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Re: Dod envelope filter 440, HELP!

11/26/1999 4:15 PM
John Greene
Re: Dod envelope filter 440, HELP!
There's definitely something not right. You did socket the opamp did you? ;^)  
check the voltage at the 2 22Ks that form the reference voltage and make sure it's 4.5v. Also, the schematic doesn't have pin numbers so if you could say which opamp is which, it may help too.  
If the reference voltage is 4.5v I would check your wiring and try another opamp.  
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11/26/1999 6:23 PM

I previously didn't use a socket so the opamp could be blown, but i am using a socket now.  
Do you mean to measure in between the 2 22ks, one going to ground and one going to + voltage?  
I used a LM358 dual opamp  
Here is the pinout :
11/26/1999 11:26 PM

Both opamps should be working as linear amplifiers in this setup. For an opamp to be working as a linear amplfier, both the inverting and noninverting inputs and the output pin must be within a few millivolts of the voltage on the noninverting input. In this pedal, all pins should be sitting at about 4.5V except pin 4 (0V) and pin 8 (9V).  
Pin 2 is at 2V while pin 3 is at 4.5V. I can't make anything out of that except (a) dead opamp or (b) error in measuring the voltage. It's very, very odd for the output to be at the correct 4.5V when the inverting pin is not.  
The 5-6-7 opamp is even stranger. If the opamp were good, I'd expect all the pins to be at 4.5V. The fact that pin 5 is at 2.5V instead of 4.5V indicates that either the input is sucking enough current to pull the reference voltage down to 2.5V, or that something is amiss with the input coupling cap (0.01uF). Again, not only are the inputs not at the same voltages, the output is sitting at 4.5V where I'd expect it and the inputs don't seem to be telling it to do that.  
"These figures seem a little wierd"
No, they seem a lot weird.  
Do this. Take out the opamp. Use your multimeter, and measure the value of every resistor in the circuit to verify they're what you think they are. Then measure the resistance through every capacitor in the circuit - they should all be open circuits. Then use the ohmmeter to verify the wiring of all the parts. Once you've done that and have corrected any flaws you find, leave the opamp out of the socket and apply power. Measure the voltage to ground at each pin of the opamp socket (which is empty, remember). You should get:  
1 - 4 to 4.5V  
2 - ditto  
3 - ditto  
4 - zero volts  
5 - 4 to 4.5V  
6 - zero  
7 - zero  
8 - 9V  
Then pop in an opamp. Measure the pin voltages again.  
You should get 4.5V everywhere except pins 4 (0V) and 8 (9V).  
It might be easier to do a fresh rebuild if this is the same board you were asking me about a year ago.  

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