Ross Compressor Debug

[DennisR]

Just finished my second pedal, a Ross Compressor using this layout with no changes except D1 is a IN4007 and I can't get it to work.
http://www.aronnelson.com/gallery/main.php/v/Toneys-Album/Rossv4.jpg.html  I've rechecked my work several times and can't find anything wrong except for the voltage at pins 2&3 on th IC. Here's my readings, any help would be greatly appreciated.






Power input:  9.5V

Q1:
C: 7.33
B: 2.39
E: 1.95

Q2:
C: 6.89
B: 2.88
E: 2.33

Q3:
C: 9.2
B: 0
E: 0

Q4:
C: 9.2
B: 0
E: 0

Q5:
C: 9.28
B: 9.04
E: 8.6

IC:
1: 0
2: .10
3: .10
4: 0
5: .60
6: 2.90
7: 9.25
8: 0

[ayayay!]

At first glance, I would check the trimpot.  Ohm it out and make sure it works in the full travel of the pot.  See what the voltage and resistance is coming into the wiper of the trimpot.   Those can be dodgy, and even when working they can sometimes have only a small workable area of travel.  Did you use a 2k or 2.5K?  It's pretty critical.

Great job on posting the debugging results, btw!

[yodude]

I can't get it to work.

Please explain what it's doing / not doing (no sound at all, distorted, etc.).

[R.G.]

can't find anything wrong except for the voltage at pins 2&3 on th IC.

You're probably 90% through debug already. You've essentially answered the question.

This thing can't possibly work with those pins at 0.1V. The only remaining question is why are they that low.

You'll notice that pins 2 and 3 are pulled down by a 1M resistor each, but pulled up by two 220k resistors in series to the wiper of that trimpot. It is very, very likely that something is not connected in the path from pin 2 (and 3) to +9V. That means possibly the wiper is open (one answer you got already) or that one of the 220ks are open, or that the capacitor in the middle of the 220Ks is shorted to ground, or there exists some other random open/short that makes the 220Ks either open or shorted to ground.

The simple way to test further is to check the voltages on all of the trimpot pins, and both pins of both 220K resistors. This will almost certainly tell you what is open and what is shorted.

[DennisR]

Thanks to everyone for the help. It did turn out be a bad pot. I'm getting around 5v now. Another thing I found wrong is that the polarity on C13 on the layout is reversed compared to the schematic. Thanks everyone, I can at least hold on to my remaining hair. Debugging it was almost as much fun as building it. Almost.

[ayayay!]

Hey that's great news!  And yeah it looks like you're correct about that cap on the layout.

Easiest way to set the trim pot now:  Crank the sustain all the way up.  If it cuts out at all, then fiddle with the trim pot until you hear it.  That's not a hard rule, as sometimes the trimpot sounds better towards one side or the other.  Just know that your output may disappear if you crank the sustain, and can be recovered setting the trimpot.  

Congrats!  

[Kinetic]

Sorry to hijack this thread, but I recently built a Ross Compressor and while it seems to work well, the output volume rises as I turn up the sustain knob.  Is this normal behaviour or is something wrong?  I used the Tonepad layout here: http://www.tonepad.com/getFile.asp?id=9

[DennisR]

Mine does the same thing. Actually the sustain knob has more effect on the volume than the level does  . With the level cranked, if I crank the sustain it will more than double the volume. I wonder if it has anything to do with the trim pot. Adjusting mine didn't make much of a difference