[duck_arse]

I stuck a bunch of 2N7000 into the basic circuit with the source resistor at 5k, 330R, 180R and 0R this eve, just to see what happened and measured all the interesting voltages. and as you can see from the string of resistors, 5k1//1M//1M between V+ and ground in each section, whatever voltage appears at the drains (which is set by and varies with the resistance in the source) sholud be halved at the gate. and from your voltages and my voltages, something is wrong with Q2. the gate voltage is too low for the drain voltage, and the drain voltage is too high for the source resistor.

so, check R5 - R9 for resistance values and connections. maybe R8 is connected to R5? we are very close. check the caps for shorts as well. it wouldn't hurt to swap Q1 with Q2 to see if the voltages change, either.

and from the amount of gain this thing has, you should be getting bulk clipping at the output, when it is working.

and as for wires, you don't seem to be short of holes on that board. there is really no need to go to the trouble of jambing 2 wires into 1. and it is just as easy to short the unused lug of a pot at the lugs, and then just run 2 wires back to the board instead of three. same goes for the switch.

[Ozsi]

Duck_arse, thanks very much for the help, it's pretty much working now!

After a lot of poking around Q2 area, I found there was a little connection between two diagonal strips - see the yellow circle below



It was a tricky one to pick up, because it was connecting the Q2 gate with the R14 resistor towards the end of the circuit - which explains the results from the audio probing too now.  But I'll know for the future what those sort of symptoms might point too.

and as you can see from the string of resistors, 5k1//1M//1M between V+ and ground in each section, whatever voltage appears at the drains (which is set by and varies with the resistance in the source) sholud be halved at the gate. and from your voltages and my voltages, something is wrong with Q2. the gate voltage is too low for the drain voltage, and the drain voltage is too high for the source resistor.

I appreciate the explanations too - it's slowly coming back to me from my school and uni days, but will take a while I see

Now to add back in the switching and fit it to the enclosure, should be a fun afternoon

[duck_arse]

you didn't really expect me to pick up that short from your first photos, were you? and hoo-ray! it works.

how does it sound, loud? clipping?

[Ozsi]

Ha, I would've been very impressed if you'd spotted that one   You did get me looking in the right area though.

With that part snipped, it sounded pretty good, but still with a few gremlins - some controls didn't seem to work as they should, some crackling and noise from the pots and switches etc.  So buoyed by having the core thing going, I decided to take what I'd learnt from the first two builds, and from your (Duck_arse) help here and have one final go at.  And third time it worked a treat - sounded great straight away, all the pots and switches did what they were supposed to.  I even included a mod from the original page - replacing R5 with another 100k pot.

Got it packed up in a pedal and I'm really liking it.  I'm not sure how closely it compares with the original Distortron, but this one is giving me a huge array of sounds.  Everything from some subtle warming and fattening for my Strat, through to light overdrive, bluesy crunch, modern rock through to real dirty grungy type sounds.  And unlike some of my other (bought) pedals, the gain control leaves the overall volume largely unchanged while increasing the distortion, which makes tweaking much easier.

Getting all the offboard wiring going was a bit of a challenge though, and I've got some work to do to get everything to fit more cleanly in the enclosures in the future.

And already got parts on the way for my next build, and thoughts in my mind on possible mods etc - I can see this getting quite addictive

Thanks again Duck_arse for the help - if we hadn't got this first one working it would have put a damper on my DIY pedal building before I'd hardly got started.

And some final pics - just pretend the fourth knob is there, and some nice decals