Zendrive. 3v at op-amp pin 3 and unpleasant decay

[nonost]

Hi guys. It's okay to have 3v at pin 3 of a Zendrive op amp? The pedal sounds good, but I'm not that impressed. I hear some unpleasant fizzy decay in the distorsion that I'm not sure if other units/builds have .

Layout used: http://tagboardeffects.blogspot.com.es/2012/01/hermida-zendrive.html

Op-amp

1 - 4.8v      8 - 9.4v (new battery)
2 - 4.8v      7 - 4.8v
3 - 3.3v     6 - 4.8v
4 - 0v         5 - 4.7v

Q1

S - 4.5v
G - 4.85v
D - 4.85v

Q2

S - 4.75v
G - 4.8v
D - 4.8v

On the bottom lug of the 470k resistor that goes to pin 3 of the op-amp my tester reads 4.75v, but after this resistor the voltage drops to 3.3v.

I've used the same components as shown in the layout except 1N34A diode for D3, and multilayer ceramic cap instead of ceramic cap. Tried different op-amps and mosfets. BS170 seem to work better and help with the fizziness in the decay that I've mentioned above.

I have to say that I put backwards the power filter cap (100uf), and to my surprise it didn't explode...Although the pedal was working fine I changed the cap for a new one and also repaced the other electro, just in case. Could that have fried another cap?

I've checked multiple times all components values, positions, links and cuts. At that point I don't know if  it's just how the pedal sounds or what .

Any help will be very welcome :)

[Frank_NH]

That voltage is not correct and should be about the same as the other inputs (4.8V) since the zendrive is a simple non-inverting gain stage.  Check the connections at the 470K resistor and 470n capacitor - all it takes is tiny solder bridge to mess things up .  I think you'll find the problem.  If all else fails, you can try changing the IC, but I think it's likely a simple connection issue.

[nonost]

Hi Frank, thanks for your help. I've checked the connections and changed the op-amp without success. It could be that my cheap tester is reading 3.3v because of its internal resistance (a low one) but the actual voltage is fine...I don't know.

[Frank_NH]

Try checking the voltage difference between pins 2 and 3.  The difference should be very small (theortically zero).

I've built the zendrive on vero and it turned out OK, so the layout should be good.  Good luck!

[Frank_NH]

One more thing - try replacing the 470n capacitor at the input.  Maybe it's bad?  If there was a short across it, that would affect the DC voltage at pin 3.

[nonost]

Between pins 2 and 3 I got 1mV (or 0.1mV). I've tried with a few capacitors since I had put sockets in that position. Anyway, I think it's time to upgrade to a better multimeter.

Thank you Frank, I will post any update.