Moen Fuzz Moo power / grounding issue

[moid]

Hello everyone

How are you all? It's been a long time, but the end of the academic year has finally happened and I have a few weeks away from students and so I can turn to matters effects pedal (finally)... like an albatross returning to land to lay an egg once a year, I hope you'll have a look at what this year's nest contains! Before I get onto various other projects that never got finished last Summer, my son has listed our number one priority as fixing one of his favourite fuzz pedals - the Moen Fuzz Moo. It's a big muff variant of some sort, and sounds great except it has started to constantly self oscillate when he lets any notes decay out - and it does so loudly in a very annoying fashion. I tried recommending just playing fast and never stopping, but hey what do I now about modern music? Apparently I need to fix this... and as I don't have any wish to inform him that I have no idea what I'm doing, I'm hoping you kind people can help me figure out what the hellis wrong with it so I can regain my position as a father that is worth knowing.

If the pedal is powered by a battery then it behaves perfectly; the issue is only with mains power and exists on different power leads (ones that work fine with every other pedal I own), so I suspect the issue is something to do with power or grounding? I took some photos of the interior, and also found a schematic if that helps. I've barely touched anything electronic for a year now, so I apologise if I've forgotten terminology / social niceties / what the hell I'm doing in this post.

The interior looks OK (nothing is on fire, my usual go to for pedal issues), one weird thing - there is a ground cable coming off the pedal that goes to a copper ring that wraps around the stomp switch (I think it's millenium bypass in case that matters). I've never seen this before so I wonder if this is the issue? It was wedged between the nut and the enclosure, so it occurred to me that maybe that was causing a short somehow, so I tried covering the interior of the enclosure with insulation tape (this has fixed self oscillating issues in fuzzes I've built before)... but no luck this time.

Any guesses where I should look? I'm not even sure what to look for - the power jack is I guess the most obvious place to start - I have a DMM; I tried to connect it to what i think are the + and - of the jack where it attaches to the PCB but could only get a tiny voltage reading 0.005v DV when the pedal is on which seems odd... so I'm probably doing that wrong.

Thanks for any enlightenment you can bring to this device! I hope it's not too hot where you are (it's way too hot here ) and that your Summer is going well.

edit: sorry about the image sizes I thought I could resize images using [img350x250]http://sort of BBcode... but it doesn't seem to work - here's a link to the originals on Imgur if that is easier to look at

edit02: aha figured the code out

Code Select Expand

[img width=250 height=350] works fine. Images should be either right click and show iamge to see them in a tab of their own, or just left click them to make them expand inside the DIYStompboxes forum.

https://imgur.com/a/mgRc9ga

PCB


ground cable connected to stomp switch


schematic

[antonis]

I should move 100R resistor right after LED current limiting resistor & add a 47-100μF shunt cap..

P.S.: Your switch grounding ring isn't at its best conductivity state..

[amz-fx]

The audio jacks are plastic so the ring with the ground wire on it is used to make sure that the enclosure and the switch body are grounded. You do not want to insulate it with tape.

There are clearly 5 transistors in the photo but only 4 on the schematic, so there is a discrepancy there.

It is also curious that there are places for two more transistors and their associated parts - probably for some related version of the pedals with extra features.

I would re-solder the connections to the power jack and any jumpers that appear related to it.

There are 3 ceramic 470p capacitors and I would make sure that their solder joints look good too.

regards, Jack

[Mark Hammer]

Not discounting the possibility that the drawing is inaccurate, I see the PCB for bypass switching shows 6 lugs, rather than the 9 we'd expect from a 3PDT stompswitch.  I'm guessing that the 5th transistor is for some type of buffered bypass used with a DPDT stompswitch.

[antonis]

Most possible some version of Millenium by-pass..
(although it should have 5^th transistor and respective circuitry on DPDT switch PCB..)

[moid]

I should move 100R resistor right after LED current limiting resistor & add a 47-100μF shunt cap..

P.S.: Your switch grounding ring isn't at its best conductivity state..

Thanks Antonis. I can't find a current limiting resistor for the LED on the PCB which is really weird... also now that I look at the rear of the PCB more closely, I can see it has a different layout of connection strips, and some components are on the underside of the board... and I'm not sure how close that schematic I posted is to this actual PCB... regarding the components you mentioned they presumably both connect to ground?

