DOD250 needs debugging help.

[aab0mb]

Hello stompboxers,
     I'm trying to build the circuit on perfboard using one of the tungngruv layouts from the gallery.  It's passing a tiny bit of sound but it's way below unity and doesn't really do anything at this point.  There is no switch, just jacks, pots, and 9v battery clip.   I (mistakenly) thought it'd be an easy build. 


Name and show link to circuit schematic.
  *DOD250  http://www.freeinfosociety.com/electronics/schempage.php?cat=1
  If you're using a layout or PCB post a link to that too.
  *http://www.aronnelson.com/gallery/v/tungngruv_0/DOD-OD250/Copy_of_1Yellow741verified.gif.html
   Is there a connection between circuit power supply V+ and Gnd. under any of all power supply 'conditions' ie...is there shorting under and 'normal' switching conditions of input jack or DC jack or any other time?
  *No
  Measure battery voltage.
  *8.71VDC
  Measure battery voltage with circuit loading
  *8.61VDC
If using a power supply, what does it say on it about voltage rating?
  *Battery only
  Also name power supply type
  *Battery
  test the voltage of the power supply.
  *VDC
  Test votlage of power supply with circuit connected
  * .vdc
  Is the circuit npn-negative ground?
  *Yes 741 singal opamp  (radioshack)
  connect DMM to ground, and test for continuity at every point in the schematic with a ground symbol, from the most 'distant' [test points should be beyond all connections and solder joints].
  Test the cables that are plugged if they can be plugged in, at the plug sleeves which connect to guitar and amp
  Is there continuity at all 'distant circuit ground points
    *yes ground tests for continuity at all distant points
  Stage 2
  Connect power supply, if there is a large current draw [ie voltage drops by 10% or more, disconnect power and post that result].
  Is there a >10% voltage drop in battery when circuit is connected?
  * No
  What voltages are measured [using ground as reference voltage =O.0V], using DMM at all active component pins?
 
-----
  *Single opamp
*1  NC
*2  .51v
*3  1.62v
*4  0.0v
*5  NC
*6  .55v
*7  8.61v
*8  NC

  List any pertinent comments and questions.

**I substituted 22k resistors for the 20k resistors.  Subbed a .1uf input cap and a .1uf for the cap hanging off of the feedback loop.  Used a 1M pot for the gain control because it's all I could find.  Left out the diodes for now until I can get it working.


  Stage 3
Audio probe, and list results and comments.

**Have not audio probed yet.


  Test that all stages are 'separated' by DC blocking capacitors,
  Can you physically see that a capacitor is properly wired to the input and output of each active component stage? ie signal path must go through the cap to enter or exit the active stage.
  The circuit debugging may have limited results if the answers with * are not known or posted.
  BTW the opamp pins for most distorters [gain stages] are more than optimal, bias is 'centered' at 1/2 the power supply, except for the power supply pins [the 9v and 0.0v ones.
 



Thanks for any support in advance,
Aaron

[JOHNO]

i just finished boxin my up right this minute, i works fine but i still dont know if i like it. ill check my voltages and let ya know

[JOHNO]

the voltages ive got are
1 nc
2;2.27v
3;2.09v
4;0v
5;nc
6;2.29v
7;8.44v
8;nc

[frankclarke]

The input pin should be close to half the supply voltage, try looking at that. You probably connected something wrong, or have a wrong resistor somewhere.

[aab0mb]

I got it working but the voltages are a little off from what JohnO posted.  I suspect that it's because I used 22k resistors for the voltage divider instead of the 1M or 20k i've seen other places.  It sounds pretty good though.  I just went to the music store to try out an MXR Dist+ and a Micro Amp.  My circuit killed the D+.  I have subbed the input cap for a .1uf and subbed another .1 for the cap to ground in the gain section of the opamp.  It sounds much fatter than the stock model.  Thanks again for the help guys. 

Now it's time to mod it a little further. 


Aaron

[aab0mb]

Mark Hammer stated in another thread that the LM741 clips w/ or w/ out the diodes in place making it a faux "double clipper".  Is there any way to stop the opamp from clipping so you could have a switch for distortion/drive or regular boost?  Will placing the diodes in the feedback loop do anything to combat this issue or just change the clipping sound to "overdrive"? 

[JOHNO]

HHHMMMM frank is right the input voltage should be half the supply voltage i better check mine again and see whats up there.as for your question aabomb i cant answer that but maybe someone else could.by the way i take back all the bad things ive said about this box. i stuck it in front of my jcm900 just running the amp clean,well as clean as a jcm can run. and it is certainly a huge sound. i love this thing now not so great in front of the fender champ though. the champ just doesnt have the bottom end for it.

[gaussmarkov]

fwiw, the IC-Based Overdrive project is about the DOD Overdrive 250 and voltages for the IC are listed there
in section 3.