Broken 70s muff fuzz

[HeavyFog]

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.

[Cozybuilder]

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?

[j_flanders]

Is 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.

Just 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)





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



Version with 680k bias resistors (no info on the rest) From here: http://www.ehx.com/forums/viewthread/23/P30/#8496
6 caps


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/

[HeavyFog]

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.

[j_flanders]

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:


New photos I have found:

[HeavyFog]

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.

[HeavyFog]

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.

[HeavyFog]

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

[anotherjim]

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.

[HeavyFog]

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.

[j_flanders]

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.

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?


BTW, not a single online schematic (and I have found quite a few) is correct.

[ElectricDruid]

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

[graemestrat]

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.

[HeavyFog]

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

[newfish]

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.

[newfish]

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.

[HeavyFog]

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