Rhythmic Pedal Popping

[magootaylor]

Hi all,

I've built a Green Russian Big Muff based on this schematic: http://www.bigmuffpage.com/images/schematics/V7BMP_Schematic_GreenCivilWar_TallFontRussian.jpg

When I finished the circuit and tested it out, the indicator LED flashed at about 100BPm, and the pedal made a rhythmic popping at the same rate. What could cause this? A bad transistor? I tried removing the 20uF cap from the circuit, and that caused the indicator light to stay on, but the circuit makes a high pitched hum and does not output any guitar. I've checked everything for shorts, correct routing, bad connections etc that are on a standard troubleshooting list.

[antonis]

Hi & Welcome..

Place a 47R resistor right after LED CLR (R1) and a 220μF to 470μF cap between 47R right leg and GND..

[magootaylor]

Thanks for the advice- I've done this, which fixed the popping. Unfortunately, now I'm not getting any output from the pedal, and the 47R is getting hot when power is connected. Seems like a short somewhere but I'm not finding it? Sadly I don't have an oscilloscope/voltmeter with me at the moment so I can't probe anything.

[bluebunny]

It does sound like a short.

I don't have an oscilloscope/voltmeter with me at the moment

Magnifying glass?

[GibsonGM]

One must ask - what changed?  The circuit worked, then you added the 47R/cap, right?  So, what changed when you were in there?   

This is where the magnifying glass and DMM come in very handy. 

The 47R should not get hot, since down the line the circuit is only calling for little current (47R on its OWN might well get hot, but not used this way, lol).

Go back and check your work very very closely, as well as any assembly/disassembly you did - anything shorting to the enclosure and so on!

[antonis]

Green Russian circuit current draw (without LED) is estimated at 3mA ( https://stinkfoot.se/archives/856 ) causing a voltage drop across 47R resistor of 140mV..

The above call for a less than 500μW(0.5mW) resistor power rating..

There are two options for your new issue:
1. Some kind of short to GND on resistor right leg (a shorted cap, perhaps)..
2. Wrong resistor value..
    e.g. 47k -> 423mW (Hot) or even 470k -> 4.23W (Inferno) - always dependend on resistor actuall wattage..

[duck_arse]

that circuit is drawn in error. the led is tied to the volume pot wiper and output when effect is on. led will come on when effect is bypassed. as drawn. how did you build it? how did you wire the footswitch?

[magootaylor]

The resistor is definitely 47R. The only change I made was adding the 47R to my 9V line before it got to my 9V rail on the board, and adding a jumper from 9V to 220uF cap to ground. I have it out of the enclosure for testing purposes and it is on a non-conducting surface.

I wired the LED to the 3DPT so that it is ON when the effect should be engaged. My understanding is that the circuit should always have power when it is plugged in, regardless of switch position? That is how I have wired it, with just the LED being switched by the foot. If the circuit should only be powered when it is engaged... Then that could be my problem. Regardless, the 47R heats up whether or not the effect is engaged.

I guess I'll mosey my way over to my college campus and make use of a scope in one of the labs- if they're open during the summer, or see if I have a magnifying glass to look closely. I don't imagine that a cap leg is shorted because of how I have them situated on my board, but at this point I'm willing to try anything.

[magootaylor]

Ok, after some probing, I decided to remove the 20uF cap again, and that seemed to get rid of my short. Why? I have no idea. I traced every line going to ground and that was the only one that could even possibly be causing that much power dissipation in the 47R. So now my pedal works, but the output is much quieter than it should be, but it at least outputs and sounds decent now. Any suggestions?

[GibsonGM]

I hate when they tack on crap like the indicator LED like that. Sorry, had to vent. 

Is it still popping?  If that's gone and it seems like it sounds the way it should but as you say, too quiet - examine the volume pot connections.
Then audio probe backward from there til you find where it 'gets quiet'...

[antonis]

The resistor is definitely 47R. The only change I made was adding the 47R to my 9V line before it got to my 9V rail on the board,

In such a case, current through 47R resistor isn't only circuit's draw (about 3mA) but the sum of it and LED current..
Anyway, it doesn't justify neither resistor heating nor low output..

Resistor heating and low output call for large current through 47R resistor and big voltage drop across it..

[magootaylor]

- examine the volume pot connections.
Then audio probe backward from there til you find where it 'gets quiet'...

I've checked the volume pot, it should be wired correctly. Do you have a useful link on audio probes? I'm a mechanical engineering student so I have a lot of theoretical knowledge about circuits but not much practical. An audio probe is essentially an input jack and a capacitor, correct?

[antonis]

https://diy-fever.com/misc/audio-probe/
https://www.diypedalgearparts.com/en/novita/26_Audio-probe-how-to-build

[magootaylor]

PEDAL FIXED! The connection between the .0039uF cap had a bad solder job to the Q2 transistor. Thanks everyone for the help, the audio probe was the key to finding the spot. Thanks Antonis for the original fix for the blinking and popping!

[GibsonGM]

Out of curiosity, did you find it with the audio probe, or 'visual inspection'? 

[magootaylor]

Audio Probe!!!!