ehx muff overdrive problems and mods

[CynicalMan]

I recently got a used electro-harmonix muff overdrive with the intention to mod it. The problem is that I can't access the circuit board since the knob seems is on very tight. Despite attempts to get it off with a screwdriver and pliers, it just wouldn't come off. Does anyone know any tricks or anything for getting the knob off?

Also, does anyone know where to find the schematic for the muff od? According to the electro-harmonix website the muff od is a "Reissue of the original 1969 Muff Fuzz". There are 2 muff fuzz schematics floating around on the internet but I don't know which one I have. I'd just look at the PCB but I can't because of my previous problem.
http://fuzzcentral.ssguitar.com/schematics/mufffuzztransschem.gif
http://fuzzcentral.ssguitar.com/schematics/mufffuzzicschem.gif

Last, are there any suggested mods for this?

[CynicalMan]

Never mind about the knob problem, I just got it off. The circuit has 2 transistors and 2 diodes but doesn't seem to match either schematic. Also on the PCB, there are 3 grey cylinders labeled as FB1, FB2, and FB3. What are these?

[bumblebee]

FB= Ferrite Bead. Although I wouldn't refer to them as "cylinders" Unless you are mistaken then they could be SMD electrolytic caps as they are cylindrical.

[CynicalMan]

They are cylinders. They look like this except more greyish.

http://rocky.digikey.com/weblib/Steward/New%20photos/28L0138-40R-10.jpg

[bumblebee]

Yep, ferrite beads.

[reverbie]

Do they ferrite beads REALLY need to be there???

[CynicalMan]

My guess is that they are part of some sort of power filter.

I can't trace the exact schematic because the effect is built on a 2-sided PCB, part of it is obscured by jacks, components, etc., and I'm no good at tracing.
However, if anyone was willing to try tracing the schematic from pictures, I'd be willing to supply some.

I think I'll mod it with a switch to turn on germanium clipping diodes.

[bumblebee]

A schematic allready exists for this effect, minus the ferrite beads of course.

[CynicalMan]

Where can I find it?
The closest schematic I could find was this:
http://fuzzcentral.ssguitar.com/schematics/mufffuzztransschem.gif
And like I said, it doesn't seem to match the circuit exactly, though it is an OK approximation.

[fuzzo]

I have to look in my files but I think I've the double muff schematic from EHX ,factory schematic (It's two muff overdrive together). I can post it if that can help you.

[bumblebee]

Where can I find it?
The closest schematic I could find was this:
http://fuzzcentral.ssguitar.com/schematics/mufffuzztransschem.gif
And like I said, it doesn't seem to match the circuit exactly, though it is an OK approximation.

What doesn't fit exactly? There's only the transistor version and the IC version of thi spedal.

[CynicalMan]

I have to look in my files but I think I've the double muff schematic from EHX ,factory schematic (It's two muff overdrive together). I can post it if that can help you.

That would be really useful.

Where can I find it?
The closest schematic I could find was this:
http://fuzzcentral.ssguitar.com/schematics/mufffuzztransschem.gif
And like I said, it doesn't seem to match the circuit exactly, though it is an OK approximation.

What doesn't fit exactly? There's only the transistor version and the IC version of thi spedal.

I think electro-harmonix changed the muff fuzz circuit a bit for the muff od. For example, the input and output caps are .068u and 1u rather than .1us. Also there are the ferrite beads and 3 or 4 100pF caps

I just soldered in the mod, a DPDT On-Off On switch that switches between adding 2, 1, or no 1N34A diodes to ground on the output. Here's a diagram of it (sorry about the crappy paint graphics).

http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=38811&g2_GALLERYSID=6311a38a7cecfe2f948834f912973899

[bumblebee]

I have the double muff schem if you want that, it may help.

[fuzzo]

<an - copyright image removed>

it's  the same values in EHX double muff (I had one)

[CynicalMan]

Thanks for the schematic!

I finished the mods today and everything is working great!

[slideman82]

http://fuzzcentral.ssguitar.com/schematics/mufffuzztransschem.gif

It's quite like this but with a 68n input cap, 1uF electro output cap, 3.3k instead of 2.7k and POSITIVE GROUND  !!!!!! I don't know why EH made this with NPN transistors, and it's full of inductive filters and 100p cap. I never listened to it, a friend bring me one to mod it. And for shure I'll make it negative ground.

[CynicalMan]

Here's the info about the mods I did:
http://sites.google.com/site/distorque/home/mods/ehx-nano-muff-od

As annoying as the positive ground is, I would avoid trying to make it negative ground. There are a lot of weird power filtering components in there, such as the ferrite beads, and I think it would be difficult to make it negative ground. Plus, that might also add noise and/or oscillations, as sometimes occur in negative ground fuzz faces.

[slideman82]

Well, I'm gonna take off some FB and a 10 ohm resistor that goes from -9V to filter cap, I've already traced a schematic, and I must say last transistor is a buffer, not a gain stage, I forgot to tell that in my last post.

[CynicalMan]

If you have a schematic available, could you please PM it to me?

I wouldn't take off that resistor if I were you. I've blown it out before when I accidentally daisy-chained the pedal with negative ground stuff, but it was the only thing that saved me from breaking harder to replace components.

[slideman82]

I've already done it, I'll change it to negative ground so my firend can use it with his multi 9V supply. I drew it in a piece of paper, the circuit is the same as the link I posted with the differencies I said, and you have to change 9V to ground and ground to -9V. Also you hace a FB in series with the 10 ohm resistor from -9V to transistor supply, some 100p filter caps and a 220uF supply filter cap.

Under supply jack it says something like "Do not daisy chain supply"!

I don't know why EH made this distortion with positive ground, really a crap.