dod tr3g(squared) double grunge troubleshooting

[sirkut]

Now this pedal has 4 sections on the pcb. 2 grunge pedals, a pitch shifter then the master volume bypass pedal. The unit lights up, each effect can be turned on and off (the led status changes when stepping on the pedal) but there is absolutely no sound in either bypass or in effect mode.  What steps should I do to troubleshoot this pedal?

On interesting bit is the dod fx69(4560s and one cd4007) grunge doesnt even look the same as in this double grunge pedal(has a lot of 4558s)..even the knobs are named differently.

[Peter Snowberg]

I would go right for the audio probe. You should be able to trace the signal through "obvious" points like opamp inputs and outputs and the source or drain of JFETs used for bypass switching.

Of course it always helps when you have a schematic to work from, but even without one I wouldn't be suprised if you found the fault in less than 10 minutes. Don't feel bad if it stumps you though.

If you trace the signal into an electrolytic cap, make sure it comes out the other side of the cap.

Good luck,
-Peter

[sirkut]

I'll have to give that a shot, there's just so much on this board, the pitch shifter has an insane IC on it that is socketed. Wish me luck. When you say JFET, how do I check those? There are quite a few on this item.

[Peter Snowberg]

If you look at a DoD switch schematic you'll see a flip-flop that takes care of the alternating off-on action. The outputs from the flip-flop go to the gates of JFETs to do the actual switching. Check out this DoD switching info http://ampage.org/hammer/files/dodswitch.gif (Thanks Mark! ).

The JFET switches are usually fairly easy to find on the circuit board because of their connections. The gate will be connected directly to the anode of a signal diode, while the drain and source will connect to a resistor going to the bias supply and a coupling cap.

I would start by touching the audio probe to the point where the audio enters the circuit board and then trace from there. At some point you'll see it enter a component and not exit out the other side. My guess is that you will run across a dead capacitor some place on the board. If you see an electrolytic cap on the board and you hear audio on one leg but not the other.... that could very well be the problem. I skip the tracing sometimes and first check for signals making it through coupling caps as a first check. Some caps will be there to shunt audio to ground, so you may find caps where audio on one side is normal too... but usually these are small caps (less than 0.5uF).

Best of luck! It may take a little time, but I'm sure you'll find it.

Take care,
-Peter