Functional diode question in OD-3

[ayayay!]

I have an OD-3 that a friend asked me to repair.  

http://www.freeinfosociety.com/electronics/schemview.php?id=2299

Works in bypass, no signal whatsoever engaged.  I open it up and someone has tried modifying it:  No diodes (or jumpers) in D6 & D7.  Forgive my ignorance, but I don't know what D6 & D7 are doing.  I would wager they're not for clipping, but rather stabilizing spikes to Q15.  

(My guess is someone removed them thinking it would make it a clean boost pedal, not knowing that would break the signal.)

...Also has a RC4558L instead of M5218AL (I saw the "fresh" on the pcb before I even flipped it over.)  Not really worried about that one.  I can test/measure/replace as needed...

[PRR]

> No diodes (or jumpers) in D6 & D7.

They are clipping diodes. When the input and output are more than 0.6V different, they limit. Since q15 has gain near -10, the output won't go over about 0.54V. If the diodes are open/omitted, Q15 can swing 3V peaks.

> no signal whatsoever

It's not D6 D7, and replacement is not urgent until you get signal flowing through.

You know the drill. There should be battery voltage many places. "VB" should be half of that. q8 has battery on drain, half-voltage on grid, and something near that (a bit high) on source. The chip opamps are probably VB on all signal pins. Q15 has zero on gate, some small source voltage, and the drain should be SOME place between zero and battery (not stuck to either extreme). Broken pots are popular. Also that plan shows half of bypass switching, and any of that may be ill. So it would be wise to feed signal and follow along listening to each stage. Conversely, plug the output to an amp and put your finger on the last stage, the stage before, going forward to the input.

[ayayay!]

Thank you Paul.  I was hoping you would pick up on this one.  I wouldn't have guessed they're indeed for clipping.  I'm adept at tracing & audio-probing circuits, so I'll just trace along until I find it.  Yeah the flip-flop circuit is no problem, I can t-shoot that too.  

So when you say,

"They are clipping diodes. When the input and output are more than 0.6V different, they limit. Since q15 has gain near -10, the output won't go over about 0.54V. If the diodes are open/omitted, Q15 can swing 3V peaks."

Did you mean gain near *roughly* 10?  I'm not sure I follow the *negative* 10.  Or is it too early and my coffee hasn't kicked in yet...?

And if I'm getting this right, the 0.6V is the diode drop right?  

[PRR]

"Negative 10" is shorthand for "gain of 10 inverting". If you do math, using +/- notation helps get things computed. In this case the inversion is not too important, I was just being particular.

[ayayay!]

I know this is old but I wanted to give an update.  I repaired this thanks to my audio probe. 

As noted before, D6 & D7 were not present.  Replaced them.  The *five* components after that all had bad solder pads beneath them.  I figure it must have been a bad job in the pcb creation.  C24, R40, R26, C16, C15 all had pads that were just crummy.  Wiring it point to point cured it. 

Pretty cool pedal.  I'd never heard one before.  It's a solid distortion.