So a friend of mine gave me his 70s op amp muff fuzz to see if i could fix it and i'm running into a few problems. First, someone "fixed it" before me and did an awful job at it since apparently, he wasn't able to make it work. The battery leads were poorly soldered back in, a few traces on the pcb were ruined and bridged, and there's a mess of flux and solder blobs everywhere that was touched up. The pedal works to a degree, but the sound is very weak and gated and you can only hear at all on the low e string if you really hammer it hard. I suspect it might be a problem with the power supply, considering that's where new wires were attached but in mot sure since i measured 9v on the lead and on the trace. Just want to be sure before i take the soldering iron to it.
The parts and values on the pcb also aren't matching up with any schematics iv'e ever seen for an op amp muff fuzz. I was expecting the values not to match up but i wasn't expecting to see an extra capacitor and resistor, both of which do look like they belong there. Any help?
Forgot to mention that the on off switch and the pot both work (the pot bring the volume down) and i do get a bypassed signal. Il try to get a picture up as soon as i can
Post up a potential schematic please.
Usually but not always. That I have to hit the lowest note possible really hard and even then it sounds gated thing. Is a biasing issue.
The fact someone's already messed with it might not help.
Rich
Here's a schematic
(http://www.beavisaudio.com/schematics/Images/Electro-Harmonix-Muff-Fuzz-Opamp-Schematic.png)
And here are some pictures of the pcb
Top (there's a resistor under the switch and a visible green capacitor just to the side)
(http://i1268.photobucket.com/albums/jj574/nwells200/20170325_193615_zpsatlmj3yz.jpg)
Bottom (keep in mind i haven't soldered anything yet)
(http://i1268.photobucket.com/albums/jj574/nwells200/20170325_193715_zpsjpigjpo7.jpg)
Know how to use a meter, Fog? Can you post voltages at each pin of the chip, as referenced from ground? (battery - terminal)
Have the unit on and plugged in but no guitar playing into it.
Dot on chip upper left side is the #1 pin, then they go down 1234, jump to right side and then it goes bottom to top 5678.....
Voltages will tell is if there is a bias issue (R3, R5) as Rich suggested (a good deduction)
Post up some voltages.
I hope you didn't do that solder job.
There's a lot of flux on there. It would probably be beneficial to clean that up with some alcohol and a tooth brush.
Il get some voltages on the ic pins, i have an old meter lying around here somewhere.
and no that soldering wasn't me :)
Quote from: HeavyFog on March 25, 2017, 07:25:51 PM
and no that soldering wasn't me :)
Phew my thoughts might have insulted you else.
Alright here are the pin voltages:
1-5.28v
2-5.17v
3-4.46v
4-nothing
5-4.46v
6-5.16v
7-5.28v
8-9.37v
Not entirely sure what to make of these values but i have a feeling something isn't right herre
They're not far off.
Do you have access to a capacitor, so you can make an audio probe? I would just probe pin1 and pin 7 while someone plucks notes to see if it still seems "gated". you should have an audio probe for this work, anyway ;) If your audio is 'normal' thruout the circuit, then something else is wrong (vol pot?)
Read on, and post your results:
http://www.diystompboxes.com/pedals/debug.html (http://www.diystompboxes.com/pedals/debug.html)
Hi, it's great to see someone with an actual IC Muff Fuzz. I'm trying to build one at the moment.
Quote from: HeavyFog on March 25, 2017, 03:54:56 PM
The parts and values on the pcb also aren't matching up with any schematics iv'e ever seen for an op amp muff fuzz. I was expecting the values not to match up but i wasn't expecting to see an extra capacitor and resistor, both of which do look like they belong there. Any help?
I also tried to build one a few years ago but, as a lot of people have reported, it sounded gated, did not clean up with the guitar volume and sounded nothing like the transistor version of the Muff Fuzz or the single Muff1 in the the Double Muff.
Back then I used the same schematic you posted. And I guess most people did, since it's the only schematic found online together with this one:
(http://fuzzcentral.ssguitar.com/schematics/mufffuzzicschem.gif)
Most suggested mods are adding a resistor and/or capacitor parallel to the clipping diodes.
You then end up with a schematic like the one below, which I currently think is actually a Muff Fuzz.