I did wonder about the insulation tape and the switch grounding ring... what can I say it's as hot as hell here, my brain isn't at it's best (probably never is, but the heat makes it worse!). The oscillation issue was occurring before I put the tape in place, so removing it presumably won't fix that issue... maybe I should run a cable from the switch ring to one of the ground points on the board? Or the ground points on the power jack (which seems to have two ground points!)

[moid]

The audio jacks are plastic so the ring with the ground wire on it is used to make sure that the enclosure and the switch body are grounded. You do not want to insulate it with tape.

Yes I did wonder about the madness of that action I'll remove it - but it isn't helping fix the oscillation.

There are clearly 5 transistors in the photo but only 4 on the schematic, so there is a discrepancy there.

I think the fifth transistor is for millenium bypass - it say C1815 on it (all the rest are clearly 2N5088)

It is also curious that there are places for two more transistors and their associated parts - probably for some related version of the pedals with extra features.

Yes there seem to be a few unused areas of the PCB... some do have components on the underside (there is a diode and a resistor on the underside)

I would re-solder the connections to the power jack and any jumpers that appear related to it.

There are 3 ceramic 470p capacitors and I would make sure that their solder joints look good too.

regards, Jack

Thanks very much, I will have a go at that as soon as it cools down here (hopefully around Friday /Saturday). It's 31 degrees centigrade here at the moment and the idea of turning a soldering iron on to make the room even hotter is not one I can entertain right now! I will report back once I've done this.

[moid]

Not discounting the possibility that the drawing is inaccurate, I see the PCB for bypass switching shows 6 lugs, rather than the 9 we'd expect from a 3PDT stompswitch.  I'm guessing that the 5th transistor is for some type of buffered bypass used with a DPDT stompswitch.

I am pretty sure you are correct - the stomp switch is a six pole switch, so that sounds like Millenium bypass to me. The schematic might not be accurate; I found it online in and posted it in case it helped anyone understand the PCB. The PCB doesn't seem to be using the same components so it might not be of much use alas.

[moid]

Most possible some version of Millenium by-pass..
(although it should have 5^th transistor and respective circuitry on DPDT switch PCB..)

There are strips connecting the DPDT switch PCB with that transistor, so I think you are right about Millenium Bypass.

[antonis]

regarding the components you mentioned they presumably both connect to ground?

100R resistor (the one right after the battery/PS) should be kept in series with whatever is fed from +9V except LED (and its respective resistor) & the cap should be connected between right resistor leg & ground..
(to form a LPF unaffected by LED current fluctuations..)

I did wonder about the insulation tape and the switch grounding ring... what can I say it's as hot as hell here, my brain isn't at it's best (probably never is, but the heat makes it worse!). The oscillation issue was occurring before I put the tape in place, so removing it presumably won't fix that issue... maybe I should run a cable from the switch ring to one of the ground points on the board? Or the ground points on the power jack (which seems to have two ground points!)

Don't mess up with a lot of grounding points..!! 

Plastic audio jacks can't ground enclosure so there should be another grounding point and this is obtained via switch ring & black cable..
(it's simply used DPDT thread & nut to make a firm connection between PCB ground & enclosure..)
PCB ground should be originated from power supply GND (either battery negative pole or power jack minus (-) jug..
There is only one "ground" point on power jack - the other two are for switching +9V between battery and external PS..

P.S.
Jack's suggestion for solder reflowing NFB loop caps is more than wise 'cause you deal with a high gain circuit which is prone to high pitch oscillations..!

[moid]

OK finally had some time to look at this PCB. I have re soldered all the power supply tabs and the legs of all the ceramic capacitors... sadly the circuit still makes a constant high pitched noise that only goes if it is powered by a battery

I also resoldered the black cable and ring that connects to the stomp switch, but that didn't help.

The schematic (having looked at it a lot more closely) is very different to the actual PCB sadly, so I don't think it should be used as much of a guide. I might try drawing an image of Antonis' suggestion about the 100r resistor (which I think the schematic has, but the PCB doesn't) and then try that? I'm rather confused... is there anywhere on the PCB I should be measuring with the DMM to try to find a fault? Thanks for your help. If there's no way of fixing this, I've found the company have re issued the pedal in a smaller format and it's quite cheap so I could buy one for my son once I get paid at the end of the month.