MXR Classic 108 Fuzz

paperhouse · Post by **paperhouse** » 18 Jul 2008, 03:36

Supposedly an Si Fuzz Face with a switchable buffer (not a bypass buffer, but a buffer make it not oscillate when using it with a wah), but it looks like there's a LOT more going on in there:

I count 7 transistors

It looks like there are some diodes and tant caps in there.

Not my pics, taken from this thread @ HC http://acapella.harmony-central.com/for ... st29112288

Fuzzer · Post by **Fuzzer** » 18 Jul 2008, 15:01

The Switch is a DPDT, and the pedal is (allegedly) True Bypass, so maybe a big bunch of those extra parts are used to accomplish the switching, I thought the were going to use relay-based switching with a simpler switch. Maybe one of those momentaries.

The buffer circutry goes before of after the Fuzz?

Torchy · Post by **Torchy** » 18 Jul 2008, 15:07

After the fuzz (as its designed to be used before the wah)?
It probably uses a RAT/Millenium style tbp.
And including the metal can 108s I count 10 trannys...

Fuzzer · Post by **Fuzzer** » 18 Jul 2008, 15:21

I'm sorry, now that I rad the HC thread ther'es the answer to my question

"The Classic 108 Fuzz uses the same configuration in circuitry design as the JHF1.

It also utilizes the same BC108 transistors. The form of the board is changed to utilize several necessary functions aside from being PCB built. It has to fit the MXR box design which includes the ability to accommodate PCB mounted input/output jacks and a power adapter.

The buffer sits in front of the fuzz and is completely removed from the circuit when it is disengaged.

The three internal trim pots are used to match the BC108 transistors to one another and to the factory specification for the Fuzz. Changing these settings is not recommended."

That was from Dunlop themselves;

Other thing is, tkae in account that you also need switching for the buffer, it is also supposed to be bypassed completely, with the little LED indicator.

And last but not least, some might have thought that I'm crazy, or one of those specimens whot think they can 'hear the grass growing', but in the thread someone also states;

First thing I noticed was that it really didn't like my Dan-echo. I've always heard about the poor buffers on the Danelectros but it's never bothered me until I found that it robbed the Classic Fuzz of a lot of it's fuzzy-ness.

I'm pretty sure he doesn't use the delay before the Fuzz, so it proves that some (most of them for what I've tried) non-TrueBypass pedals screw up a Fuzz Face sound, even placed after them. This I stated in other thread;

https://www.freestompboxes.org/viewtopic.php?f=10&t=2760

So If anyone has some thoughts on this, I'd greatly appreciate them.

nooneknows · Post by **nooneknows** » 17 Aug 2008, 22:41

Hi,
Please, is there anyone who has this pedal? If so, is it possible to know the collector voltages of the two BC108?
My last attempt to work around a fuzz face circuit is this one that I post here.
The buffer seems to work ok but I'm curious about the voltage readings of the MXR, I put mine using... ears but it doesn't work as I wanted (particularly, when I back off my guitar pot the fuzz gets way too darker than I expected, but it cuold be the cap between the collectors, too high).
Thank you

Ripdivot · Post by **Ripdivot** » 24 Feb 2012, 04:22

I borrowed this fuzz from a friend and I was able to make some progress even though I only had it for 1 night. It is a nice sounding fuzz and the buffer actually works quite well. It cleans up real nice with the guitar volume knob both with the buffer on or off. A little better without the buffer. Check out Andy’s demo here:

https://www.youtube.com/user/ProGuitarS ... nolbaSQgxA

I was able to trace the fuzz circuit but I didn’t have time to trace the rest. The buffer is what I am mostly curious about. The pedal is definitely true bypass as I was able to meter that part out. One of the pics shows the buffer switch connections for both on and off. Another pic has the component values. If you need more photos let me know, I took about 20. Transistor voltages are on the schematic...

snyder80 · Post by **snyder80** » 24 Feb 2012, 06:55

Thank you, must have been quite a lot of work. Special Thanks for the Voltages in the Schematic...

snyder

Ripdivot · Post by **Ripdivot** » 24 Feb 2012, 15:32

snyder80 wrote:

Thank you, must have been quite a lot of work. Special Thanks for the Voltages in the Schematic...

snyder

You are welcome. Yes it was a fair bit of work to do in one night.

Harold · Post by **Harold** » 20 Apr 2012, 11:09

Ripdivot wrote:... The buffer is what I am mostly curious about ...

Same here!

Any chance you can make a schematic for that too? I promise I'll make a strip/veroboard when there's a schematic available!

BAARON · Post by **BAARON** » 23 Apr 2012, 03:24

Is the 47k feedback resistor really connected to the input side of that cap rather than the transistor side, or is that an error in the schematic?

Nocentelli · Post by **Nocentelli** » 27 Oct 2012, 12:35

BAARON wrote:Is the 47k feedback resistor really connected to the input side of that cap rather than the transistor side, or is that an error in the schematic?

I was going to check out the schematic here before trying out IvIark's layout and was wondering the same.

IvIark · Post by **IvIark** » 27 Oct 2012, 13:00

I have my doubts

Nocentelli · Post by **Nocentelli** » 27 Oct 2012, 13:41

Well, it's a simple switch on the vero if the 47k does need to go direct to the base: I'll breadboard it this afternoon and see.

mirosol · Post by **mirosol** » 06 Nov 2012, 20:04

BAARON and Nocentelli are both correct. I just gave Mark's layout a try, and banged my head against the wall for some time. Then i searched for the schem and found this thread. That is most definitely an error on the schem. 47K feedback resistor needs to connect to Q1 base and not the input.

Otherwise the schematic and layout are both correct.

It's a nice fuzz. Not too different from blue face though. Although for my taste, the fuzz pot could be C1K and i went with B500K for the volume. For the transistors, i used BC108B (389 hFE) for Q1 and BC108C (517 hFE) for Q2. Only other deviation was 7R5 instead of 5R1, so that might contribute to my C1K opinion. This isn't going to get very high rank on my to-be-boxed list.. Maybe i could try to squeeze it in to an A box... :)

Anyway. Thanks guys!

mirosol · Post by **mirosol** » 06 Nov 2012, 20:12

Hmmmm... Makes me wonder about the faith of that 47p cap.. Circuit works just fine with it at the input, but i think that should also go to the base of Q1.

IvIark · Post by **IvIark** » 06 Nov 2012, 20:41

mirosol wrote:Hmmmm... Makes me wonder about the faith of that 47p cap.. Circuit works just fine with it at the input, but i think that should also go to the base of Q1.

Yes I thought it would too. Thanks for the heads up Miro, I'll update the layout

nooneknows · Post by **nooneknows** » 11 Nov 2012, 08:31

Any chance to see the traced buffer? that would be very interesting, the sound in pro guitar shop video is very clean when the volume down and the buffer in.

roseblood11 · Post by **roseblood11** » 11 Nov 2012, 20:44

nooneknows wrote:Any chance to see the traced buffer? that would be very interesting, the sound in pro guitar shop video is very clean when the volume down and the buffer in.

I would be more interested in the buffer than in the fuzz itself...

Harold · Post by **Harold** » 03 Apr 2013, 21:18

I received my MXR Classic 108 fuzz today ...

Let the tracing commence!

Ripdivot · Post by **Ripdivot** » 04 Apr 2013, 02:00

Cool Harold! I would have finished the trace but I only had the pedal for 1 night. Looking forward to seeing what the buffer looks like.

MXR Classic 108 Fuzz [schematic]