original Mutron III, only fuzz sound

[Kas2]

Hi Al,


I have a Mutron III from 1975 here and it doesn't work anymore. It just gives a fuzz distortion.

In HP the volume is ok. In BP an LP there is almost no signal (soft with fuzz though)

-The switches don't seem to do anything. All the hardware is checked and OK. The solder has been redone. Elco's seem ok.
-Input stage and GAIN works.
-Envelope following section seems to work. I just don't know if the voltages are right.
-Voltages arrive at the opamps.
-Grounding is good.
-PEAK pot makes the unit fuzz loud or softer. Closed it's clean, opening it dials suddenly from halfway fuzz tone. So no smooth transition. After halfway it gets more fuzz.

Where should I look? I hope that there is someone out there who had this too.

[Kas2]

Sorry my question is useless without this:
http://files.effectsdatabase.com/docs/schematics/musitronics_mu-tron3.jpg

[R.G.]

-Envelope following section seems to work. I just don't know if the voltages are right.
...
Where should I look? I hope that there is someone out there who had this too.

Read "Debugging: What to do when it doesn't work". Measure and report the DC voltages of all pins of all opamps.

[Kas2]

The power supply that came with the effect was not doiong so well. Giving 9,3 V DC and -11,9 DC.
I redid the power supply: rectifier -> 1000uF -> regulator 7809, and same for the negative side with a 7909 regulator.
I now get with load 8,94 an -9,07 VCD.

Electrolytics are replaced, diodes replaced, all traces resoldered, all components resoldered, PEAK pot was destroyed, replaced with 250K. All hardware tested for continuity OK,
C3 replaced, C5 and C7 replaced. opamps A3,4 and A5,6 replaced.
Doesn't change the fuzz everywhere.

HP, range HIGH, drive UP, Footswitch IN:
Voltages DC:
no signal in:           with signal in:
A 1,2:
1:  0,0
2:  0,0
3:  0,0
4: -9,07
5:  0,15
6:  0,29                  to 0,45 then drops
7: -7,94                  to -7,5 then drops
8:  8,94

A 3,4:
no signal in:           with signal in:
1:  8,66
2:  0,18                 to 7,9 then drops
3:  0,0
4: -9,07
5:  0,0
6:  0,15                 to 0,03 then drops
7:  8,76
8:  8,95

A5,6:
no signal in:           with signal in:
1:  -7,98                to 8,16 then drops
2:  -0,02                to 7,16 then drops
3:  -0,06                to 7,77 then drops            
4: -9,07
5:  0,07                 to 0,03 then rises
6:  0,08                 to -1,2 then rises
7:  0,51
8:  8,95

crossing R17 R18: yes voltage drop after rise.

Optocoupler:
A2 7:   -7,94  to  -7,7
A3 6:   0,15   to  0,03
A3 7:   8,76   no  difference
A4 2:    0,18  to  7,9
A6 1:   -7,96  to  8,14  
A6 2:   van 0  to  6,6

-Putting LED between A6#1 and R23, makes the LED work.
It acts a bit weird though. The LED lights up when there is hardly any input, playing louder makes it light less. In the UP position it does nothing. Is this nomal?
The voltage seems weird too on A6#1.
De current to before the LED in the opto measures 2,44 mA with an R23 of 2,3K that I have in there. Is that normal?

I unsoldered the LDR sides of the optocoupler. Jumpered the PEAK pot. To check the resistances.

DRIVE DOWN:          lots and loud input               no signal
A2pin7 and A3pin6:  rises slowly to 150M        14,1K
A3pin7 and A4pin2:  rises slowly to 150M        14,4K

With normal input of one or two short notes it will rise to 1M. If I keep playing the value keeps rising.

DRIVE UP:              lots and loud input               no signal
A2pin7 and A3pin6:  14,2K                    rises slowly to >200M
A3pin7 and A4pin2:  14,5K                    rises slowly to >200M

If I mute the note it will slowly rise to >200M.

