Can this work - at all? (dod280a)

[Mikkel]

Hi. Project of the week is a 2-in-1 compressor. A Ross and a DOD280a in one box, as I thought that would be quite handy for times when I'm not bringing my whole pedalboard. -I do a lot of lap-steel playing and uses compressors a lot. -Okay.
So I builded the DOD280a I found on GGG:
http://generalguitargadgets.com/index.php?option=content&task=view&id=193&Itemid=218
I have stayed with the not-modded version, used a NSL32 for the LED/LDR and 2N3904 transistors (checked their pinout) - (twice). Haven't experimented with OP-amps yet, as the thing doesn't seem to work. My question is:
On the layout the volume-pot is lug3-pcb out, lug2-to output and lug1-gnd. As far as I can comprehend this (and hear it when I connect my board, BTW), when I turn the volume-pot all the way down I have a short-out, as lug2 is the output-tip and lug1 is ground. So please help by telling me what it is I don't understand? My humble experience with these things tells me that when I get this far, if it doesn't work it's usually the human factor..

Thanks,
Mikkel

[aron]

That's correct about the volume pot. When you turn it all the way down it will silence the unit. Check out the What to do when it doesn't work thread and the Debugging page

Aron

[Mikkel]

Ah, yes. The human factor.. But for now I'm completely puzzled. The thing is all but turned down when the pot is. It hums, and loud! Can it be the output cap that's too small? I get a perfect 0.00's resitance between the output sleeve and tip when I measure it..

[petemoore]

It hums, and loud!
  Check all ground points.
  Can it be the output cap that's too small?
  See FAQ, caps begin a LF rolloff 'slowly' @ some frequency, more rolloff as the frequency gets lower/closer to DC, caps block DC.
  Smaller uf value caps attenuate lower frequencies.
  There are many things it could be, understanding what the components do, posting the voltage readings, audio probing...are included in the Debugging thread and FAQ/Links.
  The reading supplies better, more concise explanations than can be easily typed out again here, often with pictorials and graphs to help demonstrate component and circuit functions.

[Mikkel]

Okay. I've checked the debugging/FAQ pages, and I didn't get any closer to understanding this PCB. Sorry.
I checked the ground points and they look allright. So I did some measuring, and hope that something looks wrong to someone so I can get pointed in the right direction. (Pots wide open).
OP-amp (LM358P):
Pin1 - 4,66v
Pin2 - 4,66v
Pin3 - 3,14v
Pin4 - 0,0v
Pin5 - 4,60v
Pin6 - 4,63v
Pin7 - 4,63v
Pin8 - 9,33v

Transistors (2N3904)
E - 0v
B - 0v
C - 0v

(I suppose that the transistors need a signal to measure anything?)

LED-side of NSL32 - 1,70v across (again no signal)

In-jack tip to out-jack tip resistance - 98k(when vol. turned up), 0,0 when down.

Does this mean anything to anyone?
I adopted a single thing from the modded version (link in my first entry), and that would be the reversing of R14 and C2. Can't tell excactly why. 
-Could that be it? Furthermore i used a 1M pulldown resistor instead of 2M2 (I didn't have one). Was that a bad idea?

Thanks for your patience.
Mikkel

[petemoore]

Pin1 - 4,66v
Pin2 - 4,66v
Pin3 - 3,14v
Pin4 - 0,0v
Pin5 - 4,60v
Pin6 - 4,63v
Pin7 - 4,63v
Pin8 - 9,33v
  Only one I'm not sure about is Pin 3, seems a little low for a 9.3v supply, but this is a comp...which I don't really understand.

Transistors (2N3904)
E - 0v
B - 0v
C - 0v
   not sure what voltages, but some voltages...would be seen on the transistors...It looks like they're used to light/dim the LED by drawing more or less current, if all the pins are 0, I don't see how they're going to do that...you could always connect your thumb to the circuit input and retest, but I don't think that will raise current/voltage on the Q pins.
  I would measure resistances through as many solder joints as possible [distant point testing], around the Q's, as well as verify connections, it looks like they're not getting any PS.

[Mikkel]

Hey, and thanks for the input.
You were absolutely right, there were a ground-short by the transistors, so they were not getting any power. Now they are at a nice(?) 7,3V.
Now I think the pcb works as it should; when I probe around I get the guitar right to the very output, but I have a little trouble with the volume-pot.
When I plug the jacks as they're supposed to, the thing is dead-guiet. I really believe I have soldered both the vol-pot and output jack correctly, though.
I haven't put a bypass-switch in there yet (as this is supposed to be a 2-in-1 compressor), so for now it's wired vol-lug1 -> pcb out, lug2 -> output-jack tip, and lug3 -> ground. Isn't this correct?!
And by the way, that fixed resistor parallel to the LDR - would that by any chance have anything to do with the grade of compression, and if I wanted the thing to squeeze even harder (well, it IS an effect-pedal..), should this resistor be larger or smaller? In the stock-layout from GGG it's 3M, while the "modded" one uses a 1M. I ask, instead of just try it, because I'm making this thing on Vero-board and it IS getting a little messy. Right now I have a 1M parallel to the LDR.

Thanks again.

[Mikkel]

Having followed someone's advice and left the pedal alone for a while, I found the bug the least and most obvious place. A short somewhere inside the output plug. The agony! The pain! That just didn't occur to me.. It's even the open kind, where it should be quite possible to see that kind of mechanical issues. But it wasn't. So to whom it might concern: You can't trust ANYTHING when putting together your third pedal in the world!
That much learned - should I try to describe in English what the thing sounds like, I would go with: pants. It doesn't really compress as much as add some sort of distortion where the compression should have been. When set to compress pretty hard ("comp" over halfway) it distorts when I pick a note, and the distortion fades out as the note settles.
I guess it's time for another walk.

Mikkel