Weird problem debugging a Muff.

[Muadzin]

I've been building the 75#1 version of the Ramshead Muff using this layout.



Schematic used:


It didn't work so I whipped out an oscillator and an audio probe. The signal stops right at the start at the input of R2. When I desolder C1 signal will pass through R2, resolder C1 and the signal disappears again. Oddly enough when I hook the muff up to an amp and the oscillator through the audio probe signal will through the entire circuit, including C1. But just not through R2. Anyone have any idea how to solve this?

Also, how correct is the orientation of C7 in this schematic, as it seems to be contrary how C6 sits in the circuit?

[IvIark]

I'd say C7 is the wrong way round, you'd expect the higher voltage on the collector.
No idea what your C1/R2 problem could be, what voltages are you getting at Q1 when everything is connected up?

[Muadzin]

Q1:
C=9.04
B=0.03
E=0.00

Q2:
C=4.64
B=0.68
E=0.07

Q3:
C=4.15
B=0.64
E=0.04

Q4:
C=4.37
B=1.55
E=1.07

[IvIark]

The Q1 voltages look very suspect.  Are you sure about the values of the 8K2 and 120R?

[Muadzin]

I measured them and although the 8K2 is closer to 8K, it and the 120R were generally in the park. It did however lead me to finding the real culprit for the weird voltage of Q1, the 41K was not soldered properly. Reflowed the solder and the voltage on C is now a more normal 4.3. It didn't solve the main issue though, although when I turn up my little testing amp all the way up I do hear a faint heavily gated guitar sound.  

[Muadzin]

Gave it another try today. I desoldered the problem cap, put in a 1uf film cap, same problem. So I figured it had to be something that came afterwards. Desoldered the 39K and 2K resistor I had put in parallel to form a 41K resistor, replaced it with a 39K and lo and behold, there was sound.

Problem solved!

[induction]

39k and 2k in parallel is 1k9. To get 41k, you'd have to put them in series.

[Muadzin]

Really? I always thought that putting two values next to each other, as opposed to in succession would create a new combined value, as opposed to subtract.

[dbat69]

Capacitors in parallel is a summation of the two values, but not resistors.  However, resistors in series is a summation of the values.

Wiki has an easy guide: http://www.wikihow.com/Calculate-Series-and-Parallel-Resistance

[Muadzin]

Cheers mate!

[Muadzin]

Inspired by getting this Muff to work I turned my attention to some other Muffs that were in dire need for some troubleshooting and got four more to work. Two are giving me weird problems.

I have this Skreddy Pig Mine which I managed to get working. I usually testfire them using some 2n5088 or 2N5089's which I have lying about before breaking out and bending the unobtainium transistors. The Pig Mine build states that you should use 3 BC184's and 1 BC183. Installed those and nothing. Dead silence. Put the 2n5089's back in and awesome muffage again. Tried subbing individual transistors to see if one was causing the problem and each time the sound either disappeared or turned to a very soft splattery gated fuzz.

On a sidenote the Pig Mine reminded me again why I never use log volume pots anymore. Unity gain at 14:00 is not cool. Linear volume pots for the win!

The other muff still causing problems is a Green Russian which has below unity gain volume even when maxed. Using my audio probe I discovered that the signal degrades in the tone section. If I follow the signal through to the 33K resistor to lug 1 of the tone pot there is no volume drop, but when I trace the volume through the 3.9N cap to lug 3 of the tone pot the signal drops massively in volume after the 3.9n cap. I thought maybe the 3.9 cap was faulty, but when I desolder one end and apply the audio probe the volume is back. Resolder the 3.9n cap back into the circuit and the volume drops again.