DOD 250 build problem - gain pot does nothing

[deke99]

Hi all,

I just built a DOD 250 using the GGG schematic with the 'Gray' mods.  I pass signal and the volume know works, but I'm getting no change with the gain pot and really no gain is introduced, the pedal just passes something slightly above unity volume with the volume pot maxed.  I made my own layout for this as I like to keep the PCB small.  Here is a pic of my layout with part orientation:



Does anyone have any idea what could be causing this problem?  I've checked my connections on the board and confirmed my layout a few times and I'm at a loss at this point.  I think I've been looking at this too long and hopefully I'm missing something obvious.

Thanks!!

[Mark Hammer]

With no connection between the inverting ('-') pin and ground, the unit becomes a unity-gain (x1) circuit.  Maybe there is a break soewhere between that pin and your ground connection.

[deke99]

Hey Mark,

Thanks for the reply!  So the inverting pin in this case would be pin 2, correct?  I have pin 4 connected to ground and that connection is good.  The inverting pin 2 goes through the 47pf cap, then the 4.7K resistor to lug 1 of the gain pot.  Lugs 2 and 3 on the gain pot are grounded.  so no matter where the travel of the pot goes, the wiper is grounded.  I have verified that the resistance between leg 1 and the wiper of the gain pot does change between 0 and 470K over the travel of the pot.  Does all of that sound right?

Thanks again for your help!

Derek

[Mark Hammer]

Pin 2 should go to a 4.7k resistor, then the pot, then a .047uf (47nf) cap, then ground.  If you used 47pf, then no wonder you don't get any change.  47pf is 1000 times smaller value than what you need...and that's saying something because the stock value of .047uf itself is a bit too small for some folks.

The order of components, incidentally, can vary, so what you have is fine.  I just have my preferences.  But that cap seems to be either a typo or awfully small for the job.

[deke99]

Unfortunately (or not) it is not a typo, I just bought the wrong darn part!  At least now I know what I need to do to fix the circuit!

I will search my parts bin and see if I have the right value and confirm this as the fix.  Thanks again for your help!!

[deke99]

Well, I was sure psyched to have this issue licked . . . but putting the proper value capacitor in there didn't fix the issue.  One thing to note is that I didn't have a reverse log gain pot, so I used a normal analog 500K gain pot and wired it in reverse (swapped lugs 1 and 3).  With the gain on '0', the resistance is 0 and with the pot dimed, the resistance is 470K.  Could this be contributing to the issue?

[Mark Hammer]

In a word, No.  Taper doesn't change, NEVER changes, whether the circuit "works" or not. What it changes is how easily and reliably you can dial in the setting you want.  If the tonal/volume differences you want are all squeezed into a small amount of the pots rotation, then you want a different taper that will spread them out and redistribute them differently.  But whether the pot is audio, linear, reverse-audio, or some bizarre combination of log and anti-log, all those settings are "in there".

In general, the gain of such a stage increases when the resistance to ground is smaller than the feedback resistance.  So, an inverting op-amp becomes a unity-gain stage when either the resistance to ground is very large to infinite (as described earlier), OR when the resistance is very large compared to the feedback resistance.  You've checked the path to ground.  Now check the feedback path and make sure it isn't zero ohms or something darn close.

[burnt fingers]

Do you have your op amp socketed?  two things come to mind.  One, a bad op amp.  Two, op amp is in backwards. (shouldnt' work at all but worth checking.)
Can you try a different op amp? 

Scott

[deke99]

A ha, now that you explain that, I know exactly what the problem is.  I put pads into my layout to accomodate the 'yellow version' 25pf cap.  I jumpered those pads instead of just leaving them blank.  Thus I have 0 resistance in the feedback loop.  I just cut the jumper and now it works like a champ!

Thanks again for the help.  It's great to be able to understand a little more about how the circuit does what it does!

[kato]

Did you solve your problem?

I had a similar problem with a DOD250 - I was getting no distortion - just clean, unity gain or maybe slight boost.

I had the wrong value for R6.
I accidentally put in a 47k resistor. (Instead of 4.7k.)
Once I discovered and replaced with the correct value - I got good distortion.

Good luck!

[deke99]

Hi Kato,

Yes, I did resolve my issue by removing the jumper that I had in the circuit.  The pedal works fine now.  I used germanium diodes and the tone isn't wonderful right now, but it does work properly.  I'll probably try some different clipping diodes to see if I can improve the tone.

Thanks!

[Hendrik1]

And did it work out fine?

[oldrocker]

Shouldn't this thread be called the MXR Distortion + build problem?  It sounds like from the parts description that this is the MXR not the DOD 250.  For R6 on my DOD 250 it is a 47K resistor.  At least according to Tonepad.

[oldrocker]

OK.  I noticed that the GGG site does use a 4.7k so never mind.