MXR - Super Badass Variac Fuzz  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
Post Reply
User avatar
dbalu
Information
Posts: 3
Joined: 05 Jan 2018, 00:41
Been thanked: 4 times

Post by dbalu »

Hello,
As I am an avid cloner myself, I thought that it's my time to give something to the DIY community. I got ahold of the effect mentioned in the title and I am providing some gut shots, also a list with components. Capacitor values on the list have "?" due to my lack of experience with SMD components. Where there is no indication on the list will be completed tomorrow then I'll get a magnifying glass. All other info from the list are markings on the components (mostly SMD).
Here are some links to the pictures. The final picture is the components list.
https://ibb.co/k3Rwuw
https://ibb.co/ctMMuw
https://ibb.co/i931uw
https://ibb.co/hWUruw
https://ibb.co/cMgh1b
https://ibb.co/g8bRSG
https://ibb.co/imkLnG
https://ibb.co/cDMt7G
https://ibb.co/irOHZw
https://ibb.co/eyf9gb
https://ibb.co/e34D7G
https://ibb.co/jKjVnG
https://ibb.co/fywMuw
https://ibb.co/jRmnZw
https://ibb.co/k7VQMb
https://ibb.co/gjOO7G
https://ibb.co/jF7uEw

User avatar
modman
a d m i n
Information
Posts: 4890
Joined: 19 Jun 2007, 16:57
Has thanked: 4394 times
Been thanked: 2131 times

Post by modman »

Are you kidding me?
Are you kidding me?
Screenshot from 2018-01-05 18-52-39.png (60.37 KiB) Viewed 10436 times
Attachments
MXR Variac Fuzz01.jpg
MXR Variac Fuzz02.jpg
MXR Variac Fuzz03.jpg
MXR Variac Fuzz04.jpg
MXR Variac Fuzz05.jpg
MXR Variac Fuzz05.jpg (31.62 KiB) Viewed 10436 times
MXR Variac Fuzz06.jpg
MXR Variac Fuzz06.jpg (24.03 KiB) Viewed 10436 times
MXR Super Badass Variac Fuzz values
MXR Super Badass Variac Fuzz values
Please, support freestompboxes.org on Patreon for just 1 pcb per year! Or donate directly through PayPal

User avatar
dbalu
Information
Posts: 3
Joined: 05 Jan 2018, 00:41
Been thanked: 4 times

Post by dbalu »

modman wrote:
Screenshot from 2018-01-05 18-52-39.png
If you want i can give you more detailed/Hi-Res pics of the circuit, these are made with my phone

User avatar
dbalu
Information
Posts: 3
Joined: 05 Jan 2018, 00:41
Been thanked: 4 times

Post by dbalu »

The rest of the values that were not included in the list
Q1-836
U1-K6NCD
D5,6&7-B2 INFINITY
U3-G114
Q6-GY G
Q8-JAM 6 sideways
Q3-1R6
D1,D2-AD
U2-K6NCD
Q2-GY G
Q7-1E6
Q5-GY G
D3-8F 6 sideways
Q4-1E6
D9-??BC

User avatar
10goo
Information
Posts: 2
Joined: 03 Apr 2014, 04:42
my favorite amplifier: Marshall
Completed builds: Many, many...many. Fuzz man....

Post by 10goo »

Hello... I think I saw a YouTube video, where the guy explains how the "super badass" came about... It's a "Band of Gypsy" fuzz with a tone added....and then the "variac" circut added on...
You could start your search there...

Good luck

User avatar
DanGibbs
Information
Posts: 2
Joined: 27 Jul 2018, 21:44
Been thanked: 2 times

Post by DanGibbs »

Hello All :)
I'm hoping that there's still some interest in decoding this pedal as I bought one that needed help off of Reverb last week and it doesn't pass audio at all when the effect is on/LED lit up.

I bought this as I like to buy broken pedals and fix them which I'm usually successful at except for a few new(er) Dunlop/MXR/WayHuge pedals that I/we cannot get schematics for.

I therefore earlier this week started a schematic "decode" of this pedal using EasyEDA and have the core effect-function of the pedal schematicized. I ran into a brick wall with regard to what I believe to be the "variac" portion of the pedal at U3 which is marked "|G114" - I did a five minute search on google and I found nothing like it which halts me in my tracks. I was earlier able to identify U1 marked K6NCD which is actually an MMDT3904, a dual 2n3904 package. I'm still trying to ID Q1 marked 9EB which measures as a PNP transistor albeit who knows which one. Some one before me also replaced Q8 and this replacement is marked 1AM - I don't know if this is actually the correct transistor or not as the pictures on this post aren't clear enough to read with regard to Q8.

I'm willing to schematicize the whole pedal if anyone can help figure out what all of the semiconductors are on the board.

In the photo with the pencil-written parts list, U4's part number is incorrect and should be TC7660SC which is the voltage-converter IC that I believe operates the 5-15VDC "variac".