Audio probe tests:
-input clean signal,
-GAIN pot clean signal,
-A1#1 OK clean signal, GAIN works

opamp A 1,2:        
1      yes loud clean signal  
2      no signal
3      no signal  
4      very soft fuzz
5      yes loud clean, works with GAIN  
6      yes softer fuzz with kindof a changing fuzz sound  
7      yes softer fuzz with kindof a changing fuzz sound
8      very soft fuzz
       
opamp A 3,4:        
1      soft, then louder with fading of the note  
2      soft, fuzz becomes louder with fading of the note  
3      no signal  
4      very soft fuzz  
5      no signal  
6      soft fuzz
7      soft, fuzz becomes louder with fading of the note
8      very soft, fuzz becomes louder with fading of the note

opamp A 5,6:        
1      soft, fuzz becomes louder with fading of the note and gets clean sound mixed in  
2      soft, fuzz becomes louder with fading of the note and gets clean sound mixed in
3      soft, fuzz becomes louder with fading of the note and gets clean sound mixed in
4      very soft fuzz  
5      fuzz
6      different kind of fuzz
7      very loud more piercing fuzz
8      very soft fuzz

-I noticed the fuzz signal bleading in on the voltage rails. So I tried to replace the caps to the values of the Neutron clone:
http://www.generalguitargadgets.com/diagrams/neutron_filter.pdf
So 2 elco's 4,7uF and two ciramics 0,1uF
It doesn't change my signals. :-(
Is that the correct way to do it?

-Reducing the GAIN doesn't change the fuzz.
-"Bypass" works fine.

-I am testing with bassguitar, should work right?

Almost ORIGINAL GUTS from before most of the replacements are here:






Could someone please measure and post his/her voltages? I have no way of comparing.

Could there be faulty resistors making this problem?

I should also mention that it was working before. The power supply had really bad cable maybe it shorted +9 to -11V. Now I have only fuzz after A1.

So apart from my questions I am curious about your analytical conclusions.

[Processaurus]

Strange!  My Mu-Tron Vol/Wah had the exact same symptom, fuzzy sounding, and the fuzz was coming from little spikes (at the frequency of the audio) on the power rails.  For me, resoldering everything and replacing the electrolytics fixed it, without me ever learning the specific cause of the problem.  Maybe check the continuity between everything that should be ground and make sure it is indeed low resistance (it may read zero volts, but verify it is connected with no resistance).  Also you should decouple the regulators inputs and outputs with .1 film or ceramic caps, sometimes regulators will oscillate without them, because electrolytics don't absorb high high frequencies.

Otherwise, maybe there is a short somewhere from the rectified audio in the envelope detector into the audio path?

Did you get this pedal broken, or did it just stop working one day?  If it was always broken, I'd look for previous botched repairs or dumb stuff like the jacks wired wrong.

Could there be faulty resistors making this problem?

Resistors are the least likely component to fail, usually the order of likelihood of failure is
Mechanical parts
wires breaking at the solder joint
IC's
Discrete transistors
Electrolytic caps
Tantalum caps
Film/ceramic caps
resistors

[Kas2]

I will check the resistance to ground.

Are you saying that the regulators should be decoupled in the power supply extra? Or is it good like it is designed in the schematic C11, 12, 13, 14?

I checked the shorts. On the pics you can see there is one jumper very close to a diode. That's the closest things get. But has always been like that, and the unit worked.

So yes the unit just worked before.

The fact that I am testing with a passive Fender Precision should be fine right?

[Kas2]

 
It's working again!
I took away more old solder on some switches. I had remelted them but clearly that wasn't enough cause now it works! 
So I guess for me too the answer was: Mutron fuzzes? Take away all solder and replace it.

Now I decided to solder back some old parts for original vintageness, but the sound got way thinner and slower.
I put back C3, 4, 5, 6, 7, C11, C12, C13, C14 (old ceramic disc) and A5,6.
Would it be possible that the slope changes this way?
Could it be the cause of the thinning out the tone?