It does sound like the transistor version of the Muff Fuzz and cleans up almost equally well.
(http://www.montagar.com/~patj/fuzz001.gif)
But as far as I can tell from the resistors in your picture it's not entirely the same. But that isn't too uncommon for EHX effects.
Would you be so kind to list the values of all the components or take more clear pictures, or just tell the colour bands on the resistors and markings on the caps and diodes?
UPDATE
For whatever reason its actually working now but its very quiet even when the volume is all the way up. I'm getting a signal that sounds like a muff fuzz, just a quiet one with a bit less gain. Maybe its the pot? Bypassing it with wires doesn't seem to do anything
And if you short the diodes?
Tried grounding them (if that's what you meant) and it just killed the signal all together. I'm starting to think its the pot since it crackles quite a bit and because its so old. Its just odd that suddenly its working but i haven't done a thing to it yet, so maybe i wiggled the pot in such a way that it works? Had the same issue before with my russian big muff.
(edit) it definitely doesn't have as much gain as it should
That's not what I meant.
If you think it's the pot audio probe it before the pot. It should be loud.
Probing it didn't give a signal that was any louder so i don't think its that. I heard from my friend that it worked fine, then the battery lead broke, so he gave it to someone he knew to fix it, and after that i didn't work at all, so i suspect now that the source of the problem has something to do with the attempted repair. It's not the battery leads since i'm getting normal looking voltages on the ic pins but perhaps its something else
And if you probe it after R4 (- input of U1b)
Nope still no difference. I think what il try to do is trace the circuit to see if there's a cut of something touching that shouldn't be, since it starting to look like that's the case
Quote from: HeavyFog on March 26, 2017, 03:00:11 PM
Tried grounding them (if that's what you meant) and it just killed the signal all together.
To ground them,you need to connect them to Vbias (4,5V) or if you want to connect them to 'real' ground (0V) you need a capacitor to block the DC otherwise you 'kill' the signal.
But since these are diodes in the NFB loop of an inverting op amp they are already tied to 'virtual' ground (Vbias).
And jumpering/shorting them would create 0 resistance in the feedback loop thus gain of zero giving zero output.
If you want them out of the circuit you'd really need to clip at least one leg of each. I wouldn't bother.
Quote from: HeavyFog on March 26, 2017, 02:40:10 PM
Maybe its the pot? Bypassing it with wires doesn't seem to do anything
You still need a suitable load, so if you really bypass/jumper the pot you should add a 50k resistor as load instead.
Quote from: HeavyFog on March 26, 2017, 02:40:10 PM
UPDATE
For whatever reason its actually working now but its very quiet even when the volume is all the way up.
Check with a magnifying glass for bridged solder connections or traces.
Maybe a silly suggestion but check if your guitar volume is fully up. A properly working Muff Fuzz cleans up very well. Depending on your pickups and volume pot it can go from clean on 8 to overdrive on 9 to fuzz on 10. But even the cleans shouldn't be "very quiet"
Not related to your problem but, like a fuzz face, this pedal has low input impedance and doesn't like another pedal in front (unless it's unbuffered and in bypass)
Quote from: HeavyFog on March 26, 2017, 02:40:10 PM
1-5.28v
2-5.17v
3-4.46v
4-nothing
5-4.46v
6-5.16v
7-5.28v
8-9.37v
Still a considerable difference between -IN and +IN pins. I measured my pins based on the Muff Fuzz schematic posted above:
1-4.40v
2-4.55v
3-4.55v
4-nothing
5-4.55v
6-4.55v
7-4.40v
8-8.97v
BTW I'm still very much interested in the component values of your pedal. :)
Il write down all the part values next chance i get. If you have a picture of the bottom of the pcb on your muff fuzz (if its an original) that would help as well.
Just noticed that there' s a resistor coming off ic pin 7 that doesn't seem to end anywhere. It doesn't go to ground and it doesn't connect to any other pad. I'm going to have to test all traces for continuity, and when (if) i get this thing working il record all values.
The 470K looks like it connects to pin 6, and the 10K looks like it might connect to pin 2. Is there a cap between that 10K and pin 2?
Quote from: Cozybuilder on March 26, 2017, 09:49:37 PMIs there a cap between that 10K and pin 2?