I have an 94kB png image of the partial work-in-progress schematic but I don't know how to insert it here (why is it always so damned hard on a forum to just insert an image?!? -rantoff).

Dan Gibbs :)

User avatar
george giblet
Resistor Ronker
Information
Posts: 404
Joined: 30 Dec 2008, 17:27
Has thanked: 206 times
Been thanked: 309 times

Post by george giblet »

I believe to be the "variac" portion of the pedal at U3 which is marked "|G114" - I did a five minute search on google and I found nothing like it which halts me in my tracks.
U3 is some sort of regulator.

Go to the page 5 or 6 of this thread,
viewtopic.php?f=1&t=13121

So for the Variac, U3 pin 5 goes to R30 (41.2k), then R30 to U3 pin 4,
U3 pin 4 goes to R34 (34.0k), then R34 to ground.

From that I can see, the output voltage would be,

Vo = Vref * (R30 + R34) / R34

Where Vref is the reference voltage of the regulator referred to ground. I would expect Vref to be around 1.23 to 1.25V. However, it could be 1.14 following the number printed on the chip (as some regulators are marked like that). Suppose we go with 1.25V

Vo = 1.25 * (41.2 + 34.0) / 34.0 = 2.76V

That seems low but maybe the pedal is called "variac" because the voltage is reduced; as variac would do on an amp.

So keep going! It's only power rail. You will see the rest of the circuit make more sense once you know that.
Notice the output of U3 goes to a resistor R1 (10 ohm) then to maybe looks like big cap.

User avatar
george giblet
Resistor Ronker
Information
Posts: 404
Joined: 30 Dec 2008, 17:27
Has thanked: 206 times
Been thanked: 309 times

Post by george giblet »

However, it could be 1.14
Maybe even 11.4V. Don't worry about the details for now it might become clearer.

FWIW, in the other link I posted I've identified a lot of parts on that unit. You might be able to copy some of that info (the text in the thread had quite a few misidentified parts so watch out).

User avatar
DanGibbs
Information
Posts: 2
Joined: 27 Jul 2018, 21:44
Been thanked: 2 times

Post by DanGibbs »

Hello George Giblet and thanks for the info!

I've been working diligently in most of my spare time on the schematic and I think I have it all diagrammed. A crap-ton of stuff to figure out and trace for sure. I'm in the process currently of verifying traces between the actual PCB and my schematic after removing most of the through-hole components to make sure I haven't missed anything (I'm sure I have). I called and got the actual part numbers for all of the transistors and ICs last week but I shied away at that moment of asking for the diodes and SMT capacitors as I don't want to seem too greedy and burn that new bridge.

I'll definitely get back when I'm satisfied that the circuit is as correct as I can see it. The next debacle would be trying to re-arrange the schematic to where it's better than a ball of rewound yarn to where I can make sense out of the circuit sections (phwew whadda mess). I'll prolly upload it anyway once I verify the traces before I try to re-arrange.

Again, my goal here is to make a schematic to repair my non-op unit that I bought a few weeks ago. I'm not looking to clone it and I doubt it would really be worth the effort to most folks to clone it - just my opinion.

Dan :)

User avatar
george giblet
Resistor Ronker
Information
Posts: 404
Joined: 30 Dec 2008, 17:27
Has thanked: 206 times
Been thanked: 309 times

Post by george giblet »

I'm not looking to clone it and I doubt it would really be worth the effort to most folks to clone it - just my opinion.
Same here.

User avatar
Fernando JFET
Information
Posts: 8
Joined: 20 Oct 2008, 12:11
Has thanked: 7 times

Post by Fernando JFET »

Ok, John Frusciante brought me here. What is the project status? How can I help? Thanks

User avatar
G.Bisson
Information
Posts: 39
Joined: 29 Apr 2021, 13:42
Has thanked: 17 times
Been thanked: 73 times

Post by G.Bisson »

I made an attempt at tracing this pedal.

Component side of pcb
VariacFuzz_pcb_top.jpg

Bottom side of pcb
VariacFuzz_pcb_bot.jpg

Incomplete photoshop tracing. Additional connections were confirmed later on by probing for continuity with a multimeter. Magenta lines are traces on the component side of pcb. Green dots are terminations to ground plane on component side. Blue lines are traces on bottom of pcb.
VariacFuzz_pcb_trace.png

Materials list. Active components were looked up by their markings. I'm not 100% confident in U3. Haven't looked up any of the diodes markings. SMD capacitors were measured with a multimeter while in circuit.
VariacFuzz_Materials.png

Schematic, may contain errors
VariacFuzz_Schem.png

User avatar
G.Bisson
Information
Posts: 39
Joined: 29 Apr 2021, 13:42
Has thanked: 17 times
Been thanked: 73 times

Post by G.Bisson »

Revisited this one and noticed C15 was missing from my schematic. I still have this pedal and probed it to confirm connections to C15 and updated the schematic.
GBisson_VariacFuzz_Schem.png
Also double checked the values of the smallest capacitors C7, C9, C14, C15. Judging by the size of these, they should all be in the picoFarad range. Probed them again and came up with the same results, except C14 measured much higher this time at 1790pF. The in-circuit measured values are inaccurate. Values from Nocentelli's Band of Gypsys schematic are probably more accurate:
C9 = 47pF
C14 = 150pF
C15 = 22pF
C7 is not present in the Band of Gypsys fuzz.

