Mutron

[CJ]

a popular box back in the day by Mutronics,

anybody ever fix one?

we have an original version on the bench which is supposed to sound better than any reissues, how many times have you heard that?

Mr Garcia like to use it on Estimated Prophet and other gems, if you want to know how it sounds,

here is the schematic, probably a bad opto, some folks make their own with LED's and LDR's,

JCLectronics, WTF, over?  ;D

thanks for any help,

 

[Flundran]

Just so you know how much they go for... 

 

[rock soderstrom]

The Battery Eliminator is a friend of the Terminator.   

 

[L´Andratté]

Total garden variety filter with garden variety envelope follower. Although this one is fine tuned.
There´s a geofex site about it.
I build this and other topologies with vactrols, and basically they all sound good.
Vactrol bad matching is sometimes making the resonance sound very nice.

What´s wrong with it in the first place?

 

[Gus]

I have fixed a few mu-tron phasers
First check all the solder connections
Next check the wires test if they are broken
Check for broken PCB traces
I have worked on a few older effects where all that was wrong were broken connections or broken jacks or broken switches etc.

Then start to signal trace. My best advice don't assume it is a broken transistor or chip etc., check the simple things first


The more difficult ones to fix were ones someone else tried to fix before me.

 

[CJ]

thanks Gus!

 

[CJ]

replaced all 3 opamps, this thing works but not very well,

vactrols have been substituted for original opto device, if we get something like the original opto, it should work like a stock model,

 

[Gus]

Did you use the correct 4558's? this is more important for A6. 
R23? (hard to read the number) is important to adjust this sets the LED "start" current.
Did you receive the Mutron 3 with substituted optos?

 

[CJ]

used 4558's,

these opto's look a lot newer than the 70's type parts so somebody must have swapped them out,

this circuit could use some polarity protection diodes to protect the opamps,

 

[CJ]

this thing has some VTL3C vactrols installed, 2 ea,

not the best sound but they work,

did 2 mods,

1) change input resistor from 3.3K to 33K, this limits max gain when the pot is turned all the way (zero ohms, divide feedback resistor by input resistor to get gain)
original units had a tendency to distort at max gain, and with no tru-bypass footswitch, this means the clean signal suffers too,

2) put a pot on the LED drive of the vactrols. now you can get all sorts of new sounds by biasing the on current of the vactrols at different points,  a 5K current limit keeps the vactrol LED from popping, you can wire the mode sw to HP, (the only usable setting on the box ) and use the sw hole for the 10K pot,

still, does not have the max wah effect we are looking for, so maybe we will roll our own vactrol with LED's and LDR's for more resistance drop per volt,

here are the mods>