There should be or that 10k resistor should be directly connected to pin 2, but neither seems to be the case.
Quote from: HeavyFog on March 26, 2017, 09:34:30 PMJust noticed that there' s a resistor coming off ic pin 7 that doesn't seem to end anywhere.
I was trying to trace the circuit based on a photo I found online and came to the same odd conclusion.
Sorry for the MS-paint, it's all I have on this laptop. I gave up tracing eventually. Too many traces messed up/bridged.
This is the version with 1M bias resistors and 1N4148 diodes. (Many more pictures on my laptop. Can't remember where I got them)
(http://i66.tinypic.com/ra1aom.jpg)
(http://i64.tinypic.com/zvd8xw.jpg)
(http://i65.tinypic.com/4rt4ef.jpg)
Also a 1M bias resistors version (no idea about the rest). From here: http://midwestgroovewerks.blogspot.be/2014/08/fixing-old-muff-fuzz-and-lbp-1.html
4 caps: input, output, NFB, power filtering
(http://i64.tinypic.com/t9fnfr.jpg)
Version with 680k bias resistors (no info on the rest) From here: http://www.ehx.com/forums/viewthread/23/P30/#8496
6 caps
(http://i65.tinypic.com/2hoyzqu.jpg)
No idea about the components but has the best traces despite the small picture. From here: https://www.thegearpage.net/board/index.php?threads/ehx-muff-fuzz-on-the-slab.348731/
(http://i226.photobucket.com/albums/dd204/robsharer/muff.jpg)
Looks like that dead end resistor might not be specific to my example, so it looks like there wasn't a capacitor there in the first place. Thanks for posting that picture, IL trace the pcb and draw a circuit with all values as soon as i get a chance.
After looking at many more photos, you clearly miss that 100nF coupling cap.
In your version of the circuit I see it being connected to pin2 in two different ways:
(http://i350.photobucket.com/albums/q407/jan771/Mufffuzz13b_zpsgqldutrf.png)
New photos I have found:
(http://i350.photobucket.com/albums/q407/jan771/jspv8pa31h8xvec2ecn1_zpsykbio3rz.jpg)
(http://i350.photobucket.com/albums/q407/jan771/nrwefszsk3ehlkoknjim_zpsux7mwryu.jpg)
(http://i350.photobucket.com/albums/q407/jan771/nzkkc9r6tqhpazxkdakm_zpsbmjr8o6q.jpg)
(http://i350.photobucket.com/albums/q407/jan771/s-l1600_zpskebd9ffo.jpg)
(http://i350.photobucket.com/albums/q407/jan771/s-l1600b_zpsi7yfb1cz.jpg)
(http://i350.photobucket.com/albums/q407/jan771/s-l1600c_zpsaxh0qync.jpg)
UPDATE
Been busy with school so i haven't gotten a chance to look at it until now.
Still not working and i still haven't soldered anything. Working on tracing it and getting a schematic drawn up with all parts values. Still sounds weak and its giving me a loud hissing noise at all times. Connecting a battery or psu makes a very loud pop so i'm suspecting that it,might be the issue.
UPDATE 2
Just spent another few hours looking at it and tracing a schematic (IL post that later if i need to) and it looks like there is a 100n cap missing after the 10k resistor. Other than that the schematic is nothing too unusual, but it looks like the offboard wiring was redone very poorly. I'm fairly sure now that the 2 problems are that there's a 100n cap missing after the 10k resistor, and that the input wire should be connected to the input of the circuit on the dpdt, like the traced picture above shows. Bridging the 10k resistor and pin 2 makes a very loud noise. This makes sense since the input wire is actually connected to pot lug 1 (ground) and bridging pin 2 to the 10k would complete the circuit and with the improper offboard wiring, it causes the loud noise i heard. Il move the input to the correct lug on the dpdt, get rid of a ground wire on the input of the circuit that someone added, and add the 100n cap. Maybe then this thing will work.
UPDATE 3
Just redid the offboard wiring and its a bit louder, but still gated and weak. The bypass switch works and i get a bypassed signal, all the pin voltages still look good and i bridged the 10k to pin 2 (without a cap). Now i'm also getting a fuzzy hum in the background.