Band of Gypsys schematic is in this thread: viewtopic.php?p=281465#p281465

User avatar
willwaush
Information
Posts: 1
Joined: 09 Sep 2016, 13:43

Post by willwaush »

The issue here is that without knowing what U3 (G115) is, it would be very hard anyway to replicate the circuit.
Unless there is an alternative circuit which can still generate the same voltage swing that this pedal has.

User avatar
george giblet
Resistor Ronker
Information
Posts: 404
Joined: 30 Dec 2008, 17:27
Has thanked: 206 times
Been thanked: 309 times

Post by george giblet »

willwaush wrote: 13 Sep 2023, 18:56 The issue here is that without knowing what U3 (G115) is, it would be very hard anyway to replicate the circuit.
Unless there is an alternative circuit which can still generate the same voltage swing that this pedal has.
The earlier parts list had: R1154H115B

Nisshinbo Micro Devices Inc
https://www.nisshinbo-microdevices.co.j ... 154-ea.pdf

R1154Hxxx*-T1-FE
H SOT-89-5 package
xxx voltage, format x.xx volts
* B = fixed output, or, C = adjustable

R1154H115B
H SOT-89-5 package
111 1.11V
B fixed voltage

R1154H001C
H SOT-89-5 package
001 default for adjustable
C adjustable

However some things don't add up:

1) I would expect pin 3 to be connected

The datasheet says:

"Chip Enable Input
Do not make voltage level of chip enable pin keep floating level, or in between VIH and VIL .
Unless otherwise, Output voltage would be unstable or indefinite, or unexpected current would flow internally."

The CE pin, pin 3, should connect to something.
The datasheet implies a voltage between 2.1V and VIN.

2) If the part is a fixed voltage "B" part then the Adjust pin , pin 4, should be left open.

If the part is an adjustable "C" part then the part number should be 001C.

However, if this is a feedback regulator the connection of R22, RP4, Q3 etc to the input supply doesn't make sense.
The regulator output would not be able to regulate. In this case I suspect R22, RP4, Q3 part of the circuit
to connect to the regulator output.


It should be straight forward to replace that regulator, whatever it is, all we need to know is the range of voltages at the output of the regulator when the voltage pot RP4 is set to both extremes.

User avatar
manhalt
Information
Posts: 1
Joined: 03 Apr 2019, 20:33

Post by manhalt »

Nice work on the trace! I have some info share to from tinkering with the core circuit on a breadboard:

1. Q1 collector and emitter might be swapped if it's a BC860. I tried both ways with a 2N3906 and reversed sounds a lot closer. It's a bit odd since most Octavia schematics have the transistor oriented the same way as this schematic, but that way always has this weird sag that cuts out the noise after a loud note.

2. C9 looks like it goes from Q1 base to R10.

3. The circuit stops working when I ground the 1st pin of the Gain pot. I see the connection to the copper fill but this might be an island, I forgot to probe this when was checking out the PCB.

4. My tone control was acting funny, haven't verified the trace yet but will soon.

5. I measured voltages with Vc set to 9V using Voltage pot:
Q1 (C E might be swapped, sorry)
E 2.7V
B 3.4V
C 4.9V
Q2
E 2.3V
B 2.7V
C 9V
Q3
E 1.6V
B 2.3V
C 5.6V

I know the interesting part is the Variac but honestly, this feature sounds amazing but doesn't get gross enough for my taste. Going to try some good ol bias trimmers and attempt to coax out some more gated sounds.

User avatar
Qwi
Information
Posts: 3
Joined: 02 May 2009, 02:59

Post by Qwi »

This is an interesting schematic.

I have a question about it, if anyone cares to help me think it through.
I can see the fuzz at the bottom, and the charge pump, voltage regulator, and variable voltage level to be sent to the bias the fuzz at the top right. But could someone explain to me what U2, Q2, Q4, Q5, Q6 and Q7 are doing? Basically the whole top-left part of the schematic is hard for me to follow. Is it simply to switch the led, and keep the circuit stable when the input is switched out? It must be more complex than that, no?

If anyone could clear this up for me that would be much appreciated.
Thanks!

User avatar
Lani
Solder Soldier
Information
Posts: 194
Joined: 22 May 2017, 11:57
my favorite amplifier: Bassman 5f6a
Completed builds: .....ummmm yeah. Too many to list.....
Location: Ithaca, New York
Has thanked: 134 times
Been thanked: 93 times

Post by Lani »

Yeah, that's what it looks like although I think it's more for muting the input and output while switching then for stability reasons.
Also, it looks to me that the schematic is missing a (Vc) label right after the polarity protection diode (D5)

Post Reply