Strange behavior on MXR envelope filter

[Bill_F]

Just finished building this bad boy and its acting strange. When I turn the threshold to 0 I get clean guitar but no filtering at all. When I turn the threshold up I get clean guitar that last a second or so and then cuts out. The more I turn the threshold up the shorter the clean guitar rings before it completely cuts out.

Any idea guys?
Bill

[Boofhead]

The EF isn't simple to debug by posts since there's so many places for possible faults.

To start, find the threshold control on the schematic.  You will see two inverters, one before the threshold control, which goes to the input, and one after, which goes to the diodes+two 50nF caps+1M.  Check around those two inverters for shorts or bad soldering.  *IIRC*, the output of each inverter should be in the 2VDC to 3VDC region and the output of the second inverter should not  change when you adjust the threshold control.  If you don't see this then there a problem around these inverters.

Beyond that the voltage *across* the 1M resistor should be *about* 0V with no signal and go positive when you play notes.

[Bill_F]

To start, find the threshold control on the schematic.  You will see two inverters, one before the threshold control, which goes to the input, and one after, which goes to the diodes+two 50nF caps+1M.

Thank you for the reply. I'm still fairly new at this and don't understand what you mean by inverter. Could you explain that for me?

Thank you again.
Bill

[Boofhead]

The inverter is the CMOS 4069 device.

An inverter is actually a digital logic device which outputs a digital signal in the opposite sense to the input: high in -> low out, low in -> high out.  However, a number of the 4069 devices in the EF circuit aren't used as conventional digital inverters, they are used as amplifiers (be it still inverting amplifiers).   You might want to take a look at the data sheet,

http://www.philipslogic.com/products/inverters/4069/

There's two types of 4069 inverters, buffered and unbuffered. That circuit requires unbuffered devices.  If the chip marking has a U somewhere after the 4069 then you are safe.  Non-U chips might not work.

The inverters I was refererring to were  A-2 pins 10 and pins 11, and A-2 pins 9 and 8.

Unfortunately the EF circuit is complex in the sense that is uses a lot of electronics ideas.  That's going to make it fairly hard to debug if you are just starting out because what's happening in the circuit will be very foreign to you.

[Bill_F]

Thanks Boofhead for your help.

[bwanasonic]

The inverter is the CMOS 4069 device.

If I'm not mistaken, these are pretty easy to damage with static or heat. Bill, did you use a socket? Maybe the chip is zapped? Just a thought.

Kerry M

[Bill_F]

Thanks everybody for your help. I went in an re-flowed all the solder then put in different IC's and it works now! Nice little pedal.
Now because I can't leave well enough alone, I mean how could I call myself a DIY'er if I could. I am compelled to go in and do all the mods!