Here's a schematic i traced. I'm not entirely sure if this is right but i wrote down all the parts involved and after tracing this is what i got
(http://i1268.photobucket.com/albums/jj574/nwells200/20170408_215339_zpsw1nqh3xu.jpg)
Picture of the pot looks like it could have been opened up - the 4 metal folding tabs look tampered with. Maybe temporarily move the output wire to the hot end of the pot (3) to rule that out and leave it fixed at full volume for now.
Is the bottom edge of the frame of the bypass switch digging into that little red jumper wire?
Gating may well be due to our recently discussed friend - DC offset due to op-amp input bias current. One of the diodes is all or partly turned on, so either the positive or negative signal swing isn't getting fully heard. I can't see definite evidence of this in the posted voltages, but we don't know how much the DMM input resistance is changing the measurements by.
Far as I can see, reducing the gating effect involves some degree of modification.
A: Replace the 2 1M bias resistors with lower values - might reduce offset by enough - I'd try 470k. EXH have used 680k in some builds.
B: Fit a 470nF cap between pin7 and the 10k so the signal has to go though the cap - will remove DC offset on the diodes.
C: Try a different op-amp type. If that's a 1458, try an RC4558.
Just checked continuity on all suspect traces and it looks and i could't find any miss-joints or cuts in the trace. Also checked the pot for continuity and resistance and it checked out just fine in both ( i measured around 120k) so i don't think the pot is the issue. Il still bypass it to see if that works though. Also took a look at that jumper and checked it for continuity with the switch and there was none.The jumper itself still has continuity.
Quote from: anotherjim on April 09, 2017, 08:01:09 AM
Gating may well be due to our recently discussed friend - DC offset due to op-amp input bias current.
In that thread a coupling cap helped. Since the 100n coupling cap is missing here, it's worth a try.
Quote from: anotherjim on April 09, 2017, 08:01:09 AM
A: Replace the 2 1M bias resistors with lower values - might reduce offset by enough - I'd try 470k. EXH have used 680k in some builds.
Based on all the photos I've found there are two different versions of this pedal:
version 1:
1N4148 diodes and 1M bias resistors
version 2:
? diodes (see picture below) and 680k bias resistors.
The layout is different but all the other component values seem to be identical.
What diodes are these?
(http://i350.photobucket.com/albums/q407/jan771/mufffuzz-opamp_0001-1_zpsvkrfbvdm.jpg)
BTW, not a single online schematic (and I have found quite a few) is correct.
Can I just say that that is an ugly ugly piece of work by EHX? Sorry, I mean, I deeply respect some of the stuff they've done, but that is a really cheap shitty bit of pedal building. The PCB is crummy, the components are crummy, the build quality is awful, and the circuit is basic in the extreme, cashing in on the reputation they'd managed to establish with their earlier muff pedals.
I love EHX, but sheesh. That should never have left the drawing board. I realise this is hopelessly off topic and I hate off-topic posts. I can't even just let it go, it's that bad. Gagh.
Tom
Hi there,
Yeah I'd have to agree with Jim and suggest you change those 1M bias resistors to something like 10k to 200k each.
The main problem as I see it is that 0.1uF cap across one of the 1M bias resistors (the one connected to pin 3) is waaaay too small. Needs to be 10 to 100uF, together with the new 10k bias resistors to work properly. This cap is meant to be a short circuit to ac signals and at the moment it isn't.
That cap across the 1m is an area of suspicion. I wonder if it was there originally. Im certain the resistors are original and I know that the pedal worked fine before, so I'm not sure that's the issue. I won't rule out changing them though. I'm also sure that those diodes are 1n4148 so it looks like what I have is the 1n4148, 1m bias resistors version with a few different components
Honestly, if it were in front of me, I'd start by taking a lot of that solder away - and get a clearer view of what's connected to where.
This thread has piqued my interest.
Could anyone confirm which schematic is correct?
I've seen the Beavis one (without 100nF decoupling cap between the OA stages), and the other one *with* the cap - I'm confused wich one is the correct schem.
Cheers,
Ian.
Both schematics should be fine to build and both would work no problem. EHX did a lot of swapping out parts in the 70s and because of it we have lot of circuit variants, each with a slightly different sound. Iv'e sen the beavis audio version, the version with the cap, the 1m bias version, and this weird version i seem to have here. I just wish i knew what the circuit i have sounded like