Help diagnosing distortion source - Video & Pics Inside

[Winecaster]

Hey folks, this is a pedal question but I'm hoping for someone to help out and be a hero with this one.
I picked up an original Mutron III (Envelope Filter) pedal yesterday from a keys player who said he's been using it for the last 12 years without any issues.  I got it home and plugged in (Strat>Mutron>Amp) and the only sounds I'm able to get out of the pedal when switched on are very fizzy/distorted and quiet.  It seems that I can play through it despite the power switch being off and the stomp switch being "off" as well.

It essentially has three different modes of use (Low Pass, Band Pass, and High Pass) and the pedal makes a pop every time I sweet between them.  In LP or BP modes I only get that fizzy distortion that I described, but in High Pass its silent for a bit, then fizzy, then it eventually sends a clean sound through to the amp-- but it's not an altered sound that the pedal should be making, just clean.

I'm using brand new batteries and have tested my cables to make sure they're working properly.  I'm savvy with a soldering iron but am not a pro with reading a schematic or using a multimeter (I do have one).

[jonnyeye]

Hello, and welcome to the forum!

Popping when a switch is thrown is often a sign of DC across the contacts. From the pictures someone has likely been in there in the past and swapped out the electrolytic caps (and, if those two caps in the C10 slot are in parallel and not in series, it seems that they were not quite sure of what they were doing). If the caps are leaky there can be DC across them which may be the source of your popping, but there may be another problem that wasn't quite fixed when the caps were done which has now reared its head.

First test: does the popping change at all when the Drive switch is in different positions? That might tell us something.

Next, according to our general troubleshooting thread (http://www.diystompboxes.com/smfforum/index.php?topic=29816.0) get out your multimeter and take readings of the voltages on all of the pins of the ICs. (If you haven't done this before, find the notch on each IC; pin 1 is on the immediate left if it, then number the pins 2, 3, 4... counter clockwise around. Your ground reference voltage can be the junction of the two batteries.) And also check the voltages across the two sides of the electrolytic caps; is the side labeled + more positive than the side labeled - ?

Finally, a picture of the reverse side of the board may be helpful, if you can provide that too.

Hope we can get you up and running!

[deadastronaut]

. sorry wrong post.

[Kipper4]

Welcome.

This could be a tough one.
You say the previous user used it with keys. That's a possibly much bigger signal than a guitar with a single note or two might produce.

I have built several copies and the like.
The pedal has almost too many parameter changing pots for normal use.
It will produce some in my opinion unuseable sounds if not see to a sweet spot.
This could be your distortion.

The gain knob will also make it sound distorted.

I dont think thats what I'm hearing in the demo though.

Before you go thinking soldering iron.

If you could tell us some dc voltages we might gather something from that.
Have you done this before?

Did you see the unit working before you got it?       Those things can be expensive.

[Winecaster]

Thanks for the replies, everyone!  I really appreciate you taking the time to offer your help and suggestions.  I saw this pedal on craigslist and made a 2 hour drive (each way) to pick it up.  He told me it worked, gave me a tour of his impressive recording studio (I guess I kind of trusted him after seeing that) and offered me a package deal with a Mutron II Phasor (which I bought as well-- it works).  I can't believe I didn't request a demonstration of the pedal... totally slapping myself! 

To answer jonnyeye's question:  There is no popping when I alter the drive switch, just the pass selector pot and power switch.  Additionally, in either LP or BP modes if I max the Gain pot I get a "crack" sound that repeats itself at a steady tempo.  My best amateur guess is that a cap(s) are leaking or discharging too rapidly.

I see there's a company that offers a Mutron iii repair kit which consists of twelve components-- some resistors and the electrolytic caps.  I'm wondering if my problem would lay within the boundaries of those parts but will take the IC readings regardless of that curiosity and post them here once I do.

http://www.caesound.net/mutroniiirebuildkit.aspx

[Kipper4]

I'm willing to bet the kit doesnt come with the vactrols, and you could probably buy 5x as many componants on ebay for the same price.


Lets see what the dc volts tell us.

[Winecaster]

I'm willing to bet the kit doesnt come with the vactrols, and you could probably buy 5x as many componants on ebay for the same price.


Lets see what the dc volts tell us.

Alright, so starting with the top IC and working down.  These measurements are taken with the negative probe on the bare wire between the two 9V batteries as pictured in the OP.

IC1
Pin1 7.33VDC
Pin2 7.17
Pin3 0
Pin4 -7.91
Pin5 0
Pin6 -5.05
Pin7 7.30
Pin8 8.05

IC2
-6.52
-0.02
0
-7.94
0.03
0.04
0.33
8.04

IC3
0
0.01
0
-7.93
1.08
0
-5.17
8.04

[PRR]

> IC1
> Pin1 7.33VDC
> Pin2 7.17
> Pin3 0

I bet the price of a 4558 that this chip is blown.

If "Gain" is turned to Max, and you plug IN to a big loudspeaker amp, this can happen. (I do not like this input stage.)

How this happened only on the trip to you, I do now know. Since I do not know you or the seller, I am hesitant to suggest anybody abused it.

[Winecaster]

I replaced the C10 electrolytic cap and all three IC units and she's up and running!  Too bad I only use to a narrow portion of this pedals ability... sounds great though!  Thank you all for your help with this!!