Build Your Own Clone Message Board

Hey guys,

This is an interesting one because I built my OD2 over 9 months ago and have been happily using it for a long time. Recently the boost side started being a bit temperamental and I thought it was some short because it seemed like a turn-it-off-and-on-again fix seemed to work with the 3DPT switch.

So I was rehousing it and giving it a bit of a facelift and I so I thought I'd debug it. I've reflowed all my solder joints and now the boost side is consistently not working. The pedal works fine in bypass and on the OD side.

Before posting I thought I'd knock up a signal tester and see if I could isolate the problem and here's what I found from a signal trace. The signal reaches the transistor Q3 (as seen in fig1 below taken from the OD2 FAQ) to the middle leg fine but comes out of the top leg sounding like crap. Does this mean it's simply a case of getting a new transistor? I don't have any lying around to try it or I would do...

Fig1:


I've tried to annotate duhvoodoman's signal path as to where the signal comes out fine with a probe and where it goes bad:


Any help greatly appreciated!

Transistors do occasionally fail. The boost circuit is a separate single transistor amplifier that it switched by the second foot switch in between the output of the overdrive circuit and the final output of the pedal. When it is working correctly it adds a gain of about 20 times onto the output (this is for the bi-polar transistor version, I think the MOSFET version gain is lower at only 10 times).

From your description the signal is getting to the input (the base) of that transistor stage, but is distorted at the output (the collector of the transistor). The thing is, a gain of 20 in a transistor stage that is powered from 9V is quite a lot. Since the maximum output swing before clipping is less than 9V an input of less than half a volt peak to peak will cause that stage to clip and distort. Some guitar pickups can put out that much signal so even if you have only the boost switched in you might get distortion from a high output pickup. If the signal cleans up if you back off the input that stage might be working OK.

On the other hand a new transistor wont cost very much so replacing it to see if it sounds any better is not a big deal.

Sorry, having included the debug point I was stupidly vague about the nature of the signal. It's almost non-existent in level. So at the base of the transistor it sounds fine. On the output there is a huge drop in level and it also sounds very muffled and bassy. The signal is then supposed to go up to the cap C15 but I don't get anything after that with a signal probe.

I'll order another transistor in my next parts order and check. Is it best just to order the same ones again or are there better ones?

To get the most transparent boost side of this pedal, what's best to go for?

And for bonus points (sorry for all the questions)... I've swapped out the red diffuse LEDs for some ultra-bright blues which are obviously way too bright with the 4k7 stock CLR. Suggestions for a resistor value that doesn't burn my retinas? Maybe between 10k–12k?

The boost circuit is very simple. It seems very peculiar that you can pick a signal up at the collector of that transistor but you cannot follow it through C15 to the output pot and out of the pedal. Sounds like something is wrong there - maybe you have a short somewhere.

Any general purpose NPN silicon transistor in that position will give you the same results (although you will need one with the same connections to just drop straight in). The MPSA18 is a more recent design with a high Hfe but the gain of the stage is determined by the ratio of the emitter to collector resistors. If you order the same transistor you know it will drop right in.

12k or maybe even greater will do fine with the high brightness LEDs. I can't say what brightness you would prefer.

The reason I flagged this up is when I trace the signal through a transistor on the OD side it has negligible effect on the sound however I described the complete signal degradation between the poles of Q3. I'll work out something. Quite tempted to swap out the silicon based boost for MOSFET anyway so maybe in that upgrade I'll fix the issue.

Yeah, I've ordered some straight swaps and also the parts for the MOSFET upgrade so I can see how it goes. In reality I'd like as cleaner boost as possible. I tend to use a strat with single-coils which I believe are rather on the low-output side.

I've ordered a few of each so I guess it's just plug and test.

Thanks Tark!